Free · Runs in your browser
Sudoku solver
Type the grid you are stuck on into the board, then either take one digit and carry on solving, or fill the whole thing. It runs entirely on your device — the puzzle is never sent anywhere.
Enter your puzzle: click a cell, then type or tap a digit.
Take one digit, not the answer
Most of the time the reason a board has stalled is that one deduction is being missed, not that the puzzle is unreasonable. Filling the grid in tells you nothing about which one it was. Asking for a single cell hands you the fact and leaves the rest of the solve intact — and more often than not the board opens up again from there on its own.
Next digit starts from whichever cell you have selected, so you can point it at the corner you are actually stuck in rather than taking whatever the top-left happens to be.
How the solver works
It is a constraint search: for every empty cell it tracks which digits are still legal in that cell's row, column and 3×3 box, always fills the cell with the fewest options left, and backs out the moment a cell has none. On a normal 9×9 grid that finishes in milliseconds, which is why there is no waiting and no upload.
It also checks the grid twice over. If a digit repeats in a row, column or box, it points at the cell instead of reporting failure — that is nearly always a typo rather than a hard puzzle. And if the grid you entered turns out to have more than one valid answer, it says so, because a puzzle with two solutions is a broken puzzle and no amount of logic will close it.
Seventeen clues is the floor
No Sudoku with fewer than 17 givens can have a single solution — that was settled by exhaustive computer search in 2012. If you are copying a puzzle in from a newspaper and the solver says there are several answers, count your clues first; a missing digit is the usual explanation.
Learn the move instead
If the same kind of position keeps stopping you, it is worth learning the technique rather than reaching for the solver each time. Stalls in the middle of a board are usually hidden singles; stalls late on a hard board are usually an X-Wing waiting to be seen. The techniques index walks through each one on a real grid.
Frequently asked questions
How does this Sudoku solver work?
It tracks which digits are still legal in each empty cell, always fills the cell with the fewest options remaining, and backtracks the moment a cell has none. On a 9×9 grid that finishes in milliseconds, entirely in your browser.
Is my puzzle uploaded anywhere?
No. The solver runs on your own device. Nothing about the grid you type in is sent to a server or stored.
Can I get a hint instead of the whole solution?
Yes — that is what Next digit does. It reveals one cell, starting from whichever cell you have selected, so you can point it at the corner you are actually stuck in and carry on solving the rest yourself.
Why does the solver say my puzzle has more than one solution?
Because a digit is probably missing. A proper Sudoku has exactly one answer, and no puzzle with fewer than 17 givens can. If you are copying a grid in from a newspaper, count the clues before assuming the puzzle is broken.
Why does it say no solution exists?
Two of your entered digits conflict somewhere further into the grid than a simple row or column check catches. The usual cause is a digit typed into the wrong cell — re-check the row you entered last.
Keep reading
Or make it a race
Action Sudoku puts a real opponent on the same grid, live, with a three-mistake limit. Free on iOS and Android.