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 Odd-Even Sudoku?
Odd-Even Sudoku is ordinary sudoku with every square shaded one of two ways. Fill the grid so that:
- every row, column and 3×3 box holds 1 to 9 exactly once;
- and every odd square holds 1, 3, 5, 7 or 9, and every even square holds 2, 4, 6 or 8.
On this page the two shades are two tints: in the light theme odd squares are pale orange and even ones pale blue, and the dark theme uses a darker pair. The shading covers the givens too, and it never changes during a game. Because there are five odd digits and four even ones, every row, column and box has exactly five odd squares and four even ones, on every board.
In competition the variant usually marks only some squares — a square for even and a circle for odd, in the Grandmaster Puzzles form — and the unmarked squares could hold anything. Here every square is marked, so the shading tells you the parity of all 81 answers before you begin. That is why each square starts with five candidates or four instead of nine, and why boards on this page stay unique with far fewer givens than an ordinary sudoku.
What does the shading tell you before you place anything?
Start with your pencil marks as usual, from the row, column and box of each empty square, then strike everything of the wrong parity. On a real board it looks like this:
That strike is the whole first move, and it is a big one. Across 40 boards per level, the shading strikes a median of 56 pencil marks on Easy and 156 on Expert before any digit is placed, and settles 13 squares outright on Easy, 3 on Expert. An empty square on Easy starts with 3.1 candidates from its row, column and box and 1.9 once the shading is counted; on Expert, 5.3 and 2.9. From there it is ordinary sudoku on a grid where a square has a little over half the options it would otherwise have.
Two habits make it faster. Treat the odd and even squares of a house as two small houses: an even square can only take a missing even digit, so a row whose even squares still need a 4 and an 8 has a pair before you look anywhere else. And check both kinds of single — a square down to one candidate, and a digit with one square of its parity left in a house.
Does the shading actually do any work?
Every board this page deals is built so that the answer depends on it: the generator removes digits until the puzzle has one answer with the shading, then checks the same givens without it and keeps digging until they no longer have one. On the board above:
We checked 160 fresh boards, 40 per level: every one has a single answer with its shading and more than one without it. Without it, Easy givens have a median of five answers and up to 41, Medium a median of 198, and Hard and Expert more than 200 on every board we tried. A solver that ignores the shading stalls on every board at every level.
This page’s generator already got that right before this rewrite; its check was complete, and 160 of 160 old boards passed the same test. What changed is the levels. With every square shaded, the old targets of 38, 30, 25 and 21 givens left every level to singles — 39 of 40 Expert boards fell without a single pair or locked candidate — so the levels differed only in how much was handed over. They now give 34, 27, 21 and 17. At counts that low the old search took up to 572 milliseconds for one board on a desktop; the new one propagates as it goes and takes about 2 milliseconds at the median, under 6 at worst.
How do the four levels differ?
We ran 40 fresh boards per level through a solver that knows one technique at a time, then took techniques away to see which boards stopped. The shading is not a technique here; it is where every square’s candidates start.
| Level | Given digits | Marks the shading strikes at first look | Squares it settles | Singles finish it | Needs one more step | Beyond our solver |
|---|---|---|---|---|---|---|
| Easy | 31–34 | 56 | 13 | 100% | 0% | 0% |
| Medium | 27 | 87 | 10 | 98% | 0% | 3% |
| Hard | 21 | 125 | 6 | 95% | 0% | 5% |
| Expert | 17 | 156 | 3 | 68% | 8% | 25% |
40 boards per level from this page’s generator, fixed seed; medians, and shares rounded. “One more step” is any of locked candidates, naked pairs and triples, or the X-Wing; “beyond our solver” boards still have exactly one answer. Easy comes in a few givens under 34 when that is what it takes to make the shading load-bearing.
Singles carry most of this variant. Naked and hidden singles finish every Easy board, 39 of 40 Medium and 38 of 40 Hard. Expert is where it changes: two boards in three still fall to singles, but three in forty need one more step and one in four goes beyond everything our solver knows. This page used to promise that Hard needed pairs and box/line reduction and Expert needed X-Wing and Swordfish; measured, neither was true at the old levels, and at the new ones Hard is still singles nineteen times in twenty.
The generator fills a grid, reads the shading off it, and removes digits one at a time, keeping a removal only while the puzzle still has exactly one answer with that shading; a search it cannot finish answers “not proven” and the digit stays. We re-solved 160 fresh boards with a separate counter written for the purpose: every one had exactly one answer. The deepest uniqueness check took 29 search steps of the 4,000 it is allowed. With all 81 squares shaded, boards stay unique down to about 14 givens.
Which level should you play?
If you already play ordinary sudoku, start on Medium. Twenty-seven givens, and the shading does real work on every board — the same givens have close to 200 answers without it — yet singles finish 39 boards in 40. Easy is for learning to read the two tints at a glance: it settles 13 squares before you start.
Hard gives 21 givens and is still singles nineteen times in twenty; it is the level for speed. Expert gives 17, the same count as the fewest an ordinary sudoku can have, and it is the one level that asks for more than singles. Every board still has exactly one answer, and none needs guessing.
What goes wrong on an Odd-Even Sudoku
- Forgetting the shading on a given. Givens are shaded too, and they count: a 6 in a row is one of that row’s four even digits, so the row’s other even squares need only the other three.
- Writing both parities into a square. A pencil mark of the wrong parity is never right. Strike them first and the grid shrinks by about half.
- Missing the last odd square. A house’s five odd squares hold 1, 3, 5, 7 and 9 once each. When four of them are placed, the fifth odd square is settled, however many candidates its row and column still leave it; the same goes for the fourth even square.
- Misreading the tint in the dark theme. The two dark tints are closer together than the light pair. If you are unsure of a square, switch themes, or place a digit and watch: one of the wrong parity is marked straight away.
- Treating it as ordinary sudoku. Without the shading every board here has several answers, so a solve that ends in two possible digits for the same square went past a parity it did not use.
Where does Odd-Even Sudoku come from?
It is one of the oldest variants in competition. The first World Sudoku Championship, held in Lucca, Italy, on 10–12 March 2006, set eighteen variants alongside classic sudoku, and Odd/Even Sudoku was one of them, with Diagonal, Irregular and Toroidal. In competition the variant marks only some squares: on the Grandmaster Puzzles blog, for instance, a square means even and a circle odd. Shading every square, as this page does, gives away more and makes the variant a natural first step from ordinary sudoku.
What do the buttons do?
Select a square, then a digit from 1 to 9; the page opens on Medium. Selecting a square highlights its row, column and box. A digit of the wrong parity for its square is marked as an error 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; a wrong one leaves your 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.
Digits 1–9, the arrow keys, Backspace or Delete and Ctrl+Z all work from a keyboard. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
Odd-Even Sudoku is the variant that tells you something about every square. Battenburg Sudoku keeps the idea of parity but gives far less of it away: a marker between four squares says only that they form an odd-even checkerboard. Kropki Sudoku works with neighbouring squares too, with dots for digits one apart or one double the other, and Anti-Knight Sudoku adds an invisible rule instead of a visible one.