Rules · 1 player

How to play Sudoku

Fill the nine-by-nine grid so every row, every column and every three-by-three box contains the digits one to nine exactly once.

The rules, step by step

1. 1

   Some digits are given at the start and cannot be changed. Everything else is yours to work out.

2. 2

   Select an empty square, then pick a digit from the keypad below the board — or just type it if you are on a keyboard.

3. 3

   Switch to notes mode to pencil in the candidates for a square when you are not certain yet.

4. 4

   A digit that would clash with another in the same row, column or box is flagged so you can spot mistakes early.

5. 5

   The puzzle is finished the moment all eighty-one squares are filled without a single conflict.

Three tips that change how you play

• Scan for the digit that already appears most often on the board — it is usually the easiest one to place next.
• Work a box, then the row and column that cross it. Sudoku rewards finishing a region rather than wandering.
• If you have not placed a digit in several minutes, switch to notes and fill the candidates for one box. The answer is usually sitting there.

Placing a digit by scanning

Suppose the top-left box has a seven, the top-middle box has a seven, and the top-right box does not. Look at the rows those two sevens sit in — say the first and the second. Within the top-right box, the seven cannot go in the first or second row, because those rows already contain one. That leaves only the three squares in the third row of that box. If two of them are already filled with other digits, the seven has exactly one home and you can write it in with certainty. This is cross-hatching, and it will place the majority of the digits in an easy or medium puzzle before you ever need a pencil mark.

Three mistakes that cost beginners the game

Guessing a digit to see whether it works out.

A well-formed sudoku never requires a guess. If you cannot justify a digit from the rows, columns and boxes you can see, the deduction you need is elsewhere on the board — go and find it rather than committing.

Filling in pencil marks for the whole grid before doing anything else.

Eighty-one squares of candidates takes longer than solving the puzzle. Pencil in one box at a time, and only when scanning has stopped producing placements.

Reading a row and a column but ignoring the box.

Most missed placements come from forgetting the third constraint. When you test a digit, check all three regions every time — the box is the one people skip.

Common questions about Sudoku