Sudoku Rules: How to Play Sudoku

The rule in one sentence

Every digit appears exactly once in each row, each column and each box. A box is a block with a thick border. A 9×9 sudoku uses the digits 1 to 9; a 6×6 sudoku uses 1 to 6.

A sudoku starts with some digits already filled in. Those stay fixed. Your job is to fill every empty square so the rule holds everywhere. There’s no arithmetic involved: the digits are just symbols that can’t repeat.

Rows, columns and boxes

• Row: all squares side by side, from left to right.
• Column: all squares stacked from top to bottom.
• Box: a block with a thick border. In a 9×9 sudoku it’s 3 squares tall and 3 wide; in a 6×6 sudoku it’s 2 tall and 3 wide.

Every square belongs to exactly one row, one column and one box at the same time. A digit only fits if all three are still missing it.

Step by step on a 6×6 sudoku

The small grid is a good place to learn. Columns are labeled A to F and rows 1 to 6.

1. One digit missing from a row

Look at row 1: it already has 1, 2, 4, 5 and 6. Only the 3 is missing, and it goes in the one empty square, C1.

2. Row, column and box together

Now square B2. Row 2 already has 6 and 2. Column B has 2, 6 and 4. The top-left box has 1, 2, 3 and 6. Together, that rules out 1, 2, 3, 4 and 6. The only digit left for B2 is 5.

3. The only spot in a box

Sometimes it helps to flip the question: where can a particular digit still go in a box? Take the 2 in the bottom-right box (D5 to F6). Row 5 already has a 2 in C5, so D5 and E5 are out. Column E has a 2 in E2, so E6 is out. D6 and F5 are filled. That leaves F6 as the only spot for the 2.

These three ways of looking are enough to solve the whole puzzle. Go through each row, column and box and ask: which digit is missing, and where can it still go?

The same rule on a 9×9 sudoku

The classic sudoku works exactly the same way, just bigger: 9 rows, 9 columns and nine boxes of 3 × 3 squares, filled with the digits 1 to 9. Each one appears exactly once in every row, column and box. The solving steps don’t change; it just takes a little more patience.

For more techniques for harder puzzles, from notes to the X-Wing, see our sudoku tips.

Common mistakes

• Guessing. A good sudoku can be solved without guessing. When you’re unsure, pencil in the possible digits and only fill in what’s certain.
• Checking rows only. A digit can be missing from a row and still sit in the same column or box already. Always check all three.
• Overlooking the box. In a 6×6 sudoku, the boxes are wider than they are tall. Watch the thick lines.
• Forgetting to clean up notes. When you fill in a digit, erase it from the notes in the same row, column and box.

Sudoku in a murder mystery

In Sudocrime, every chapter comes with a sudoku: a 4×4 puzzle in the practice case and a 6×6 puzzle in “Silence at Lindenrain”. In Lindenrain, every solved sudoku earns you a code digit and a snapshot for your evidence wall. Then you place the suspects on the floor plan with a similar rule: exactly one person per row and column. See how the two fit together in How a murder sudoku works.

You can check the rules in the app at any time through the question mark, and that doesn’t count as a hint. The hint itself works in three steps: it shows where to look, then explains the next step, and only at the end enters the correct number.