Basic Sudoku Rules

Welcome to Sudoku! A classic Sudoku is a 9×9 grid. The given numbers are fixed clues and cannot be changed. Let’s start with the only rules you need to remember—everything else is just a way to follow these rules more easily.

Basic Sudoku Rules

Welcome to Sudoku! A classic Sudoku is a 9×9 grid. The given numbers are fixed clues and cannot be changed. Let’s start with the only rules you need to remember—everything else is just a way to follow these rules more easily.

Row

A row is a horizontal line of 9 cells. Rows are numbered 1 to 9 from top to bottom.

Row example

Column

A column is a vertical line of 9 cells. Columns are numbered 1 to 9 from left to right.

Column example

Box

The grid is also divided into nine 3×3 boxes (sometimes called “blocks”). Boxes are usually numbered 1 to 9 from left to right, top to bottom.

Box example

How we refer to a cell

In this app, we write a cell position like r1c1:

r = row
c = column

Examples:

r1c1: row 1, column 1
r9c9: row 9, column 9

What counts as an error?

In any row / column / box, a digit cannot appear twice. If a number repeats, it’s an error and something needs to be corrected.

Error example

Your goal

Fill every empty cell with a digit from 1 to 9, so that:

each row contains 1–9 exactly once
each column contains 1–9 exactly once
each box contains 1–9 exactly once