Fill each row, column, and 3×3 box with the numbers 1–9. No number may repeat within the same row, column, or box. Use pencil marks to track candidates before committing to a value.
Controls
- Click / Tap – select a cell
- Keyboard 1–9 – place numbers
- Shift + number – toggle pencil marks
- Backspace / 0 – clear cell
- Ctrl+Z / Ctrl+Y – undo / redo
Sudoku variants
- Anti-knight / Anti-king: identical digits cannot sit a knight’s move apart, or touch even diagonally.
- Non-consecutive: edge-adjacent cells cannot contain consecutive digits.
- Kropki: white dot = consecutive; black dot = double (1–2, 2–4, 3–6, 4–8).
- XV: V = digits sum to 5; X = digits sum to 10.
- Thermo: digits strictly increase from the bulb to the tip.
- Arrow: digits on the arrow sum to the circled digit.
- Open Variants to combine any of these in one puzzle.
Strategies
- Single Candidate: when only one number fits a cell.
- Single Position: when a number can only go in one cell of a row, column, or box.
- Pencil Marks: quickly highlight potential digits to aid deductions.
- Notes & Hints: toggle auto-check or use hints for guidance.
Teacher Mode Tips
- Enable pencil marks and strict mode for classroom lessons.
- Use Mini 4×4 puzzles to introduce Sudoku logic to younger learners.
- Track weekday streaks to encourage consistent practice.