Sudoku
Complete a classic 9×9 number puzzle directly in your browser.
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empty squares
36 to 39 pre-filled; 42 to 45 empty.
Rules
- Fill every empty square with a number from 1 to 9.
- Each row must contain every number exactly once.
- Each column must contain every number exactly once.
- Each outlined 3×3 box must contain every number exactly once.
- Pre-filled squares are fixed and cannot be changed.
Difficulty levels
- Beginner: 36 to 39 pre-filled squares and 42 to 45 empty squares.
- Intermediate: 30 to 32 pre-filled squares and 49 to 51 empty squares.
- Hard: 23 to 28 pre-filled squares and 53 to 58 empty squares.
Keyboard controls
Select a square, use number keys to enter values, arrow keys to move, and Backspace or Delete to clear a square.
A brief history
Howard Garns designed the puzzle that appeared as Number Place in 1979. Japanese publisher Nikoli introduced its version and the Sudoku name in 1984. The modern puzzle reached a broad international audience after appearing in The Times in 2004.
Sources: Cornell University Mathematics and Wolfram MathWorld.
Puzzle integrity
Puzzles are generated locally with sudoku-gen. Before a board is shown, Instruo verifies the completed grid, checks every pre-filled value against it, and confirms that the puzzle has exactly one solution.