Sudoku Solver
Solve any Sudoku with constraint propagation and backtracking, confirm whether the solution is unique, and see which techniques a human would need and how many guesses were required.
Last reviewed by the Radiatus Cloud team
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Constraint propagation solves most puzzles without guessing
Two rules do the bulk of the work. If a cell has only one candidate remaining it must take it, and if a value can go in only one cell of a row, column or box, it goes there. Applying these repeatedly until nothing changes solves every puzzle rated easy or medium outright, which is why those ratings exist: they describe puzzles that never require a guess. Only when propagation stalls does a solver need to try a value and backtrack, and the number of times it has to do that is a reasonable proxy for how hard a human will find it.
A proper Sudoku has exactly one solution
Uniqueness is part of the definition, not a nicety, and a puzzle with two solutions is broken even if both look valid. Finding a second solution is the only way to know, which means a solver that stops at the first answer cannot tell you whether the puzzle is well-formed. That matters for generated or transcribed puzzles more than published ones: a single mistyped clue frequently produces a puzzle with many solutions rather than none, and it still solves.
Seventeen clues is the proven minimum
An exhaustive computational proof published in 2012 established that no valid Sudoku with a unique solution has fewer than seventeen given numbers, and examples with exactly seventeen exist. The count is a poor guide to difficulty though: a puzzle with thirty clues can require far more work than one with twenty-two, because what matters is where the clues sit rather than how many there are.
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Frequently Asked Questions
What makes a Sudoku hard?
How often constraint propagation stalls and a guess becomes necessary. Easy and medium puzzles never require one; harder ratings describe how deep the search has to go.
Can a Sudoku have two solutions?
Then it is not a proper Sudoku, since uniqueness is part of the definition. A single mistyped clue usually produces many solutions rather than none, and the puzzle still solves.
What is the fewest clues possible?
Seventeen, established by exhaustive computational proof in 2012. Fewer than seventeen cannot have a unique solution.
Does more clues mean easier?
Not reliably. Where the clues sit matters more than how many there are, and a thirty-clue puzzle can be harder than a twenty-two clue one.
What formats can I paste?
Eighty-one characters using 0 or a dot for blanks, with or without line breaks and spaces. Grids copied from most sources paste directly.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Paste a puzzle to solve it and check whether the solution is unique.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.