How does the backtracking algorithm work?

The solver finds the first empty cell, tries digits 1–9, validates each against row/column/3×3 box rules, and recurses. If a path fails, it backtracks and tries the next digit. This guarantees a solution if one exists.

What if my puzzle has conflicts?

The solver checks for conflicts before solving. If any cell violates Sudoku rules (duplicate in row, column, or box), those cells are highlighted in red and you're asked to fix them.

Can it solve any puzzle?

Yes — backtracking solves every valid Sudoku puzzle. Hard puzzles may take longer (a few hundred milliseconds) but the algorithm always finds a solution if one exists.

What if there's no solution?

If the puzzle is unsolvable (contradictory givens that the conflict check missed), the solver reports “No solution” and explains that the puzzle has no valid completion.

Does it generate puzzles?

This tool solves puzzles you provide. To play a fresh puzzle, use the Word Search game or try the Sudoku Solver with the “Load sample” button for a classic challenge.

Sudoku Solver

Enter a 9×9 puzzle and watch backtracking solve it instantly — with conflict detection.

100% private — no uploads Free & unlimited Read tutorial

How to use Sudoku Solver

Follow these steps — everything happens instantly in your browser.

1
Step 1. Click cells and type digits 1–9 to fill in the puzzle. Leave blanks for empty cells.
2
Step 2. Click “Load sample” to try a classic puzzle if you don't have one handy.
3
Step 3. Click “Solve” — the solver validates conflicts first, then finds a solution via backtracking.
4
Step 4. Given digits appear bold; solved digits appear green. Use “Clear” to start over.