Sudoku Solver
Enter any valid 9×9 Sudoku puzzle and get the solution instantly. Click a cell, type a digit. Leave blanks for empty cells. Hit Solve.
- Click a cell and type 1–9 to enter a clue digit.
- Backspace or Delete to clear a cell.
- Arrow keys to move between cells.
- Paste to load an 81-character string (0 for empty).
- Blue digits in the solution were filled in by the solver.
This solver handles standard 9×9 Sudoku only. We're building solvers for more variants.
Explore All Variants Try Killer SudokuHow the Sudoku Solver Works
Our solver uses a backtracking algorithm - The same approach competitive programmers and AI systems use for constraint-satisfaction problems.
- Find an empty cell. Scans top-left to bottom-right.
- Determine valid candidates. Checks row, column, and 3×3 box.
- Try each candidate. Places a digit and recurses.
- Backtrack on contradiction. Undoes the last placement if no candidates remain.
- Return the solution. When all 81 cells are validly filled.
| Metric | Value |
|---|---|
| Typical solve time | < 50 ms |
| Worst-case (near-empty) | < 500 ms |
| Max backtrack iterations | ~200 |
| No-solution detection | Instant |
When Should You Use the Solver?
- Verify your completed puzzle
- Escape a dead end when you're completely stuck
- Solve puzzles from newspapers or books
- Generate answer keys for printable Sudoku worksheets
- Check that a hand-crafted puzzle is actually solvable
- Skipping a puzzle - You lose the cognitive benefit
- Learning - No explanation of which technique was used
- Leaderboard scores - Solver-completed puzzles don't count
- Daily challenges - Use in-game hints instead
Frequently Asked Questions
Can the solver solve any Sudoku puzzle?
Yes - Any standard 9×9 Sudoku that has at least one valid solution, regardless of difficulty. If your clues contradict each other and no solution exists, the solver tells you so instead of guessing. Variants such as Killer Sudoku or Diagonal Sudoku are not supported yet.
What if my puzzle has more than one solution?
The solver returns one complete, valid solution. It does not list every possibility, so a grid with too few clues may produce a different valid answer each time you solve it. A properly constructed Sudoku has exactly one solution - Which requires at least 17 clues.
Does the solver show the solving steps?
No. It returns the finished grid in one step, with the digits it filled in highlighted in blue. If you'd rather learn the step-by-step logic, our guide to Sudoku solving techniques explains every method from Last Free Cell to X-Wing.
Is using a Sudoku solver cheating?
It depends on context. Using it to check a finished grid, create answer keys, or escape a hopeless dead end is perfectly normal. Using it in the Weekly Competition is against the rules, and solver results never count toward leaderboard scores.
Is the Sudoku solver free?
Yes - Completely free, with no account required and no usage limits.
Our techniques guide covers all ten solving methods - From Last Free Cell to X-Wing. Then put them to the test: practice on Hard Sudoku, or grab printable Sudoku puzzles to solve offline.