Sudoku Think sudoku is just a 9×9 grid with the numbers 1–9? Think again. Since the classic puzzle exploded in popularity in the mid-2000s, puzzle designers have created an extraordinary universe of variants — from tiny 4×4 grids for kids to monstrous 25×25 behemoths, from cage-sum arithmetic challenges to thermometer paths and skyscraper clues.
This is the definitive sudoku variations list. We've catalogued every major sudoku type into clear categories, explained the rules of each, and linked to free playable versions wherever we offer them. Whether you're a beginner looking for your next challenge or a seasoned solver hunting for something entirely new, this guide has you covered.
Variants marked with a "Play Now" button are available as free online games right here on Sudoku Online Puzzles — no sign-up, no ads-wall, just puzzles.
🔢 Classic & Size Variants
These are the foundations of sudoku. The rules stay the same — fill every row, column, and region with unique digits — but the grid size and structure change dramatically.
Classic Sudoku
The original and most famous logic puzzle in the world. Fill a 9×9 grid so that every row, column, and 3×3 box contains the digits 1–9 exactly once. No maths required — pure deduction. Classic Sudoku remains the starting point for all variants and is published in newspapers, apps, and puzzle books worldwide.
▶ Play Classic SudokuMini Sudoku
A general term for any sudoku smaller than the standard 9×9. Mini grids — typically 4×4 or 6×6 — use fewer digits and smaller regions, making them ideal for children, beginners, or anyone wanting a quick puzzle fix in under a minute.
4×4 Sudoku
The tiniest standard sudoku. A 4×4 grid divided into four 2×2 boxes, using only the digits 1–4. With just 16 cells, puzzles are solved in seconds — perfect for introducing the concept of sudoku to young children or absolute beginners.
▶ Play 4×4 Sudoku6×6 Sudoku
A step up from the 4×4. The 6×6 grid is divided into six 2×3 (or 3×2) rectangular regions, using digits 1–6. It offers a satisfying challenge without the complexity of a full grid — great for kids, commuters, and warm-up exercises.
▶ Play 6×6 Sudoku8×8 Sudoku
A rare grid size using digits 1–8 in a grid divided into 2×4 rectangular regions. Less common than 6×6 or 9×9 because the region shapes are less visually elegant, but it appears in some puzzle magazines as a mid-range challenge.
9×9 Sudoku
This is standard Classic Sudoku — the 9×9 grid with nine 3×3 boxes. It's listed here for completeness. When people say "sudoku" without qualification, this is the size they mean. The vast majority of all sudoku variants are built on top of this 9×9 foundation.
▶ Play 9×9 Sudoku12×12 Sudoku
A larger grid using digits 1–12, divided into twelve 3×4 rectangular regions. The bigger number set increases the difficulty moderately and makes each puzzle a more time-consuming affair. Popular in puzzle books aimed at experienced solvers.
16×16 Sudoku
Also known as Super Sudoku or Hexadoku. Uses a 16×16 grid with sixteen 4×4 boxes and the symbols 1–9 plus A–G (or 0–9 and A–F in hexadecimal notation). A significant step up in complexity — a single puzzle can take 30–90 minutes even for experienced solvers.
25×25 Sudoku
The Giant Sudoku or Monster Sudoku. A 25×25 grid with twenty-five 5×5 boxes, using digits 1–25 (or letters A–Y). These behemoths can take hours to solve and test endurance as much as logic. They're the ultimate size-based challenge.
Samurai Sudoku
Five standard 9×9 grids overlapping in a cross formation — one central grid with four corner grids sharing a 3×3 box each. This creates a massive 369-cell puzzle where the overlapping regions create dependencies between grids. Solving requires juggling information across all five puzzles simultaneously.
▶ Play Samurai SudokuGattai Sudoku
An extension of the Samurai concept. Gattai-N refers to N overlapping sudoku grids arranged in various configurations. Gattai-3 uses three grids, Gattai-5 is equivalent to Samurai, and you can find Gattai-8 or even Gattai-13 in puzzle magazines. The more grids, the more cross-grid deductions required.
Overlapping Sudoku
A general term for any puzzle where two or more sudoku grids share cells or regions. The shared area must satisfy the rules of both grids simultaneously. Samurai and Gattai are the most common examples, but designers create countless creative overlap patterns.
Multi-grid Sudoku
The broadest umbrella term for puzzles containing more than one sudoku grid. The grids may overlap (like Samurai), sit side-by-side connected by shared clues, or interact through other constraint mechanisms. Multi-grid puzzles are among the most time-intensive sudoku challenges.
If you're new to sudoku, start with a 4×4 or 6×6 grid to learn the fundamental logic. Once you can solve a 9×9 on Easy, you're ready to explore the variant categories below.
🧩 Region & Shape Variants
These variants modify the shape of the regions within the grid or add entirely new regions that must also contain unique digits.
Jigsaw Sudoku
Replaces the nine 3×3 boxes with nine irregularly shaped regions (like jigsaw puzzle pieces). Each region still contains exactly 9 cells and must hold the digits 1–9. The irregular shapes break your spatial intuition and force you to think creatively about which cells share a region.
▶ Play Jigsaw SudokuIrregular Sudoku
Another name for Jigsaw Sudoku. The term "irregular" simply highlights that the regions are not standard 3×3 boxes. You'll see this name in European puzzle magazines and some apps.
Squiggly Sudoku
Yet another common name for the same concept — the curvy, squiggly boundaries of the irregular regions give this name its character. Squiggly, Jigsaw, and Irregular Sudoku all refer to the same variant.
Hyper Sudoku
Adds four extra 3×3 shaded regions overlapping the standard boxes. These four additional regions (positioned at rows 2–4/cols 2–4, rows 2–4/cols 6–8, rows 6–8/cols 2–4, and rows 6–8/cols 6–8) must also contain 1–9 with no repeats. The extra constraints make puzzles easier to start but require careful coordination.
▶ Play Hyper SudokuWindoku
Identical to Hyper Sudoku. The name "Windoku" comes from the four extra regions resembling window panes on the grid. The term is more common in European puzzle communities, while "Hyper Sudoku" dominates in the US and Asia.
NRC Sudoku
Named after the Dutch newspaper NRC Handelsblad, which popularised this format. It's the same as Hyper/Windoku — four overlapping 3×3 regions — but the name stuck in the Netherlands and parts of Europe.
Girandola Sudoku
Adds one extra region of 9 cells arranged in a pinwheel pattern around the grid: the four corner cells, the four edge-centre cells, and the centre cell (R1C1, R1C9, R2C5, R5C2, R5C5, R5C8, R8C5, R9C1, R9C9). These 9 cells must also contain 1–9 with no repeats.
Asterisk Sudoku
Adds a single extra region of 9 cells arranged in a star or asterisk shape, typically centred on the grid. The exact pattern varies by publisher, but the most common uses positions like R2C5, R3C3, R3C7, R5C2, R5C5, R5C8, R7C3, R7C7, R8C5. These cells must contain 1–9.
Center Dot Sudoku
The centre cell of every 3×3 box forms one extra region. These nine "centre dot" cells (R2C2, R2C5, R2C8, R5C2, R5C5, R5C8, R8C2, R8C5, R8C8) must also contain 1–9 with no repeats. It's a subtle but elegant additional constraint.
Hyper Sudoku, Windoku, and NRC Sudoku are all the same puzzle with different names, depending on which country you're in. The variant was independently popularised in multiple countries with different branding!
➕ Arithmetic & Cage Variants
These variants add mathematical clues — usually sums — to groups of cells called "cages." They blend sudoku logic with arithmetic reasoning.
Killer Sudoku
The king of cage-based variants. The grid is empty and divided into coloured cages, each showing a target sum. The digits in each cage must add up to that sum with no repeats. Standard sudoku rules apply on top. You'll need to master cage combination analysis and the powerful Rule of 45 (every row, column, and box sums to 45).
▶ Play Killer SudokuKiller X Sudoku
Combines Killer Sudoku with X-Sudoku. In addition to cage sums and standard rules, both main diagonals must also contain 1–9 with no repeats. The diagonal constraint adds a third layer of logic and makes these puzzles significantly harder — but also provides more entry points for deduction.
Mini Killer Sudoku
A smaller version of Killer Sudoku, typically on a 6×6 grid. Cages still show sums, but with fewer cells and digits 1–6, the puzzles are gentler. Ideal for learning Killer logic before moving to the full 9×9.
Calcudoku
Like Killer Sudoku but more flexible with operations. Cages show a target number and an operation (+, −, ×, ÷). You must apply that operation to the cage's digits to reach the target. Grids can be any size (4×4 to 9×9). The trademark version is known as KenKen.
KenKen-style Sudoku
Functionally identical to Calcudoku. "KenKen" is a registered trademark by Nextoy LLC, so many publishers use "Calcudoku" or "MathDoku" instead. The puzzle was invented by Japanese maths teacher Tetsuya Miyamoto in 2004 as an educational tool.
Sum Sudoku
A simplified version of Killer where cage operations are always addition. Some publishers use "Sum Sudoku" to distinguish it from Calcudoku-style puzzles that use multiple operations. Effectively interchangeable with Killer Sudoku in most contexts.
Kakuro Sudoku Hybrid
Blends the crossword-grid structure of Kakuro with sudoku's row/column uniqueness rules. Clue cells along the edge provide sum targets for runs of consecutive cells, but the grid maintains standard sudoku regions too. A rare and challenging combination from specialist puzzle magazines.
In any cage-based variant, always start with extreme cages — those with only one possible combination of digits. A 2-cell cage summing to 3 must be {1,2}, and a 2-cell cage summing to 17 must be {8,9}. These are your golden entry points.
📐 Line & Path Variants
These variants draw lines, arrows, or other paths across the grid. The digits along each line must satisfy a specific mathematical or relational constraint.
Arrow Sudoku
Arrows are drawn on the grid, each with a circle (bulb) and a line. The digit in the bulb must equal the sum of all digits along the arrow's line. This creates equations embedded in the grid. Arrows can bend and vary in length, and a bulb may have multiple arrows extending from it.
▶ Play Arrow SudokuThermo Sudoku
Thermometer shapes are drawn on the grid. Digits must strictly increase from the bulb (round end) to the tip of the thermometer. A 4-cell thermo bulb-to-tip means the values go up (e.g., 2-5-7-9). Thermos provide powerful ordering constraints that eliminate candidates quickly.
▶ Play Thermo SudokuRenban Sudoku
Purple or highlighted lines are drawn on the grid. The digits on each Renban line must form a set of consecutive digits in any order. For example, a 4-cell Renban line might contain {3,4,5,6} in any arrangement. No repeats are allowed on a single line.
Between Lines Sudoku
Lines with circles at both endpoints are drawn on the grid. Every digit on the line must be numerically between the two endpoint digits. If the endpoints are 2 and 8, every digit along the line must be 3, 4, 5, 6, or 7. A beautifully constrained variant.
Palindrome Sudoku
Lines are drawn on the grid where the sequence of digits reads the same forwards and backwards. A 5-cell palindrome line with digits A-B-C-D-E requires A=E and B=D. This creates mirroring constraints that are surprisingly powerful for elimination.
German Whispers Sudoku
Green lines where any two adjacent digits must differ by at least 5. This means only certain pairs can sit next to each other on the line: (1,6), (1,7), (1,8), (1,9), (2,7), (2,8), (2,9), (3,8), (3,9), (4,9). The digit 5 can never appear on a German Whispers line.
Region Sum Lines Sudoku
Lines cross the grid, and wherever a line passes through a different region (box), the digits in each segment within that region must sum to the same total. If a line crosses three boxes, the sum of digits in each box-segment must be equal. Highly constraining and logic-rich.
Entropic Lines Sudoku
Lines where every set of three consecutive digits must contain one low (1,2,3), one medium (4,5,6), and one high (7,8,9) digit. This creates a fascinating three-set constraint that ripples across the grid.
Lockout Lines Sudoku
Lines with diamond endpoints. The digits along the line must not be between the two endpoint values (inclusive). If the endpoints are 3 and 7, then no digit from 3–7 can appear on the line — only 1, 2, 8, or 9. The endpoints must also differ by at least 4.
Double Arrow Sudoku
A line with circles at both ends. The digits in both circles must sum to the same total as the digits along the line between them. A variant of Arrow Sudoku with two bulbs instead of one.
Line variants are incredibly popular in the competitive sudoku community. If you enjoy logic chains and cascading deductions, start with Thermo Sudoku — its "strictly increasing" rule is intuitive and immediately rewarding.
⬡ Diagonal & Extra Constraint Variants
These variants add global constraints that apply across the entire grid — diagonal uniqueness, chess-move restrictions, parity rules, and more.
X-Sudoku
Also called Diagonal Sudoku. Standard sudoku rules apply, plus both main diagonals (top-left to bottom-right, and top-right to bottom-left) must also contain 1–9 with no repeats. This adds 18 additional cell constraints and is one of the most widely published variants after Classic.
▶ Play X-SudokuDiagonal Sudoku
The same variant as X-Sudoku. "Diagonal Sudoku" is the more descriptive name, while "X-Sudoku" or "Sudoku X" references the X-shape formed by the two diagonals on the grid.
Anti-Knight Sudoku
No two cells that are a chess knight's move apart (an L-shape: 2 cells in one direction, 1 in the perpendicular) may contain the same digit. This global constraint is devastatingly powerful — it eliminates candidates you'd never suspect and leads to satisfying "aha" moments.
▶ Play Anti-Knight SudokuAnti-King Sudoku
No two cells that are a chess king's move apart (including diagonals) may contain the same digit. Every cell "sees" its 8 orthogonal and diagonal neighbours. This is stronger than Anti-Knight in some ways, as it prevents any two identical digits from touching, even diagonally.
Non-Consecutive Sudoku
No two orthogonally adjacent cells (sharing an edge) may contain consecutive digits. If a cell contains 5, none of its four direct neighbours can be 4 or 6. Also known as Anti-Consecutive Sudoku. This constraint has sweeping implications for candidate elimination.
Consecutive Sudoku
White bars or dots between cells indicate that those two cells do contain consecutive digits. Critically, the constraint works both ways: if there is no marking between two adjacent cells, they are not consecutive. This negative constraint is the real power of the variant.
Anti-Consecutive Sudoku
Identical to Non-Consecutive Sudoku — the naming varies by publisher. No orthogonally adjacent cells may contain digits that differ by exactly 1.
Disjoint Groups Sudoku
Also called Disjoint Sets. In addition to standard rules, cells in the same relative position within each 3×3 box (e.g., all top-left corners of every box) must also contain 1–9 with no repeats. This creates nine hidden extra regions based on positional index.
Odd-Even Sudoku
Certain cells are shaded or marked to indicate whether the digit must be odd (1,3,5,7,9) or even (2,4,6,8). The markings immediately halve the candidates for each marked cell and create powerful interactions with row/column/box constraints.
▶ Play Odd-Even SudokuEven Sudoku
A focused version of Odd-Even where only even-cell indicators are given. Shaded cells must contain even digits (2,4,6,8), while unshaded cells are unconstrained (can be odd or even). The asymmetric constraint creates surprising deductions.
Odd Sudoku
The reverse of Even Sudoku — shaded cells must contain odd digits (1,3,5,7,9). Since there are five odd digits and only four even ones in the set 1–9, the distribution asymmetry adds an extra layer to the logic.
Anti-Knight Sudoku exploded in popularity after being featured on the YouTube channel Cracking the Cryptic. The constraint is so powerful that Anti-Knight puzzles often need very few given digits — sometimes as few as 2 or 3!
⚖️ Comparison & Inequality Variants
These variants use symbols between cells to indicate size relationships, ratios, or specific mathematical connections between adjacent digits.
Greater Than Sudoku
Inequality signs (> or <) are placed between horizontally and vertically adjacent cells, indicating which digit is larger. Combined with standard sudoku rules, these relational clues create chains of deductions. Every pair of adjacent cells has a sign — making this a highly constrained and satisfying variant.
▶ Play Greater Than SudokuLess Than Sudoku
Functionally identical to Greater Than Sudoku with the arrows simply flipped. Some publishers present the inequality from the smaller side, others from the larger side. The solving logic is the same.
Futoshiki Sudoku
A hybrid of the Japanese puzzle Futoshiki (inequality puzzle) and Sudoku. The grid uses standard sudoku regions, but inequality signs between cells are selectively placed (not all pairs have signs). This selective placement makes it harder than full Greater Than Sudoku because you have less relational information.
XV Sudoku
Markings between adjacent cells: an X means those two digits sum to 10, a V means they sum to 5. Critically, all X and V markings are given — if there's no marking between two cells, they do NOT sum to 5 or 10. The negative constraint is extremely powerful.
Kropki Sudoku
Black and white dots between adjacent cells. A white dot means the two digits are consecutive (differ by 1). A black dot means one digit is double the other. All dots are given — no dot means neither condition holds. This creates intricate webs of constraints.
▶ Play Kropki SudokuRatio Sudoku
Similar to Kropki but generalised. Markings between cells indicate a specific ratio between the two digits (e.g., 1:2, 1:3). Some versions show the ratio explicitly; others use different coloured dots for different ratios.
In Kropki Sudoku, the 1-2 pair is special — it satisfies BOTH the white dot (consecutive) and the black dot (double). When you see a dot between two cells, always consider whether 1-2 could be the pair, because it's the only duo that works for both conditions.
🏙️ Positional & Visibility Variants
These variants add clues outside the grid that provide information about the digits inside — sums of specific positions, visibility counts, or boundary constraints.
Skyscraper Sudoku
Clue numbers surround the grid. Each clue indicates how many "skyscrapers" you can see looking into that row or column from that direction, where each digit represents a building's height. A taller building blocks the view of shorter ones behind it. For example, if a row is 2-1-4-3, looking from the left you see 2 skyscrapers (2, then 4 — the 1 is blocked by 2, and 3 is blocked by 4).
Sandwich Sudoku
Clues outside the grid show the sum of the digits between the 1 and the 9 in that row or column. For example, a clue of 10 means the digits sandwiched between 1 and 9 (exclusive) add up to 10. The positions of 1 and 9 are unknown, creating deep deductive puzzles.
▶ Play Sandwich SudokuOutside Sudoku
Clues are placed outside the grid indicating which digits appear in the first three cells (the nearest 3×3 box) of that row or column. Multiple digits per clue are common. The constraint helps you place digits within the correct box without specifying exact positions.
Little Killer Sudoku
Clues sit at the grid's corners and along its edges with arrows pointing diagonally. Each clue gives the sum of all digits along that diagonal. Unlike X-Sudoku, these diagonals extend from the edge and can be any length. Digits may repeat on Little Killer diagonals (unlike in standard Killer cages).
Sudo-Kurve
A creative variant where rows and columns are curved rather than straight. The grid may be circular, wavy, or irregularly shaped, but the fundamental rule remains: each "row," "column," and region must contain unique digits. The visual distortion makes it tricky to track which cells share a constraint.
Border Sudoku
Clues appear on the borders (edges) between cells or boxes, providing information about the digits on either side. For example, a border clue might indicate the sum or difference of the two adjacent cells. The format varies widely between publishers.
Clueless Sudoku
A sudoku puzzle with no given digits at all inside the grid — all information comes from external clues (like cage sums, outside clues, or other constraint types). The term is used loosely for any puzzle where the grid starts completely empty.
In Sandwich Sudoku, a clue of 0 is incredibly powerful — it means the 1 and 9 are adjacent to each other in that row or column, with no digits between them. Similarly, a clue of 35 means all seven remaining digits (2+3+4+5+6+7+8 = 35) are sandwiched, so 1 and 9 must be at opposite ends.
🔄 Clone, Repetition & Relation Variants
These variants create relationships or copies between groups of cells, forcing them to mirror, repeat, or mathematically relate to one another.
Clone Sudoku
Two or more regions of the same shape are highlighted. These "clones" must contain the exact same digits in the exact same positions. If one clone has a 7 in its top-left cell, every clone of that shape also has 7 in its top-left cell. This creates powerful cross-grid deductions.
Twin Sudoku
Two complete sudoku grids share some relationship. Often they share the same solution pattern rotated or reflected, or specific rows/columns of one grid must match the other. The interconnection between the grids requires solving them in parallel.
Mirror Sudoku
Cells in mirrored positions (reflected across a central axis) share a constraint — they must contain the same digit, or they must sum to a target, or one is a specific transformation of the other. The symmetry makes for visually beautiful and logically elegant puzzles.
Modular Sudoku
Digits are divided into three modular groups based on their remainder when divided by 3: low {1,4,7}, mid {2,5,8}, and high {3,6,9}. Shaded regions or markings indicate which modular group a cell belongs to. This replaces the odd/even constraint with a three-way partition.
Battenburg Sudoku
Wherever four cells meet at a corner in a 2×2 pattern, a Battenburg marking (a chequerboard square) indicates that the four cells alternate between odd and even in a chequerboard pattern. If there's no marking, they do NOT form a chequerboard. The negative constraint is key.
Quadruple Sudoku
Clues between four cells that meet at a corner list digits that must appear in at least one of those four cells. For example, a clue of "1,3,5" at a 2×2 junction means those four cells must collectively contain at least 1, 3, and 5 (among their four digits).
Battenburg Sudoku is named after the Battenburg cake, a British cake with a distinctive 2×2 chequerboard pattern of pink and yellow sponge. The puzzle's constraint mimics this alternating pattern!
🔤 Digit & Set Variants
These variants swap out the standard digits 1–9 for letters, symbols, or alternative number systems.
Alphabet Sudoku
Replaces digits with 9 letters of the alphabet. The logic is identical — every row, column, and box must contain each letter exactly once — but using letters instead of numbers adds a surprising cognitive twist that makes the puzzle feel fresh and different.
Letter Sudoku
Functionally identical to Alphabet Sudoku. Some publishers prefer this name for puzzles that spell out a word or phrase using the selected 9 letters, adding a wordplay element to the logic puzzle.
Symbol Sudoku
Uses pictographic symbols (shapes, icons, emojis) instead of digits or letters. Popular in children's puzzle books where colourful stars, hearts, and animals replace abstract numbers. The logic is unchanged but the visual experience is more engaging for younger solvers.
Wordoku
A special type of Alphabet Sudoku where the 9 letters chosen spell out a specific 9-letter word along one row, column, diagonal, or region. Solving the puzzle simultaneously reveals the hidden word. A clever blend of word games and logic.
Roman Numeral Sudoku
Uses Roman numerals (I, II, III, IV, V, VI, VII, VIII, IX) instead of Arabic digits. The puzzle logic is identical, but reading and comparing Roman numerals requires more mental processing — especially for higher values. An interesting cognitive challenge.
Hexadoku
A 16×16 grid using 16 symbols — typically 0–9 plus A–F (hexadecimal). The grid is divided into sixteen 4×4 boxes, and every row, column, and box must contain all 16 symbols. These puzzles are substantially harder and more time-consuming than standard 9×9 sudoku.
Working with letters or symbols instead of digits removes the temptation to "feel" for mathematical relationships. This can actually sharpen your pure logic skills — try a Wordoku to see if your solving approach changes when numbers are gone!
🔧 Rules-Changing Variants
These variants modify the fundamental rules of sudoku itself — changing which digits are available, altering the constraints, or introducing new logic systems.
No 1 Sudoku
A puzzle where the digit 1 does not appear anywhere in the grid. Instead, you use digits 2–9 plus one blank cell per row, column, and box. The blank acts as a phantom digit and creates intriguing absent-value logic. A surprisingly disorienting twist on the classic.
No Repeats Outside Region Variants
An umbrella term for puzzles where cells outside a defined region (e.g., outside the central box) carry additional non-repetition constraints. These modify where uniqueness rules apply, creating extra layers of restriction beyond the standard rows, columns, and boxes.
Missing Digits Sudoku
The set of available digits is unknown — you must deduce which 9 numbers or symbols are being used as you solve. The given clues hint at the set, but part of the puzzle is figuring out the "alphabet" itself. A meta-puzzle wrapped in a sudoku.
Zero Sudoku
Uses the digits 0–8 instead of 1–9. The logic is identical, but the inclusion of zero changes mental arithmetic in cage and sum variants (since 0 contributes nothing to sums). Some find it psychologically disorienting.
Greater Area Sudoku
The grid is divided into regions of unequal sizes. Larger regions contribute more cells, and constraints may differ based on region area. This breaks the symmetry of standard sudoku and can require entirely different solving strategies.
Consecutive Pairs Sudoku
Markings indicate which pairs of adjacent cells contain consecutive digits. Unlike full Consecutive Sudoku, only some pairs are marked, and unmarked pairs may or may not be consecutive. The partial information forces careful logical deduction.
🧪 Hybrid & Modern Logical Variants
These variants emerged from the modern competitive puzzle community, blending multiple constraint types in creative ways. Many were popularised through online puzzle-setting platforms and YouTube solving channels.
Chaos Construction Sudoku
The ultimate puzzle-within-a-puzzle. The regions themselves are undefined — you must simultaneously determine the shape of the nine regions AND fill in the digits. The rules state that you'll end up with nine 9-cell regions following standard sudoku rules, but discovering those regions is the core challenge. Widely considered the most difficult sudoku variant.
Domino Sudoku
Cells are grouped into dominoes (2-cell pairs). Each domino has a constraint — its two digits might need to be consecutive, sum to a target, or form a specific relationship. The domino pairing adds a tiling puzzle element to the sudoku logic.
Quad Sudoku
Small clue circles at the intersection of four cells contain digits that must appear in those four cells. Similar to Quadruple Sudoku but with subtle rule variations depending on the publisher — some require all listed digits appear, others treat them as a subset.
Triple Dot Sudoku
Dots at the intersection of three cells indicate a relationship among those three digits — they might sum to a target, form a consecutive set, or share another mathematical property. The three-cell grouping creates more complex constraint networks than simple pairwise clues.
Fortress Sudoku
Shaded cells must contain digits that are greater than all orthogonally adjacent unshaded cells. Unshaded cells near shaded ones are forced to be smaller, creating a topography of high and low values across the grid. A visually striking and logically satisfying variant.
Deficit Sudoku
Clues indicate the difference between a cell's digit and some reference value. Solvers must work backwards from the deficits to determine actual digit values. A mathematical twist that requires careful bookkeeping.
Maximum Sudoku
Cells marked with an arrow or symbol must be larger than all their orthogonal neighbours. If a cell is marked as a local maximum, every directly adjacent cell must contain a smaller digit. This creates peaks and valleys of value across the grid.
Minimum Sudoku
The reverse of Maximum. Marked cells must be smaller than all their orthogonal neighbours. Local minima create valleys that force surrounding cells upward, with dramatic effects when combined with row/column/box constraints.
Colored Sudoku
Cells are coloured and the colours carry constraints — cells of the same colour might need to contain the same digit, different digits, or satisfy some mathematical relationship. The colour groups create cross-grid connections beyond standard regions.
Capsule Sudoku
Capsule-shaped markings (ovals) enclose two cells. The two digits within each capsule must satisfy a constraint, often summing to a given value or being consecutive. Like Domino Sudoku but with distinct visual presentation and sometimes different rules.
With modern hybrid variants, always start by identifying which constraint is the most restrictive. For example, in a puzzle combining Anti-Knight and Thermo, the thermometer paths will usually give you the strongest early deductions — solve those constraints first, then layer in the Anti-Knight eliminations.
📰 Puzzle-Book & Newspaper Style Names
You'll also encounter these names in newspapers, airport puzzle books, and apps. Most are alternate names for variants listed above, or marketing terms for specific difficulty levels.
Sudoku X
The newspaper name for X-Sudoku / Diagonal Sudoku. Both main diagonals must contain 1–9. The Times newspaper in London was one of the first to publish "Sudoku X" as a regular feature.
▶ Play Sudoku XColor Sudoku
Can refer to either Colored Sudoku (colour-based constraints) or simply a classic sudoku with a visually colourful design. In children's puzzle books, it often means Symbol Sudoku with coloured shapes instead of numbers.
Extreme Sudoku
A marketing term for very hard classic sudoku puzzles. There are no rule changes — it's standard 9×9 sudoku but with minimal given digits and requiring advanced techniques like X-Wings, Swordfish, and forcing chains. Also called "Diabolical" or "Evil" by some publishers.
Monster Sudoku
Usually refers to very large grids — 16×16 or 25×25 — though some publishers use the name for any especially difficult or complex puzzle. The sheer size of these grids is what makes them "monstrous."
Squiggly Sudoku
The puzzle-book name for Jigsaw / Irregular Sudoku. The squiggly, curved boundaries of the irregular regions make it visually distinct from standard sudoku's rigid box lines.
Hyper Killer Sudoku
A hybrid combining Hyper Sudoku (four extra 3×3 window regions) with Killer Sudoku (cage sums). The four extra regions provide additional uniqueness constraints on top of the cage arithmetic. Rare but rewarding for advanced solvers.
Giant Sudoku
Any grid larger than 9×9, but most commonly used for 16×16 or 25×25 puzzles. The term emphasises the sheer scale and the additional time investment required to solve these massive grids.
Many of the names in this section are alternate names for variants described in the categories above. The same puzzle can have completely different names depending on whether you encounter it in a British newspaper, a Japanese puzzle magazine, or an American app.
🏁 Which Sudoku Variation Should You Try First?
With over 100 variants to explore, choosing can feel overwhelming. Here's a quick guide based on your experience level:
- Complete beginners: Start with 4×4 Sudoku or 6×6 Sudoku, then graduate to Classic 9×9.
- Confident classic solvers: Try X-Sudoku or Hyper Sudoku — they add one simple rule to the familiar grid.
- Ready for more complexity: Killer Sudoku and Arrow Sudoku add arithmetic that changes how you think about the puzzle.
- Experienced variant solvers: Anti-Knight Sudoku, Thermo Sudoku, and Sandwich Sudoku will push your logic skills.
- Looking for a real challenge: Kropki Sudoku, Samurai Sudoku, or the Chaos Construction puzzles will test even expert solvers.
The best way to learn a new variant is to start on the easiest difficulty and read the rules carefully before placing your first digit. Every variant on our site includes difficulty settings — use them!
The world of sudoku variations is vast and constantly growing. New variants are invented every year by creative puzzle designers, and the online community continues to push the boundaries of what's possible within a 9×9 grid. Whatever your skill level, there's always a new challenge waiting for you.
Happy puzzling! 🧩
Frequently Asked Questions
There are well over 100 documented sudoku variations. This list covers the major ones organised into 12 categories, from simple grid-size changes to complex hybrid puzzles. New variants continue to be invented by the puzzle community every year.
Chaos Construction Sudoku is widely considered the most difficult, since you must deduce both the regions and the digits simultaneously. Other extremely challenging variants include 25×25 Giant Sudoku, complex Samurai puzzles, and hybrids like Killer X Sudoku.
After Classic 9×9 Sudoku, the most popular variants are Killer Sudoku, X-Sudoku (Diagonal), Jigsaw Sudoku, and Samurai Sudoku. In the modern online community, Thermo Sudoku and Arrow Sudoku have gained massive followings.
Sudoku types include: size variants (4×4 to 25×25), region/shape variants (Jigsaw, Hyper), arithmetic/cage variants (Killer, Calcudoku), line/path variants (Arrow, Thermo), diagonal/constraint variants (X-Sudoku, Anti-Knight), comparison variants (Greater Than, Kropki), positional variants (Skyscraper, Sandwich), and many hybrid and modern variants.
You can play over 15 sudoku variants for free right here on Sudoku Online Puzzles! We offer Classic Sudoku, Killer Sudoku, Jigsaw Sudoku, Arrow Sudoku, Thermo Sudoku, X-Sudoku, Anti-Knight Sudoku, Sandwich Sudoku, Kropki Sudoku, Greater Than Sudoku, Odd-Even Sudoku, Hyper Sudoku, Samurai Sudoku, and more — all with no sign-up required.