Sudoku rules

Classic Sudoku starts with a 9-by-9 grid containing fixed clues. Fill the empty cells with digits 1–9 while following all three rules below.

The three rules

1. Rows: each horizontal row contains every digit from 1 to 9 once.
2. Columns: each vertical column contains every digit from 1 to 9 once.
3. Boxes: each of the nine outlined 3-by-3 boxes contains every digit from 1 to 9 once.

The printed clues stay fixed. Diagonals do not have an extra restriction in classic Sudoku; that belongs to Diagonal Sudoku.

A small example

Consider this almost-complete row:

1 · 2 · 3 · 4 · 5 · 6 · 7 · 8 · □

The missing digit is 9. Before entering it, check the empty cell's column and box too. If either already contains a 9, something in the current grid is wrong; choosing another digit would not fix the row.

What does “possible” mean?

A candidate is a digit that has not yet been ruled out for an empty cell. Small pencil marks record those options; they are not final answers. A digit that fits locally is not necessarily the correct placement. Look for a reason that forces it.

Learn, print or play

• Take the interactive beginner course to practise applying the rules.
• Download the rules cheat sheet in A4 or US Letter.
• Open the daily puzzle and choose a comfortable difficulty.
• Browse practical solving tips when you know the basics.

Do I need arithmetic?

No addition or multiplication is needed for classic Sudoku. The digits act as nine different symbols. Sum-based variants have additional rules, explained in the variants guides.

New to the game? Choose a first format on Sudoku for beginners. For quick answers about playing and downloads, see the FAQ.