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Every valid sudoku puzzle has exactly one solution, and a computer can find it systematically using backtracking — trying each possible digit in an empty cell, and undoing ("backtracking") whenever a choice leads to a dead end.
This solver finds the complete solution to any valid 9×9 sudoku puzzle.
How it works
Enter your puzzle as 81 characters (digits 1-9 for filled cells, 0 or a period for blank cells), reading left to right, top to bottom. The solver finds the next empty cell, tries each digit that doesn't conflict with that cell's row, column, or 3×3 box, and recurses — backtracking whenever every digit fails — until the entire grid is filled.
- Enter puzzle (81 characters, 0 or . for blank cells).
- Click Calculate to see your results.
Examples
A classic example puzzle
A well-known sample sudoku puzzle (often used to illustrate the rules) solves completely in a fraction of a second using this backtracking approach.
Who should use it
- Checking your progress or getting unstuck on a difficult sudoku puzzle.
- Verifying a puzzle actually has a valid solution before spending time on it.
Industry applications
- Puzzle and game solving
- Computer science education (backtracking and constraint satisfaction algorithms)
Advantages
- Solves any valid puzzle instantly, regardless of difficulty rating.
- Uses a standard, well-understood backtracking algorithm rather than puzzle-specific shortcuts.
Limitations
- Only verifies that a solution is internally consistent — it can't detect if you transcribed the original puzzle incorrectly.
Common mistakes to avoid
- Miscounting cells when transcribing a puzzle from a newspaper or app, resulting in fewer or more than 81 characters.
- Using letters or symbols other than 0-9 and the period for blank cells.
Best practices
- Double-check your puzzle transcription against the original before solving — a single mistyped digit will either produce an unsolvable puzzle or an incorrect solution.
Tips
- Backtracking search is a foundational technique in computer science, used far beyond sudoku — it's the same general approach behind solving N-Queens, maze generation, and many other constraint satisfaction problems.