Rules & objective
Fill each row, column and 3×3 box with 1–9, without repeating a number.
- A digit must satisfy its row, column and box at the same time. A locally possible digit is not necessarily the final answer.
- Notes let you record candidates in empty cells. They are pencil marks, not completed entries; keep them updated as the board changes.
- Check highlights repeated-number conflicts. A ‘no conflicts’ message does not prove every entered digit is correct. Completion requires a full valid grid.
Key terms
- Candidate
- A digit still possible in an empty cell.
- Single
- A cell with only one candidate, or a digit with only one possible place in a unit.
- Unit
- One row, column or 3 × 3 box.
Difficulty levels
| Level | What changes |
|---|---|
| Easy | More starting clues. Notes and checks stay available. |
| Medium | Fewer starting clues, with one verified solution. |
| Hard | The fewest starting clues. Plan several deductions ahead. |
Changing difficulty starts a fresh round. Personal bests are tracked separately. Daily Sudoku stays on Medium.
This version on puzzle free online
Generated puzzles are checked for a unique solution. Daily Sudoku uses a shared Medium puzzle that resets at 00:00 UTC. Your unfinished board and notes save locally; daily completions contribute to your browser’s streak.
Common questions
Can I change a starting clue?
No. Only cells that began empty are editable.
Does Hard change the grid size?
No. All tiers use 9 × 9 grids; harder puzzles begin with fewer clues.
Explore this game in a new way
Keep exploring
Put the rules into practice.
Use the companion guide for a worked example, strategy and a small practice challenge.
Read the Sudoku guide ↗Browse all wiki articles