The Rules of Sudoku

Sudoku has only three core constraints. Once you know them, every puzzle is about calm deduction, not guessing.

Step-by-step visual walkthrough

  1. Step 1: Know the grid

    A Sudoku board is a 9×9 grid split into nine 3×3 boxes. Some cells are already filled; these givens are fixed.

    Blue = row Green = column Yellow = 3×3 box

  2. Step 2: Apply the three constraints

    Every row, column, and 3×3 box must contain 1–9 exactly once. No duplicates in any group.

  3. Step 3: Use candidates (pencil marks)

    When a cell has multiple possible values, note candidates lightly and eliminate them as new numbers appear.

    1 4 7 9 7

  4. Step 4: Solve with logic, not guesses

    Start where there are fewer empty cells. Place certainties first, then revisit tougher areas.

Helpful links

Download the printable Sudoku Rules Cheat Sheet (PDF) Explore Sudoku strategies Practice with printable Sudoku puzzles Play daily in the Sudoku a Day app
Quick start: Begin with naked singles and hidden singles. These two techniques solve most beginner puzzles.

Sudoku rules FAQ

What are the basic rules of Sudoku?

Each row, column, and 3×3 box must contain 1 to 9 exactly once.

Is Sudoku a math puzzle?

No. It uses numbers as symbols, but solving is pure logic.

How do you solve Sudoku step by step?

Scan the grid, write candidates, eliminate options, and place values when only one remains.

Can you guess in Sudoku?

Good Sudoku puzzles are designed to be solved without guessing.

What is a pencil mark?

A pencil mark is a small candidate note in an unsolved cell.

How long does one puzzle take?

Beginners may take 20–30 minutes on easy grids; experts can finish much faster.

What is a naked single?

It is a cell with only one possible value after elimination.

Can Sudoku have multiple solutions?

Proper published Sudoku should have one unique solution.