How to play sudoku — the rules
Sudoku is a number puzzle on a 9×9 grid. There is just one rule — you can learn it in a minute and enjoy solving for years.
The rule
The grid has 81 cells: 9 rows, 9 columns and 9 boxes of 3×3 (outlined with thick lines). Some cells are already filled in — these are the givens. Fill in the rest with digits 1 to 9 so that:
- every row contains all nine digits with no repeats;
- every column contains all nine digits with no repeats;
- every 3×3 box contains all nine digits with no repeats.
No arithmetic is involved: the digits are just nine different symbols. The puzzle is solved when every cell is filled and the rule holds everywhere.
How to start
- Pick a digit and scan the boxes. Take 5, for example, and look at a box without a 5. Rule out the cells whose row or column already has a 5. If only one cell is left, you've found it.
- Look for nearly complete rows, columns and boxes. When only two or three digits are missing, their places are usually easy to work out.
- Check single cells. See which digits are already in the cell's row, column and box. If only one digit is left unused, that's the answer.
- Use pencil marks. When the obvious moves run out, write the possible digits in empty cells in small print. Notes reveal pairs and eliminations — see How to solve sudoku.
Playing on this site
- Digits: select a cell and press a digit on the number pad or keyboard. Pressing the same digit again erases it.
- Notes: turn on "Notes" or press N — digits will be entered as pencil marks. Shift + digit adds a note without switching modes.
- Mistakes: a digit that doesn't match the solution is shown in red. You can turn checking off in the settings.
- Hint reveals the correct digit in the selected cell, Undo takes back your last move.
- Progress is saved in your browser: an unfinished puzzle will wait for you on the same page.
Difficulty levels
Difficulty depends not only on the number of givens but on the techniques needed to solve the puzzle. Easy and medium puzzles fall to singles, hard ones need candidate elimination, pairs and triples, and expert puzzles require X-Wing, XY-Wing or Swordfish.
A bit of history
The forerunner of sudoku, Number Place, appeared in American puzzle magazines in the late 1970s. The name "sudoku" was coined in Japan in the 1980s — it abbreviates a phrase meaning "the digits must be single". The puzzle became a worldwide craze in 2004–2005 when British newspapers started printing it.