Basics
Sudoku is a 9×9 square grid split into nine small 3×3 boxes. Place the digits 1–9 so that three rules hold.
- Every row contains each of the digits 1–9 exactly once.
- Every column — once each too.
- And every small 3×3 box — once each.
How to place a digit correctly
Worked example
On the left is the grid with clues, on the right the finished solution. Below you can step through the solution.
Difficulty
A sudoku's difficulty is set by the hardest technique you must use to solve it logically — not by how many digits are given at the start. We use three difficulty levels.
- Easy
Naked and hidden singles, crosshatching and candidate elimination. Every step reveals a cell where only one digit fits.
- Medium
Naked and hidden pairs, triples and quads, pointing pairs and triples, box/line reduction. Most of the work is with candidates.
- Hard
X-Wing, Swordfish and Jellyfish, wings (Y-, XYZ-, WXYZ-Wing), chains, unique rectangles and, as a last resort, trial and error.
How difficulty is measured
- The number of starting clues is a weak guide: a board with 30 digits can be harder than one with 24 if it needs advanced techniques.
- A puzzle is rated by the single hardest technique it requires — one harder step lifts the whole puzzle to that level.
- A well-formed puzzle has exactly one solution. The known minimum number of clues for that is 17.
- Every puzzle here can be solved by pure logic; trial and error is a last resort for the very hardest boards.
Interactive lesson
What a 9×9 Sudoku is
In a 9×9 grid the digits 1–9 appear once in each row, each column and each small 3×3 box.
The nine small boxes are highlighted.