Sudoku Game — Play Free Online, 4 Difficulty Levels

How to play Sudoku

Fill the grid so every row, column and box contains each digit exactly once.

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.

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.

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).

The full guide, with a worked example and common mistakes: How to play Sudoku.

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.