What Is Sudoku?

Sudoku is a logic puzzle played on a 9×9 grid where every row, column and box must contain the digits 1–9 exactly once.

Sudoku is a number-placement logic puzzle. You start with a grid that is partly filled in, and your job is to complete it so that every row, every column and every box contains each digit exactly once. There is no arithmetic involved — the numbers are just symbols, and the entire puzzle is solved by deduction.

A well-made Sudoku has exactly one solution that can always be reached by logic alone, never by guessing. That guarantee is what separates a real Sudoku from a random grid of numbers, and it is why the puzzle has stayed popular for decades.

The one rule that matters

Everything in Sudoku comes from a single constraint: each of the three kinds of group — rows, columns and boxes — must hold the digits 1 through 9 once each, with no repeats. Place a digit, and you simultaneously rule it out of the rest of its row, its column and its box.

  • Each row contains 1–9 with no repeats.
  • Each column contains 1–9 with no repeats.
  • Each 3×3 box contains 1–9 with no repeats.

Why it is a logic puzzle, not a maths puzzle

The digits could be replaced by any nine distinct symbols — letters, colours or shapes — and the puzzle would be identical. You never add, subtract or count; you only reason about where a symbol can and cannot go. That is why Wordoku (a letter variant) plays exactly like classic Sudoku.

Try it yourself

1
6
9
7
2
1
8
3
3
9
2
2
5
9
7
4
3
9
1
7
8
2
6
5
5
8
1
5
9
8
7
4
6
2
3
3
4
2
8
1

Tap a cell, then a number, to practise.

Frequently asked questions

Do I need to be good at maths to play Sudoku?
No. Sudoku uses digits as labels, not quantities — there is no arithmetic. Every move is a logical deduction about where a symbol can legally go.
Is every Sudoku guaranteed to have one answer?
A properly made puzzle has exactly one solution reachable by logic alone. Random or hand-made grids may have several solutions or none; published puzzles are checked to be unique.

Related reading

Practice online

Put it into practice on free puzzles with hints, notes and four difficulty levels.

Play Sudoku

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