Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. Some cells start filled; your job is to complete the rest so that every row, every column, and every box contains the digits 1 through 9 exactly once. There is no arithmetic and no guesswork — a well-formed puzzle always has exactly one solution reachable by reasoning alone.
Every puzzle on this site is generated fresh and checked for a unique solution before you ever see it, across five difficulty levels from Easy to Evil.
How do you solve a Sudoku puzzle?
Start by scanning one digit at a time. Pick a number, look at a 3×3 box, and check which rows and columns crossing that box already contain it — each appearance eliminates a whole line of cells, and often only one cell survives.
When scanning stops producing answers, switch to pencil marks: note every candidate for a cell, then look for pairs. If two cells in the same unit share exactly the same two candidates, those digits are locked into those cells and can be erased everywhere else in that unit.
Five tips for harder grids
- Work one digit at a time. Scanning cell by cell is slower and misses the patterns that scanning by digit reveals.
- Keep your notes honest. A stale pencil mark is worse than none — this site clears them for you when you place a digit.
- Look for the last empty cell in a unit. When a row, column, or box has eight digits, the ninth is decided.
- Learn one advanced pattern at a time. X-Wing first, then Swordfish; both are covered in our strategy guides.
- Resist the hint button for two minutes. The technique you are about to learn is usually one careful scan away.
Is Sudoku good for your brain?
Sudoku exercises working memory and pattern recognition, and unlike most puzzle games it gives immediate, unambiguous feedback: a contradiction is always your own mistake, never bad luck. That makes it an unusually honest way to practise careful reasoning — and a solved grid is its own reward.
New to the game? Start with our rules and beginner techniques, then try a solving technique.