The puzzle that needs no introduction
Classic 9×9 Sudoku is the purest distillation of constraint-based reasoning. A blank grid is sprinkled with 25–35 starting digits ("givens") and your task is to complete the grid so that each row, each column, and each of the nine 3×3 boxes contains the digits 1–9 exactly once.
What makes Sudoku timeless is its zero-luck, zero-knowledge nature. There is no vocabulary to memorise, no hidden information, no timing pressure — only deduction. Every digit you place either follows necessarily from the rules or it doesn't, and a well-formed puzzle has exactly one solution that can be reached by logic alone.
Why it works
The 9×9 grid hits a sweet spot: small enough to hold in working memory, large enough to require real strategy. Difficulty is controlled entirely by the givens — fewer givens and trickier deduction chains make harder puzzles. This infinite tunability is why Sudoku has, for forty years, refused to go out of style.
How to play
- Open the 9×9 grid — some cells are pre-filled (the "givens").
- Fill every empty cell with a digit from 1 to 9.
- Each row must contain 1–9 with no repeats.
- Each column must contain 1–9 with no repeats.
- Each bold 3×3 box must contain 1–9 with no repeats.
- When every cell is filled and all three constraints hold, the puzzle is solved.
Strategy tips
- Scan rows and columns first. If a digit already appears three times in a column, the remaining six cells in that column can't take it.
- Use pencil marks. Track candidate digits per cell; eliminating candidates is 80% of the work.
- Look for naked singles. When a cell has only one possible candidate, place it — no logic chain needed.
- Hidden singles in a box. If a digit can only go in one cell of a 3×3 box, that's its home.
- Never guess. A proper Sudoku has a logical path. If you're stuck, re-scan, don't speculate.
Play now
InteractivePrefer to read first? Scroll up for the rules and strategy tips.
Other Sudoku variants
Adds two more constraints: both main diagonals must also contain 1–9. A small rule change that completely reshapes the deduction landscape.
Replace digits with nine distinct colors. Same constraints, different brain region — visually intuitive and surprisingly tricky.