How this solver works
Most online sudoku solvers fill in the grid and stop there. This one does that too — press Solve and you get the answer in a fraction of a second — but the more useful trick is the replay. Hit play and the solver walks back through the puzzle one deduction at a time, naming the technique it used at each step. If you got stuck on a particular cell, you can scrub forward to that cell and see the move that revealed it.
That's the whole pitch: an answer machine when you need one, a study tool when you want one. No sign-up, no daily limit, no rewarded ads to watch first. Paste a grid in or type your puzzle into the board and press Solve.
Reading the replay
Each step in the replay shows three things: the cells the solver was looking at, the technique it applied, and either a digit it placed or a set of candidates it eliminated. Cells in the foreground get a coral outline; cells whose pencil marks were trimmed get a yellow tint. The scrubber lets you jump anywhere in the trace, and the speed slider controls how fast autoplay moves.
If the puzzle is easy, the replay is short — usually a chain of naked and hidden singles. As difficulty climbs, more elaborate techniques appear, and a single hard step can unlock a cluster of easy ones in its wake. That's how harder puzzles work in human solving too: the cracks come from finding the one move you didn't see.
Techniques the solver knows
These are the deductions the solver walks through. Each one has a short explanation in our learn section.
- Naked single — a cell where eight digits are already taken in its row, column, and box. The ninth is the only candidate, so it goes there.
- Hidden single — a digit that has only one possible cell in its row, column, or 3×3 box. Even if that cell still has several candidates, the digit must go there because nowhere else fits.
- Naked pair and naked triple — two or three cells in a unit that share exactly the same two or three candidates. Those digits are locked into those cells, so they can be eliminated as candidates from every other cell in the unit.
- Locked candidates — when every candidate cell for a digit inside a 3×3 box lines up on a single row or column, that digit can be eliminated from the rest of that row or column. The mirror case (“claiming”) works the other way around.
- X-wing — a four-cell rectangle on two rows where the same digit only has two candidate cells in each row, and those cells line up on the same two columns. The digit must use one cell on each row, which eliminates it from the rest of those two columns.
When the solver says "advanced deduction"
Some puzzles need techniques that aren't on the list above — chains, unique-rectangle patterns, XY-wings, colouring. When the solver hits one of those, the replay shows a single "advanced deduction" step at the end and skips to the solved grid. We're adding more techniques over time, so this fallback comes up less and less.
If you want to see how a hard puzzle plays out from the human side, play a classic puzzle on the site — the play surface uses the same engine, with hints and pencil marks, so you can practise the techniques you saw in a replay.