Rules
The rules of sudoku.
Fill a 9×9 grid so no digit repeats in any row, column or 3×3 box. That's the whole game. The rest of this page just spells those rules out, plus a few terms you'll see around the site.
The rules
The rules, one at a time.
Fill every cell with a digit from one to nine
A sudoku is a 9×9 grid of 81 cells. Every cell ends up holding a single digit, and the only digits used are 1, 2, 3, 4, 5, 6, 7, 8 and 9. There's no math involved: it's all about where the numbers go.
Every row holds each digit exactly once
There are nine rows, each nine cells wide. A finished row contains all nine digits, with no digit repeated and none missing. So once a digit appears in a row, it can't go anywhere else in that row.
Every column holds each digit exactly once
The same rule runs down the grid. Each of the nine columns contains every digit from one to nine exactly once, so a digit seen anywhere in a column can't appear again in that column.
Every box holds each digit exactly once
Heavier lines divide the grid into nine boxes of 3×3 cells. Each box also contains all nine digits exactly once. Boxes are the third rule, and together with rows and columns they're what make the puzzle solvable by logic.
The given digits are fixed
A published puzzle starts with some cells already filled in. These are the givens. They're part of the puzzle, not a hint: they never change, and any solution that contradicts one is wrong.
There's exactly one solution
A proper sudoku has a single arrangement that satisfies all three constraints at once. That's why every deduction can be certain: if two answers were possible, you could never prove a move was right.
Never guess
Guessing isn't against the rules, but it breaks the puzzle. A sudoku is designed to be finished by deduction alone, and the eight techniques the game teaches are enough for almost every puzzle you'll meet. And even a lucky guess leaves you unsure, for the rest of the grid, which moves were logic and which were luck.
Boxes aren't the only units
A unit is any group of nine cells that must hold all nine digits: a row, a column or a box. Most techniques are described in terms of units, because anything that works for a row works for a column or a box too.
The words
What the terms mean.
Seven words you'll see all over this site. None of them are hard, and together they cover most of what sudoku players talk about.
Given
A digit printed in the puzzle before you start. Also called a clue.
Cell
One of the 81 squares. A cell is named by its row and column, and it belongs to exactly one row, one column and one box.
Unit
A row, a column or a box: nine cells that between them must hold every digit once.
Peers
The twenty cells that share a unit with a given cell: eight in its row, eight in its column and four more in its box, after the overlaps are counted once. A digit placed in a cell removes itself from all twenty.
Candidate
A digit a cell could still take. Written into a cell as a small mark, it's called a pencil mark.
Single candidate
A cell with exactly one candidate left. Also called a naked single. It's the one move that needs nothing but counting.
Hidden single
A digit with exactly one possible cell left in a unit, even though that cell can also take others.
The next step
Now try a puzzle.
Knowing the rules isn't the same as solving a puzzle, but the gap is smaller than it looks. The walkthrough takes you through a first grid using three simple moves, and the technique pages explain each one in detail.
Common questions
What people ask about the rules.
Is sudoku a math puzzle?
No. The digits are just symbols: swap every 1 for a 9 and every 9 for a 1 and you'd have an equally valid puzzle. The only thing that matters is that no symbol repeats in a row, a column or a box.
Can a sudoku have more than one solution?
A properly made puzzle has exactly one. A puzzle with several solutions counts as broken, because you can only be sure of a move if there's just one possible answer.
Do I have to fill in pencil marks?
Not on an easy puzzle, where they mostly slow you down. On harder ones they're what makes the grid readable, and every technique past the first two needs a complete set.
How long does a sudoku take?
An easy one often takes about ten minutes; a hard one can take an hour or more. Don't worry about the clock. It's done when it's done.
What's the difference between sudoku and killer sudoku?
Killer sudoku adds dashed regions whose digits must add up to a printed total, which adds math to a puzzle that otherwise has none. Standard sudoku, and everything on this site, uses only the row, column and box constraints.
Rules learned. Time to play.
Sudoku Mori is a sudoku game for iPhone and iPad with no ads, no account and no internet needed. It teaches the eight techniques on this site as lessons, then gets out of your way. On iPad the board and controls sit side by side, as the sudoku on iPad page shows.