Sudoku Solver

How to use the solver

Click the first square and type the digits row by row; the cursor moves on as you type, and the arrow keys move it anywhere. Leave blank squares empty. Press Solve to fill in the rest, or Check to see whether what you have entered so far still leads to exactly one solution, without revealing it. Example fills the grid with a freshly made hard puzzle if you want to try it out, and Clear empties it.

A tip for typing in a puzzle from paper: enter it one row at a time and glance back at the paper at the end of each row. Most failed solves come from one digit in the wrong square.

What the messages mean

• "Solved. This puzzle has exactly one solution." A proper puzzle. The grid now shows its answer.
• "Solved, but this puzzle has more than one solution." The puzzle is badly made, or a digit is missing from what you typed. The solver shows one valid answer, but a different one would be equally correct, which is why such a puzzle can't be finished by logic alone.
• "This puzzle has no solution." No grid fits the digits you entered. Usually it's a typing slip, a digit in the wrong square rather than a clash you can see.
• "Row 3, column 5 clashes with another 7" (for example). The same digit appears twice in a row, column or box. The message names the square so you can fix it.
• "A sudoku needs at least 17 given digits to have a single solution." Mathematicians proved in 2012, with a very large computer search, that no sudoku with 16 or fewer givens has a single solution. So the solver asks for at least 17 before it tries.

How the solver works

The solver doesn't try to imitate a person. It searches. It finds the empty square with the fewest possible digits, tries each of them in turn, and moves on to the next most constrained square, backing up whenever it reaches a dead end. Because it always starts where the choices are fewest, it finishes most puzzles in a fraction of a second.

It also doesn't stop at the first answer. It keeps searching until it either has proof that there's only one answer (the search runs out) or finds a second. That second check is what lets it tell you whether a puzzle is properly made, and it's the same check our generator uses on every puzzle we publish. Our guide on how sudoku puzzles are made explains that side.

The solver runs entirely in your browser. The puzzle you type isn't sent to us.

Checking your own progress

Partway through a puzzle from a book and worried you've gone wrong? Type in the original givens plus the digits you've added, and press Check. If it says "No solution from here", one of your digits is wrong. Remove your entries a few at a time and check again to find out which. If it says "Valid: exactly one solution", you're on track, and nothing has been revealed.

Stuck, not finished?

If you only want a nudge, look for the step you're missing instead of solving the whole thing. The techniques page lists every method in the order you'll need it, and our guides go deeper with worked examples, from naked and hidden singles to the X-Wing. If you keep getting stuck at the same point, common mistakes and how to get unstuck may help.

Or play one of our own puzzles, at easy, medium, hard or expert level, where the hint button gives one square at a time and names the technique behind it.