Sudoku encyclopedia · Rules and variants

Sudoku rules, explained

Updated 2026-10-05

The three rules

A Sudoku is a 9×9 grid divided into nine 3×3 boxes. Some squares already hold a digit. Your job is to fill the rest so that:

each row holds every digit from 1 to 9 once,
each column holds every digit from 1 to 9 once,
each 3×3 box holds every digit from 1 to 9 once.

Sources: Wikipedia: Sudoku

Words players use

A cell is one square. A unit, sometimes called a house, is a row, a column or a box: any group of nine cells that must hold 1 to 9. The givens, or clues, are the digits printed at the start. Candidates are the digits that could still go in an empty cell; many players pencil them in small, as notes.

Sources: HoDoKu: Introduction and terminology

No arithmetic needed

In classic Sudoku the digits are symbols, not quantities. No arithmetic is needed, and nine different letters, colours or shapes can replace them without changing the rules. Our picture Sudoku for young children uses shapes this way.

Sources: Wikipedia: Sudoku, Nikoli: Sudoku

Exactly one answer

A well-made Sudoku has exactly one solution. If two solutions fit, the givens and the rules do not decide every cell, so there is no single right answer to reach. For the standard 9×9 grid, a puzzle needs at least 17 givens to have a single answer, a limit proved by an exhaustive computer search announced in 2012. Having 17 or more does not by itself guarantee a single answer.

Sources: McGuire, Tugemann and Civario, There is no 16-Clue Sudoku (arXiv 1201.0749), Wikipedia: Mathematics of Sudoku

Sources

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