The Rules of Sudoku
Sudoku has a reputation for being tricky, but its rules could not be simpler. There is just one rule, and no maths is involved. This page sets out exactly how the grid works, what counts as a finished puzzle, and how the popular variants change the rule. For a guided walkthrough of your first solve, see how to play Sudoku.
The one rule of Sudoku
Fill every row, every column and every 3×3 box with the digits 1 to 9, using each digit exactly once in each. That single rule is the whole game. The puzzle starts with some digits already placed — the “givens” — and from those you work out every remaining cell by logic alone.
The digits are just symbols; there is no adding or counting, so being good at maths makes no difference. You could play with nine colours or nine letters and the logic would be identical.
Rows, columns and boxes
A standard Sudoku has three kinds of group — called units — and each must contain 1 to 9 exactly once:
- Rows — the nine horizontal lines of nine cells.
- Columns — the nine vertical lines of nine cells.
- Boxes — the nine 3×3 blocks marked by the thicker borders.
Every cell belongs to one row, one column and one box at once, so a digit you place rules itself out of all three groups. A digit may never repeat within the same row, column or box.
What makes a valid solution
A Sudoku is solved when every cell is filled and every row, column and box holds the digits 1 to 9 with no repeats. A properly made puzzle also has exactly one solution reachable by logic — so you never have to guess, and there is only one correct grid to find. Every board here is generated and checked to guarantee that single solution.
The rules of Sudoku variants
Variants keep the one-of-each rule and add a twist:
- Killer Sudoku — dotted “cages” must sum to a given total, with no digit repeated inside a cage.
- Jigsaw Sudoku — the nine boxes are irregular shapes instead of 3×3 blocks.
- X / Diagonal Sudoku — the two main diagonals must also contain 1 to 9.
- Mini Sudoku — smaller 6×6 or 4×4 grids with the same idea, great for a quick game.
Sudoku Rules: FAQ
What are the rules of Sudoku?
Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. No digit may repeat within the same row, column or box. That single rule is the entire game.
Can a number repeat in Sudoku?
Not within the same row, column or 3×3 box. The same digit appears nine times across the whole grid — once in each row, once in each column and once in each box — but never twice inside one of those groups.
Does a Sudoku always have exactly one solution?
A properly made Sudoku does. The single-solution guarantee is what lets you solve by pure logic without ever guessing. Every puzzle here is checked to have one and only one solution.
Do you need to be good at maths to play Sudoku?
No. Sudoku never asks you to add or count — the digits are just nine distinct symbols. It is a pure logic puzzle, so anyone who can spot patterns can play.