How to Play Classic Sudoku

The complete rules of standard 9×9 Sudoku, explained for absolute beginners.

Classic Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. Some cells start filled with "given" digits; your job is to fill the rest using reasoning alone — no arithmetic and no guessing required.

A correctly made puzzle has exactly one solution that can always be reached by logic.

The rules

  1. Fill every empty cell with a digit from 1 to 9.
  2. Each row must contain all nine digits with no repeats.
  3. Each column must contain all nine digits with no repeats.
  4. Each 3×3 box must contain all nine digits with no repeats.
  5. The given digits cannot be changed; a valid puzzle has one unique solution.

The grid

The board has 81 cells arranged as nine rows, nine columns, and nine 3×3 boxes. Together these are called "units". Every unit must end up containing the digits 1 through 9 exactly once.

The three kinds of unit, each holding 1–9 exactly once: a row, a column and a 3×3 box.

Getting started

Begin by scanning for cells that can only hold one digit (naked singles) and digits that can only go in one cell of a unit (hidden singles). These two moves solve most easy and medium puzzles on their own.

  • Look along rows, columns and boxes for missing digits.
  • Use pencil marks to track remaining candidates.
  • Place a digit only when the logic is certain.

From singles to techniques

Once singles dry up, switch to elimination methods. Naked and hidden pairs lock candidates out of a unit; pointing pairs and box/line reduction trade candidates between a box and a line. Harder grids then call for fish (X-Wing, Swordfish) and wings (XY-Wing, XYZ-Wing), each of which exists only to create a new single somewhere else.

A reliable order of attack is: singles → locked candidates → naked/hidden subsets → fish → wings and chains. Reach for the simplest move that still progresses the grid.

  • Singles first, always — they need no pencil marks.
  • Then locked candidates and subsets to thin the field.
  • Save fish, wings and chains for when nothing simpler works.

Try it yourself

7
6
1
9
3
5
5
6
9
5
1
8
6
5
2
8
5
1
7
6
2
3
4
6
6
3
7
4
1
4
5
7
6
2
1
9
4

Tap a cell, then a number, to practise.

Frequently asked questions

Do I need maths to solve Sudoku?
No. The digits are just symbols — Sudoku is pure logic. You could play it with nine different colours instead of numbers.
Is guessing ever required?
Not in a properly constructed puzzle. Every step can be reached by logic, though hard puzzles need advanced techniques.
What is the best way to improve at Sudoku?
Learn techniques in order of difficulty and practise spotting each one until it is automatic. Start with hidden singles and naked pairs, then add locked candidates, then fish and wings. Solving a few puzzles a day at a difficulty that makes you think — but not stall — improves you fastest.

Play Classic Sudoku online

Free Classic Sudoku puzzles in your browser — no download, no account needed.

Play Classic Sudoku

We use Google Analytics to understand how people use the site. No tracking cookies are set unless you accept. Read our analytics cookie policy.