Learn Sudoku

Sudoku Rules: Everything You Need to Know

CalmSudoku GuideUpdated August 27, 2026

The objective

The objective of standard Sudoku is to fill every empty cell in a 9x9 grid so that the digits 1 through 9 appear exactly once in every row, every column, and every 3x3 box. The starting numbers are clues, or givens, and they remain fixed while you solve.

Rule 1: each row contains 1 through 9

A completed row contains nine different digits. If a row already has a 6, no other cell in that row can contain 6. When solving, list the missing digits and use the crossing columns and boxes to determine where each one can go.

Rule 2: each column contains 1 through 9

Columns follow exactly the same restriction. A digit cannot repeat within a column. This means a candidate that looks possible from the row alone may still be impossible because the same digit already appears above or below it.

Rule 3: each 3x3 box contains 1 through 9

The nine bold 3x3 regions are equally important. Each must contain every digit once. Checking the box is what often eliminates the final candidate for a cell after the row and column have been considered.

You do not add the numbers

A common beginner misconception is that Sudoku is a math puzzle. The digits behave more like nine symbols than quantities. There is no addition, subtraction, multiplication, or division required. The challenge is constraint-based logic: determining which symbol can legally occupy each position.

Can you guess?

You can physically enter a guess, but disciplined Sudoku solving does not require blind guessing. A better approach is to record candidates and look for logical eliminations. If a move cannot be justified by the row, column, box, or a valid solving technique, keep investigating rather than gambling on a number.

What are pencil marks or Notes?

Pencil marks are small candidate numbers written in an empty cell. They are not final answers. They record which digits remain possible after checking the row, column, and box. CalmSudoku calls this feature Notes. Candidates should change as the board changes, so review them as you make progress.

When is a Sudoku finished?

A Sudoku is complete when every cell is filled and all three constraints are satisfied: no repeated digit in any row, column, or 3x3 box. A properly constructed standard puzzle is intended to lead to a consistent completed grid. CalmSudoku verifies its generated puzzles for a unique solution so the board is not asking you to choose between multiple valid endings.

Rules in one sentence

Place 1 through 9 once in every row, once in every column, and once in every 3x3 box. Everything else - notes, scanning, pairs, advanced patterns, speed - is a method for reaching that result.