Sudoku

1-9 with no repeats in any row, column or box - always exactly one solution

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About this tool

Sudoku fills a 9x9 grid with 1-9 so that no digit repeats in any row, column, or bold-outlined 3x3 box, and each puzzle is generated inside your browser from a random seed. For example, picking the default "Normal" difficulty tries to empty up to 50 cells out of a solved grid (40 for Easy, 56 for Hard), but any cell whose removal would create a second valid solution gets refilled, so only puzzles with exactly one solution are ever served.

How to use

  1. A puzzle appears right away. Pick easy, normal or hard.
  2. Tap a cell then a number below, or type 1-9 on the keyboard. Backspace clears.
  3. Tapping the same number again removes it.
  4. Not sure? Turn on Notes and jot down candidates. Confirming a number clears the notes in that cell.
  5. The row, column and box of the selected cell are highlighted, as is every matching digit on the board.
  6. Stuck? "Show answer" reveals it, but since that cannot be undone, you get an extra confirmation before it fills in.
  7. Share the link and your friend gets the exact same puzzle.

Frequently asked questions

Could a puzzle have more than one solution?
No. While generating, each removed cell is checked for a second solution and put back if one appears. So if you fill the grid legally, that is the one and only answer.
What do the red cells mean?
The same digit appears twice in that row, column or 3x3 box. It does not tell you whether a cell differs from the answer โ€” only rule violations are shown.
Does the shared link really give the same puzzle?
Yes. The address carries a puzzle number and the same puzzle is regenerated from it. Only the number travels, not all 81 cells, so the link stays short.
Do I lose my progress on refresh?
The digits you entered are lost, but the puzzle is not โ€” the number in the address brings back the same grid.
How many cells does each difficulty leave empty?
Easy tries to empty up to 40 cells, Normal up to 50, and Hard up to 56, out of 81 total. But any cell whose removal would break the single-solution rule gets refilled, so the actual number of empty cells can end up a bit lower than the target.
Can I undo after pressing "Show answer"?
No. It fills the entire grid with the solution and cannot be undone, which is why you get a confirmation step before it happens.
Do notes (pencil marks) count toward solving?
No. Notes are only for jotting down candidate numbers, and they clear from a cell as soon as you confirm a digit there. Whether the puzzle is solved is judged only by the confirmed digits.
Can I save several puzzles and come back to them later?
No, only the one puzzle you currently have open is tracked. Starting a new puzzle discards the old grid, though if you saved its puzzle number from the address, you can regenerate that same puzzle again later.
Do I need to be online to generate a puzzle?
No. Puzzle generation is all computed inside your browser, so you can create new puzzles even while offline.

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