The rules of sudoku

Sudoku has fewer rules than almost any game that holds an adult's attention for half an hour. You can learn them in two minutes, and nothing you learn later replaces them — every technique in existence is a shortcut for applying these same three sentences faster.

The three rules

Fill the grid so that every row contains the digits one to nine exactly once, every column contains them exactly once, and each of the nine three-by-three boxes contains them exactly once. That is all. The digits already printed are fixed and cannot be changed, and a properly made puzzle has exactly one solution.

What the rules do not say

There is no arithmetic in sudoku: the digits are symbols and could be nine colours or nine letters without changing anything. Nothing has to add up. Diagonals do not matter in classic sudoku, however tempting the shape looks. And no correct puzzle ever requires a guess — if you find yourself picking one of two options and hoping, there is a deduction you have not spotted yet.

Where to make the first move

Look for the row, column or box that already has the most digits, because the fewest possibilities remain there. Then take one digit at a time and ask where it can still go within that unit: often only one cell survives. That single habit will finish an easy puzzle from start to end without any named technique at all.

Notes, and when to bother

Writing small candidate digits into empty cells is not cheating and not required. On easy grids they mostly slow you down. From medium upwards they become the working memory that makes patterns visible: a pair of cells sharing the same two candidates is invisible in your head and obvious on paper.

Ready to try it

Easy grids are built so that the scanning habit above is always enough. Nothing there needs a technique with a name.

Play Easy Sudoku for kids How to Play

Frequently asked questions

Can a sudoku have more than one solution?

A properly constructed one cannot. If a grid allows two different completions it is considered broken, which is why every puzzle here is verified to have exactly one solution before it reaches you.

Do I need to be good at maths?

No. The digits never get added, subtracted or compared in size. Replace them with nine animal pictures and the puzzle is unchanged.

How many digits does a sudoku need to start with?

At least seventeen. That is a proven mathematical minimum for a unique solution; below it, more than one completion always exists. Most published puzzles start with between twenty-two and thirty.

Is guessing ever necessary?

Not in a correct puzzle. Every step can be justified by the three rules. If nothing is deducible, the missing piece is a pattern you have not learnt yet rather than a licence to guess.