Sudoku Solver

Type the clues of your puzzle into the grid (or load an example) and press Solve. Hint fills one cell and names the rule that proves it, so you can learn the logic instead of just copying the answer.

How to use the sudoku solver

1. Click a square and type a digit 1 to 9. Arrow keys move around the grid.
2. Press Solve for the full answer, Hint for one cell with its reason, or Check to test your progress.
3. Conflicting digits turn red. The solver also tells you if a puzzle has no solution or more than one.

The rules of sudoku

Fill the 9 × 9 grid so that every row, every column and every 3 × 3 box contains the digits 1 to 9 exactly once. A proper puzzle has exactly one solution.

Solving techniques

• Naked single: a cell where only one digit is still possible after looking at its row, column and box.
• Hidden single: a digit that has only one possible cell left in a row, column or box.
• Pairs and triples: when two cells in a unit can only hold the same two digits, those digits can be removed from the rest of the unit.
• Trial and error (backtracking): guess a digit, follow the consequences, and back up if you reach a contradiction. That is how a computer finishes the hardest puzzles.

The math of sudoku

There are 6,670,903,752,021,072,936,960 completed sudoku grids (Felgenhauer and Jarvis, 2005), and a puzzle needs at least 17 clues to have a unique solution (McGuire, Tugemann and Civario, 2012).

Sudoku logic quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

Can this sudoku solver solve any puzzle?

Yes, any valid 9 × 9 sudoku. It uses logic first, then a fast backtracking search for the hardest puzzles.

How do I get a hint instead of the whole answer?

Press Hint. It fills one cell and explains the rule (naked single or hidden single) that proves it.

What if my puzzle has more than one solution?

The solver tells you and shows one of them. Published puzzles should have exactly one.

Why does it say no solution?

Two clues conflict, or a typo made the puzzle impossible. Red squares show direct conflicts.

Is sudoku math?

Sudoku uses logic rather than arithmetic, but it is closely tied to Latin squares, combinatorics and graph coloring.

Sources

• Wikipedia: Mathematics of Sudoku
• McGuire, Tugemann and Civario, There is no 16-clue Sudoku (2012), arXiv:1201.0749

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