Sudoku checker: check your grid and find the mistake

How to use it

1. Choose Puzzle numbers and type every number that was printed with the puzzle.
2. Choose My numbers and type the numbers you filled in.
3. Press Check. Press Show where to see the wrong ones, or Next step when you are stuck.

What the results mean

• Exactly one answer: a proper Sudoku. The checker compares each of your numbers with that answer.
• More than one answer: a printed number may be missing or copied wrong, or the puzzle itself has more than one answer, so there is no single answer to compare with.
• No solution: at least one printed number is wrong, or some of your own numbers were typed as printed ones.
• Fewer than 17 printed numbers: no Sudoku with that few has exactly one answer. A computer search proved this in 2012.

Why the checker needs the printed numbers

A wrong number only shows up against the right answer. With the printed numbers, the checker works out that answer and compares each of your numbers with it. A check for repeats alone finds a wrong number only when it repeats in a row, column or box, and many wrong numbers don't repeat yet.

When you are stuck

Next step finds a square you can fill with a single: the only square left for a number in a row, column or box, or the only number left for a square. When no single is left, the puzzle needs a pattern such as a pair, an X-Wing or a chain; our lessons explain them, and the hint in our app shows the one your grid needs.