How to play Sudoku
- The goal: fill every empty square so each row, each column and each 3×3 box contains the numbers 1 to 9, with no number repeated.
- Pick a square by tapping it, or move with the arrow keys.
- Enter a number with the number pad or your keyboard. Tap the same number again, or press Erase, to clear it.
- Use Notes to pencil in small numbers you think might fit. Press N on a keyboard to switch notes on and off.
What the buttons do
| Button | What it does |
|---|---|
| Notes | Numbers you enter become small pencil marks instead of answers. Placing an answer removes that number from the notes in its row, column and box. |
| Hint | Fills in the selected square with the right number. If that square is already right, it fills the first wrong or empty square instead. |
| Check | Tells you how many of your numbers don't match the solution, without showing which ones. |
| New puzzle | Makes a fresh puzzle at the chosen difficulty. |
How the puzzles are made
Each puzzle is made in your browser when you press New puzzle. We fill a complete, valid grid at random, then remove numbers one by one. After each removal we check the puzzle still has exactly one solution; if it would have two, the number goes back. So you never need to guess.
| Difficulty | Numbers given at the start |
|---|---|
| Easy | 40 |
| Medium | 32 |
| Hard | 26 (sometimes a few more, if removing more would allow a second solution) |
Tips for solving
- Look for the last empty square in a row, column or box. Only one number can go there.
- Scan one number at a time. Pick a number, say 5, and look at each box. If every square but one is blocked by a 5 in the same row or column, the 5 goes in that square.
- Use notes on harder puzzles. When a square has only one possible note left, that is its answer.
- Pairs help. If two squares in a row can only be 3 or 7, then no other square in that row can be 3 or 7.
Frequently asked questions
Does every puzzle have only one answer?
Yes. Before a puzzle is shown, it is checked to make sure there is exactly one way to complete it, so you can always solve it by logic.
Why does a square turn red?
Red means the same number appears twice in that square's row, column or box. A number can also be wrong without turning red, because it may break no rule yet but still not match the solution. Use Check to find out.
Can I save a puzzle and finish it later?
No. We don't use cookies or browser storage, so the puzzle is gone if you close or reload the page. Leave the tab open if you want to come back to it.
Can I play with a keyboard?
Yes. Tab to the grid, move with the arrow keys, type 1 to 9 to fill a square, press Backspace or Delete to clear it, and press N to switch notes on or off.