How to play Sudoku

Fill the grid so every row, column and box contains each digit exactly once. Also known as Number Place.

▶ Play Sudoku ⊞ Open the solver

The rules

A standard grid is 9×9, split into nine 3×3 boxes. Here you can also play 4×4, 6×6 and 12×12.
Write one digit in every empty cell — 1 to 9 on a 9×9 grid (1 to N on an N×N grid).
No digit may repeat in any row, any column or any box.
The digits already printed (the givens) cannot be changed. A good puzzle has exactly one solution.

Playing Sudoku on Puzzle Solvers

Choose a grid size and a difficulty, then press Generate Puzzle.
Click a cell and type a digit, or use the on-screen keypad. Notes switches to pencil marks.
Undo steps back; Force places one digit that has only one possible value; Check marks mistakes in red without giving away the answer.
Stuck? Send to Solver opens the same puzzle in the step-by-step solver. Send to Slate opens it as a board on the Slate whiteboard, to solve in front of a class.

Open the Sudoku game →

Using the Sudoku solver

Type or paste the puzzle — use Import puzzle string for an 81-character line of digits and dots.
Press Solve step by step. Each move names its technique (Naked Single, Pointing Pair, X-Wing …) and highlights the cells involved.
Use the arrows to move one step at a time, or Solve all remaining.
Every technique is explained on the Sudoku techniques page.

Open the solver →

A worked example

Say the top-left box is missing a 7. Row 1 already has a 7 further along, and so does row 2. That leaves only row 3 of the box for the 7.

Column 1 also has a 7 lower down, so of the three cells in row 3 of the box only columns 2 and 3 remain — and column 2 holds a given 4. The 7 must go in row 3, column 3. That is a hidden single: the cell could hold other digits, but it is the only home left for 7 in that box.

Strategies

Scan for hidden singles. Pick a digit and look where it already sits. Inside one box, if every cell but one is ruled out by that digit's rows and columns, it must go in the last cell.
Find naked singles. A cell whose row, column and box already use eight different digits can only take the ninth.
Use pencil marks. Once easy placements dry up, write the possible digits (candidates) in each cell. Most intermediate techniques work on those notes.
Look for pairs. Two cells in a unit that can only hold the same two digits lock those digits there — remove them from every other cell in that unit (a naked pair).
Pointing pairs. If a digit inside a box can only go in one row (or column), it cannot appear in that row anywhere else outside the box.
Advanced patterns. X-Wing, Swordfish and XY-Wing remove candidates across several rows and columns at once. The step-by-step solver shows each of them when a puzzle needs it.

Common mistakes

Guessing when stuck. A well-made Sudoku never needs a guess — look for a pair or a pointing pair instead.
Forgetting to update pencil marks after placing a digit. Every placement removes that digit from its row, column and box.
Checking only rows and columns and forgetting the box.

Where it comes from

Number-placement puzzles of this kind appeared in American puzzle magazines in the late 1970s. A Japanese publisher named them Sudoku in the 1980s, and they became a worldwide newspaper craze in 2004–05.

Frequently asked questions

How many givens does a Sudoku need?

A standard 9×9 Sudoku needs at least 17 givens to have a unique solution. Most newspaper puzzles give 25 to 30.

Is Sudoku a maths puzzle?

No arithmetic is involved — the digits are just symbols. It is a logic puzzle about placement; letters or colours would work the same way.

What does Devilish mean here?

The hardest level. Devilish puzzles need intermediate and advanced techniques such as pairs, X-Wings and chains, not just singles.

Can I check a puzzle from a newspaper?

Yes. Type it into the Sudoku Solver and it solves it one step at a time, naming the technique for every move.

Why do 6×6 and 12×12 grids have rectangular boxes?

Their boxes are 2×3 and 3×4, because 6 and 12 are not square numbers. The rule is the same: each box holds each digit once.

Keep going

All guides: How to play every puzzle and game.