The Rules of Sudoku
The complete rules of classic Sudoku in plain language: fill the grid so each row, column and box holds the digits 1–9 once.
The rules of Sudoku fit in a single sentence, but it is worth stating them precisely. Fill every empty cell with a digit from 1 to 9 so that no digit repeats within any row, any column, or any of the nine 3×3 boxes. The digits printed at the start — the "givens" — can never be changed.
Because the givens are fixed and the three constraints overlap, a correct puzzle has exactly one completion. You do not need to try every possibility: each placement is forced by what is already on the board.
The rules, step by step
Read the rules as a checklist you can verify at any moment during a solve. If a placement would break one of them, it is wrong — even if it looks plausible.
- Use only the digits 1–9.
- Each row must contain every digit exactly once.
- Each column must contain every digit exactly once.
- Each 3×3 box must contain every digit exactly once.
- The given (pre-filled) digits stay fixed.
- A finished grid satisfies all three constraints at once.
What "valid" means
A grid is valid while every filled cell still respects the three constraints, and solved once every cell is filled and no constraint is broken. You can always check your work by scanning each row, column and box for a repeat.
Try it yourself
Tap a cell, then a number, to practise.
Frequently asked questions
- Can a digit repeat on the diagonal in classic Sudoku?
- Yes. Classic Sudoku only constrains rows, columns and boxes — diagonals are unconstrained. The diagonal rule applies only to the Sudoku X variant.
- What happens if I place a digit that breaks a rule?
- The grid becomes invalid and can no longer be completed. On our boards an illegal repeat is flagged so you can correct it before going further.
Related reading
Practice online
Put it into practice on free puzzles with hints, notes and four difficulty levels.
Play Sudoku