Sudoku Rules

The three rules

Fill every empty cell of the 9×9 grid with a digit from 1 to 9 so that:

1. Row rule: each row contains the digits 1 to 9 exactly once.
2. Column rule: each column contains the digits 1 to 9 exactly once.
3. Box rule: each of the nine 3×3 boxes contains the digits 1 to 9 exactly once.

The starting numbers are fixed and cannot be changed. Everything else you need follows from these three rules.

The rules in action

Suppose a row already contains 1, 2, 3, 4, 5, 6, 7 and 8 with one empty cell left. That cell must be 9, because the row rule needs every digit exactly once.

In practice you combine the rules. A digit that already appears in a cell's row, column or box cannot go in that cell. When a cell has only one digit left, you place it. This is the core idea behind every solving technique on our Sudoku techniques page.

What makes a puzzle valid

• One solution only. A proper Sudoku has exactly one valid completed grid. Every puzzle on intosudoku is checked for this.
• Solvable by logic. Because the solution is unique, you never need to guess. If you feel you do, there is a technique you have not spotted yet.
• At least 17 givens. Mathematicians proved that a 9×9 puzzle with 16 or fewer givens cannot have a unique solution.

Common misunderstandings

• "Rows must add up to a number." Sums do not matter in classic Sudoku. They only matter in Killer Sudoku.
• "Diagonals must also contain 1 to 9." Not in the classic game. Only rows, columns and boxes count.
• "A digit can appear only once in the grid." No. Each digit appears nine times, once per row, column and box.
• "You need to guess sometimes." No. A valid puzzle is always solvable by deduction.

Rules of Sudoku variants

The same idea scales to other sizes. The digits used and the box shape change, but the three rules stay the same.

Killer Sudoku adds one more rule: the grid is divided into cages, each showing a target sum. The digits in a cage must add up to that sum and cannot repeat inside the cage.

Checking your solution

You have solved a Sudoku when there are no empty cells and every row, column and box contains each digit exactly once. To verify a position or get unstuck, try the Sudoku solver. To learn the process from scratch, follow how to play Sudoku.

Sudoku Rules FAQ

What are the basic rules of Sudoku?

Fill the grid so that every row, every column and every 3x3 box contains the digits 1 to 9 exactly once. The starting numbers cannot be changed.

Can a number repeat in a Sudoku?

A digit cannot repeat within the same row, column or box. It appears once in each, so nine times in the whole grid.

Do the diagonals matter in Sudoku?

Not in classic Sudoku. Only a special variant called Diagonal (or X) Sudoku adds the diagonals as extra units.

Does every Sudoku have only one solution?

Every properly made Sudoku does. This is what lets you solve it by logic alone, without guessing.