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About this tool
Solve any Sudoku instantly or step-by-step with technique explanations. Generate new puzzles with unique solutions across four difficulty levels.
The Sudoku Solver takes any 9×9 grid you type in and either fills it instantly using backtracking with a fewest-candidates-first search, or walks the solution one deduction at a time, naming the technique behind each placement as a Naked Single or a Hidden Single in a row, column or box. It also generates fresh puzzles at four difficulties — 40, 32, 26 and 22 clues — each verified to have exactly one solution. Use it to finish a puzzle you are stuck on, or to see the reasoning you missed.
Open Sudoku Solver on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Step mode names the technique for every placement — naked single or hidden single by row, column or box — instead of dumping a finished grid.
A solution counter stops at the second answer, so an ambiguous or contradictory grid is reported rather than silently guessed at.
The backtracking search always branches on the cell with the fewest remaining candidates, which prunes the tree far faster than scanning left to right.
Yes — the instant solve uses backtracking, which is exhaustive, so it finds an answer for any solvable grid regardless of difficulty. Step-by-step mode is different: it only applies naked and hidden singles, so on very hard puzzles it will place what it can and then tell you no further logical step is available.
A naked single is a cell where only one digit remains legal after eliminating its row, column and box. A hidden single is a digit that has only one legal home left within a single row, column or box, even though that cell still looks like it could take several values.
You will be told the grid has no solution rather than getting a wrong answer, because the solver detects when a cell runs out of legal candidates. If your grid has more than one valid completion, the uniqueness check flags that too.
Easy grids keep 40 given clues, medium 32, hard 26 and expert 22, and each is carved from a full solution with every removal reverted unless the puzzle still solves exactly one way.