Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Renban Sudoku?
Renban Sudoku is sudoku with groups. Some squares are joined into groups of two to five, each outlined with a dashed line and tinted, and you fill the 9×9 grid so that:
- every row, every column and every 3×3 box holds 1 to 9 once, as usual;
- and the digits in each group are all different and make a run of consecutive numbers, in any order — a group of three might hold 5, 7 and 6, but never 5, 6 and 8.
Between half and two thirds of the squares belong to a group; the rest carry no extra rule. “Renban” is Japanese (連番) for consecutive numbers. This page uses the region form, as the World Puzzle Federation’s Sudoku Grand Prix does — its rule, as clarified in 2021, is that “digits placed in each shaded region must form a consecutive, non-repeating set” — while many puzzles today draw renban as a line through the squares instead, under the same rule.
Take the groups away and no board here has one answer any more: on Easy a median of 30 grids fit the same givens, and on every other level 200 or more. The other way round, when we dug 40 Easy boards as far as they would go, singles with the groups read still finished them from as few as 7 given digits, and from 10 at the median.
How do you read a group?
A group of k squares holds exactly the k digits of one run, so start by asking which runs it can still hold. A group of two holds one of 8 runs (1–2 up to 8–9), a group of three one of 7, a group of four one of 6 and a group of five one of 5. A digit already in the group, or one its squares cannot take, rules runs out, and each square keeps only the digits of the runs that are left. The plainest case is a group of two with one digit given:
Two more things follow from the runs. A digit placed in the group cannot appear in it again, even in a square that shares no row, column or box with it. And a digit that every possible run contains must be in the group: when only one of its squares can take it, it goes there, a hidden single inside the group. With those, singles finish every Easy and Medium board on this page. Selecting a square lights up the rest of its group, which makes the runs quick to check.
What do Hard boards add?
Locked candidates, and the groups make their own. When singles run out, put the digits each empty square can still hold in its pencil marks. A digit the group must hold is somewhere in the group; if all of its places in the group share a row, column or box, that house’s copy of the digit is the group’s, so it comes out of the rest of that house:
The ordinary kind works here too: a digit whose places in a box all lie in one row or column leaves the rest of that line. Hard boards are dug so that singles, the groups and locked candidates finish them.
And Expert boards?
Naked pairs. Two squares of one row, column or box that can each hold only the same two digits take those two between them, whichever way round, so no other square of that house can have either:
Expert boards are dug so that singles, the groups, locked candidates and naked pairs finish them, and they start with the fewest digits.
Were the boards on this page fair?
Nearly always, but not every time. Until October 2026 the generator blanked squares at random down to the level’s count, with no check as it went, then kept the board if a search found exactly one answer; after six boards that failed, it dealt a seventh with no check at all. That last board is where it went wrong: we dealt 400 boards per level from that generator, and 19 of the 1,600 had more than one answer — 5 Hard and 14 Expert, one of them with 40 — though the page promised exactly one solution, reachable without guessing. A player who found a different answer was marked wrong, because the game checks every digit against one stored answer. The levels also meant less than they said: 158 of 160 boards we dealt fell to singles with the groups read, so Hard and Expert differed from Easy mainly in how many digits they gave.
The generator was rewritten in October 2026. It now takes a digit out only while the level’s own steps, groups read, still finish the board, which also proves the board has exactly one answer, and it keeps digging until the groups matter. Three faults in how the board was drawn went at the same time. On the dark theme the groups kept their pale tints under near-white digits, which left the digits all but unreadable; the groups now have dark tints there. The tints also hid the selection: a group square in the selected row, column or box did not light up. And the groups were never outlined, so two touching groups could only be told apart by colour; each now has a dashed outline.
How do the four levels differ?
By how many digits are given, by the size of the groups, and by the steps a board is dug for. We dealt 40 fresh boards per level from the new generator:
| Level | Given digits | Group sizes | Dug for | Singles alone finish it |
|---|---|---|---|---|
| Easy | 30 (was 38) | 2–3 | singles | 40 of 40 |
| Medium | 24 (was 32) | 2–4 | singles | 40 of 40 |
| Hard | 16 (was 26) | 2–5 | + locked candidates | 25 of 40 |
| Expert | 12 (was 22) | 2–5 | + naked pairs | 10 of 40 |
40 boards per level, fixed seed; singles always with the groups read. On Hard, 7 boards could not be finished without locked candidates; on Expert, 22 needed locked candidates and 5 needed pairs. An Easy board sometimes starts with a few digits fewer than 30, when its givens alone would already fix the grid as ordinary sudoku and digging goes on until the groups matter.
The levels changed because the old ones all asked the same thing. At 38, 32, 26 and 22 digits almost every board fell to singles once the groups were read, so the new counts are lower, and Hard and Expert now need the moves above on a good share of their boards. Reading the groups at first look settles a median of 12 squares on an Easy board and 3 on an Expert one.
A digit comes out only while the level’s steps — singles with the groups read on Easy and Medium, plus locked candidates on Hard, plus naked pairs on Expert — still fill the grid, and each step only strikes digits that cannot go where they are struck. We re-counted 1,600 new boards, 400 per level, with a separate counter written for the purpose: every one had exactly one answer, and every one had more than one without its groups.
Which level should you play?
Start on Easy if renban is new to you, or on Medium, where the page opens, if you know sudoku well. Easy gives 30 digits and groups of two or three, Medium 24 and groups of up to four, and both fall to singles once you read the groups.
Read the small groups first: a group of two with a given 1 or 9 in it has only one choice for its other square, and any group with two givens far apart, say 3 and 6 in a group of four, is already fixed. Hard and Expert then add the two moves above, on boards with far fewer digits to start from and groups of up to five.
What do the buttons do?
Select a square, then a digit; the page opens on Medium. Selecting a square highlights its row, its column, its box and the rest of its group, and a wrong digit is marked the moment it goes in. Under the grid:
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column and its box (not the rest of its group); a wrong one leaves the other squares’ notes alone.
- Erase clears the selected square, and pressing the same digit twice does the same.
- Undo takes back the last change to a square, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one square with its answer, picked at random rather than the square you could work out next.
- Solve asks first, then fills the grid and stops the timer.
- New Game deals another board at the level you are on; the level buttons deal one at theirs.
From a keyboard: 1–9, the arrow keys, Backspace or Delete and Ctrl+Z; shortcuts such as Ctrl+1 are left to the browser. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
Other sudoku with drawn constraints: Killer Sudoku outlines its groups the same way but gives each a total, Thermo Sudoku makes digits rise along a line, and Arrow Sudoku makes a circle the sum of its arrow.