Sudoku Rules: The 12 Rules Every Player Must Know

Sudoku looks simple from the outside — a grid of numbers, some filled in, most blank. But understanding exactly how the rules work is what separates players who struggle from players who solve consistently. This guide covers every rule you need to know, from the absolute basics to the subtle constraints that make sudoku work as a puzzle.

The Basic Setup

Every standard sudoku puzzle is played on a 9×9 grid. The grid is divided into nine rows, nine columns, and nine 3×3 boxes. Some cells already contain numbers when you start — these are called givens or clues. Your job is to fill in the rest.

The numbers used in sudoku are 1 through 9. No other numbers are used, and no cell is left blank when the puzzle is complete.

The 12 Rules of Sudoku

Rule 1: Every row must contain the numbers 1 through 9

Each of the nine horizontal rows must contain every number from 1 to 9, each appearing exactly once. No number can appear twice in the same row.

Rule 2: Every column must contain the numbers 1 through 9

Each of the nine vertical columns must contain every number from 1 to 9, each appearing exactly once. No number can appear twice in the same column.

Rule 3: Every 3×3 box must contain the numbers 1 through 9

Each of the nine 3×3 boxes must contain every number from 1 to 9, each appearing exactly once. No number can appear twice in the same box.

Rule 4: No number can repeat within a row

This follows from Rule 1 but is worth stating explicitly. If the number 5 already appears in row 4, it cannot appear again anywhere else in row 4.

Rule 5: No number can repeat within a column

If the number 7 appears in column 3, it cannot appear again anywhere else in column 3.

Rule 6: No number can repeat within a 3×3 box

If the number 2 appears in the top-left 3×3 box, it cannot appear again anywhere else in that box.

Rule 7: Every cell must contain exactly one number

No cell can be left empty when the puzzle is complete. No cell can contain two numbers. Every cell has exactly one number from 1 to 9.

Rule 8: The given numbers cannot be changed

The numbers printed in the grid at the start are fixed. They are there to define the puzzle and provide the starting information you need to deduce the rest. You cannot remove, move, or change them.

Rule 9: Every puzzle has exactly one solution

A valid sudoku puzzle has one and only one correct solution. If you reach a state where two different completions of the grid are both valid, something has gone wrong — either the puzzle is faulty or you have made an error somewhere.

Rule 10: The solution must satisfy all three constraints simultaneously

Rules 1, 2, and 3 all apply at the same time. Every number you place must be valid for its row, valid for its column, and valid for its 3×3 box simultaneously. Meeting two of the three constraints is not enough.

Rule 11: No guessing is required or needed

Every valid sudoku puzzle can be solved through logical deduction alone. You never need to guess. If you feel stuck, it means there is a logical technique you have not applied yet — not that the puzzle requires a guess.

Rule 12: The order in which you fill in numbers does not matter

There is no required sequence for solving sudoku. You can fill in any valid number at any time. The only constraint is that each placement must follow the three core rules. Experienced solvers often work on whichever area of the grid has the most information available, rather than starting from the top left and working through systematically.

The Three Constraints Summarised

Everything in sudoku flows from three constraints:

Rows: Each number 1-9 appears exactly once per row. Columns: Each number 1-9 appears exactly once per column. Boxes: Each number 1-9 appears exactly once per 3×3 box.

These three constraints interact with each other across the entire grid, which is what creates the logical challenge. A placement in one cell affects the possibilities in every other cell in the same row, column, and box.

Common Misconceptions

Sudoku is a maths puzzle. It is not. The numbers 1 through 9 are used as symbols only. You could replace them with letters, colours, or shapes and the puzzle would work identically. No arithmetic is involved at any point.

You need to be good at maths to solve sudoku. You do not. Sudoku requires logical reasoning, not mathematical ability. Many excellent sudoku solvers have no particular affinity for maths.

Harder puzzles have different rules. They do not. Every sudoku puzzle — from the simplest easy grid to the most extreme expert level — follows exactly the same three rules. The difference between difficulty levels is how many givens are provided at the start, not any change in the rules themselves.

What Makes a Valid Sudoku Puzzle

For a sudoku puzzle to be valid it must meet two criteria. First, it must be possible to complete the grid following the three rules. Second, there must be exactly one way to do so. A puzzle with multiple solutions is considered faulty because it cannot be solved by logic alone — the solver would need to guess which solution is intended.

According to research by mathematician Gary McGuire, the minimum number of givens required to create a valid sudoku puzzle with a unique solution is 17. Any puzzle with 16 or fewer givens will either have no solution or multiple solutions. Most published puzzles have between 22 and 35 givens depending on the intended difficulty level.

Learn more about the mathematics of sudoku from the American Mathematical Society.

Now You Know the Rules — Start Playing

Understanding the rules is the first step. The real learning happens when you apply them to actual puzzles. Play a free easy sudoku puzzle to put the rules into practice, or read the full how to play guide for tips on getting started. When you are ready to go beyond the basics, the sudoku strategies guide covers every technique from beginner to advanced.