Classic Sudoku

Classic Sudoku is the full 9×9 puzzle: fill every row, column and 3×3 box with the digits 1 to 9. It needs no arithmetic — only logic.

Each puzzle is generated on your device and verified by a solver to have exactly one solution. The difficulty sets how many clues you start with: around 40 on Easy, 33 on Medium, 28 on Hard and as few as 24 on Expert, where you will need techniques such as pairs, pointing and X-Wings.

Your unfinished puzzle is saved automatically, so you can close the page and continue later.

1. Select a cell by clicking it or using the arrow keys.
2. Enter a digit with the number pad or your keyboard.
3. Turn on Notes to add small candidate digits instead.
4. Use Undo, Redo and Erase to correct entries.
5. Duplicates in a row, column or box are highlighted in red. Turn on Mistakes to also highlight digits that don't match the solution.
6. Stuck? Hint reveals the selected cell (or the first wrong or empty cell).
7. The puzzle is complete when every cell is filled correctly.

• Cross-hatch each digit from 1 to 9 before adding notes.
• Finish nearly-full rows, columns and boxes first.
• Add notes box by box, starting with the fullest boxes.
• Hunt for hidden singles: a digit that fits only one cell in a unit.
• On Hard and Expert, check for pointing pairs, naked and hidden pairs, then X-Wings.

Guides: Sudoku Strategies for Beginners, Advanced Sudoku Techniques and How to Solve Difficult Sudoku Puzzles Step by Step.