How the sudoku solver works

The solver uses a backtracking algorithm combined with the Minimum Remaining Values (MRV) heuristic. Instead of trying cells in order, it always picks the empty cell with the fewest valid candidates. This dramatically cuts the search space and allows even the hardest puzzles to be solved in under a millisecond.

Sudoku rules

A standard 9×9 sudoku requires that every row, every column, and every 3×3 box contains each of the digits 1–9 exactly once. The puzzle starts with some cells pre-filled (the "givens"), and you must deduce the rest.

Tips for entering puzzles

Click a cell and type the digit. Use Backspace or 0 to clear.
Navigate with Arrow keys or Tab / Shift+Tab.
Click Load Example to try Al Escargot, rated one of the hardest ever published.

Try to solve it yourself first

This solver is optimized for speed - it finds the solution in milliseconds via backtracking, not by mimicking human logic. If you're working through a puzzle to improve your skills, attempt it yourself before using the solver. Use the solver to check your answer or get unstuck on a specific step, not as a shortcut to skip the challenge entirely.

Unsolvable vs. invalid puzzles

There are three distinct outcomes when you submit a puzzle:

Unique solution: a properly formed sudoku has exactly one valid solution. The solver will find and display it.
Multiple solutions: if the puzzle was generated with too few givens or incorrectly designed, multiple valid solutions may exist. This is technically not a proper sudoku. The solver will find one solution and report that others may exist.
No solution: conflicting constraints make the puzzle impossible. This usually results from a typo when entering the puzzle. The solver will report that no solution exists.

Step-by-step solving mode

The current solver reveals the full solution at once. A step-by-step mode - which shows one logical deduction at a time with an explanation of which technique was applied - would be far more educational for players learning to solve harder puzzles. This is a potential future enhancement.