Sudoku

How to play

Click an empty cell and type a digit from 1 to 9; the pre-filled clue cells are locked and can't be edited. Every puzzle is generated fresh — the game builds a complete valid grid, then strips cells out one at a time, keeping each removal only when a solver confirms exactly one solution still remains, so there's never a puzzle with multiple valid answers or one that requires a guess. Use Check for mistakes at any point to flag incorrectly filled cells in red, without giving away the right answer or touching cells you've left empty. A timer runs in the corner from the moment a new puzzle loads and stops automatically the instant your entries exactly match the solution. Reveal solution fills in every remaining cell and stops the timer too, but unlike solving it yourself, it doesn't count toward your solved total.

Frequently asked questions

Is my solved count saved?

Yes — the number of puzzles you've completed by filling every cell correctly yourself is saved to your browser's local storage and will still be there the next time you open this page in the same browser. It only increments when your own entries exactly match the solution; using Reveal solution to finish a puzzle stops the timer but does not add to the count. Because it's stored locally rather than in an account, it won't carry over to a different browser or device, and clearing your browser's site data resets it to zero.

Are the puzzles really unique?

Yes. The generator first builds a complete, valid 81-cell grid using backtracking, then removes cells one at a time in random order. Before each removal is kept, a separate solver checks how many solutions the resulting puzzle would have; if removing that cell would allow more than one solution, the cell goes back in and generation moves to the next candidate. The result is a puzzle with exactly one valid solution at whatever clue count your chosen difficulty targets, never a guess-and-check grid with multiple valid fills.

Any solving tips?

Start with “naked singles” — cells where only one digit is possible once you rule out what's already in that row, column, and 3×3 box. Then scan each box for digits that can only fit in one remaining cell even though other cells look open at first glance. On harder grids with fewer clues, pencil-marking candidate digits in the margins (on paper, or just mentally) makes it much easier to spot these patterns instead of re-scanning the whole board from scratch each time.