Why does it say the puzzle has more than one solution?

A proper sudoku has exactly one solution. With too few clues or a digit missed while typing, several solutions exist; the tool fills in one of them and warns you. Double-check the puzzle, especially easily confused digits such as 1 and 7 or 6 and 8.

Can it solve the hardest sudokus?

Yes. It combines bitmask candidate tracking with backtracking that always tries the cell with the fewest candidates first, so even puzzles that need heavy trial and error are solved in a few milliseconds — and the search continues far enough to confirm whether the solution is unique.

How are the difficulty levels defined?

By the number of clues and the techniques required. Easy has about 40 clues and Medium about 32, and both can be finished with naked and hidden singles alone; Hard has about 26 and Expert about 22 and usually need more advanced techniques. Every generated puzzle has a unique solution.

What is the 81-character format?

The cells written left to right, top to bottom in one string: digits 1–9 for givens and . or 0 for blanks, e.g. 53..7....6..195…. Spaces, line breaks and separators such as | - + are ignored on import, so you can paste straight from a web page or text file.

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🔢Sudoku Solver - Solve Any Sudoku, Get Hints and Generate New Puzzles

Stuck on a sudoku? Enter the puzzle in the 9×9 grid (or paste an 81-character string) and press Solve to get the answer in milliseconds — even the puzzles billed as the world's hardest. The solver flags repeated digits in rows, columns and boxes and tells you when a puzzle has no solution or more than one. If you would rather not see the whole answer, use Hint: it finds the next cell that can be deduced with a naked single or a hidden single and explains why, while Show candidates pencils in every digit still possible for each empty cell. Check compares your entries with the unique solution and marks mistakes in red. It also works as a sudoku game: generate a fresh puzzle in Easy, Medium, Hard or Expert, every one guaranteed to have exactly one solution, with Easy and Medium solvable using basic techniques only.

Free · No sign-up · Runs in your browserLast updated: 2026-10-26

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How to use Sudoku Solver

1Enter the puzzle: click a cell and type 1–9, use the arrow keys to move, or paste an 81-character string
2Press Solve for the answer, Hint for one logical step at a time, or Check to verify your entries
3To play, choose a difficulty and press New puzzle

FAQ

Why does it say the puzzle has more than one solution?: A proper sudoku has exactly one solution. With too few clues or a digit missed while typing, several solutions exist; the tool fills in one of them and warns you. Double-check the puzzle, especially easily confused digits such as 1 and 7 or 6 and 8.
Can it solve the hardest sudokus?: Yes. It combines bitmask candidate tracking with backtracking that always tries the cell with the fewest candidates first, so even puzzles that need heavy trial and error are solved in a few milliseconds — and the search continues far enough to confirm whether the solution is unique.
How are the difficulty levels defined?: By the number of clues and the techniques required. Easy has about 40 clues and Medium about 32, and both can be finished with naked and hidden singles alone; Hard has about 26 and Expert about 22 and usually need more advanced techniques. Every generated puzzle has a unique solution.
What is the 81-character format?: The cells written left to right, top to bottom in one string: digits 1–9 for givens and . or 0 for blanks, e.g. 53..7....6..195…. Spaces, line breaks and separators such as | - + are ignored on import, so you can paste straight from a web page or text file.