How to use it
- Type the clues into the grid (arrow keys move, Backspace clears), or paste the puzzle as 81 digits with 0 or a full stop for each empty cell and press Load puzzle.
- Press Solve to fill every cell (solved cells appear in teal), or click an empty cell and press Hint to reveal just that one.
- If two clues clash, both turn red. If the puzzle has no solution, or more than one, the page says so.
How it solves
The solver keeps track of which digits are already used in each row, column and 3×3 box, picks the empty cell with the fewest possible digits, tries each of them in turn and backs up when it reaches a dead end. Always choosing the most constrained cell first is what makes this fast: a well-known very hard puzzle takes about 25,000 steps and a few hundredths of a second. The solver keeps searching after the first answer until it finds a second one or runs out of options, which is how it can tell you whether a puzzle has exactly one solution, as a proper sudoku should.
What we tested
On October 15, 2026 we generated test puzzles with a separate solver written in Python: 300 with exactly one solution, 150 with more than one, 100 with no solution even though no two clues clash, and 50 with clashing clues, plus a hard example from Peter Norvig’s essay on solving sudoku (solved in 3,723 steps). The page agreed on every one: the same single solution where there is one, two different valid grids where there are several, “no solution” where there is none, and the clash where clues collide. All 601 puzzles together took about 3011 ms. In Chromium on a 390 px screen, a pasted very hard puzzle was solved into a valid grid that keeps every clue, a hint filled the chosen cell with the correct digit, and a typed duplicate was flagged.
Sources
- Peter Norvig, Solving Every Sudoku Puzzle (constraint search with the most-constrained cell first).
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools