How to play
- Choose a difficulty; a fresh puzzle appears immediately. The given digits are black and cannot be changed.
- Click or tap a cell (or move with the arrow keys) and type a digit, or use the digit pad. The pad shows how many of each digit are still missing.
- Press N or the Notes button to switch to pencil marks, then type the candidates you want to remember; type the same digit again to remove it.
- Digits that break a rule turn red at once. Press C (Check) to highlight entries that differ from the solution, H for a hint, Z to undo.
- The puzzle is solved when all 81 cells match the unique solution. Your time appears and, if no hint was used, is compared with your best.
Example
Suppose row 4 already holds 1, 2, 5, 6, 7, 8 and 9, leaving two empty cells for 3 and 4. One of those cells sits in a column that already contains a 3; that cell must therefore be the 4, and the other the 3 – a naked pairresolved by a column. With Notes on, type 3 and 4 into both cells first; after you place the 4, the tool erases the 4 from the other cell’s notes, leaving only the 3. The same reasoning inside a 3×3 box (“where can the 7 go in this box?”) – a hidden single– solves most Easy and Medium puzzles without any guessing.
Rules and scoring
The generator first builds a random complete grid (the three diagonal boxes are filled independently, the rest by a backtracking solver), then removes clues in random order and keeps each removal only if a solver still counts exactly one solution. The solver always branches on the cell with the fewest candidates, so the uniqueness check takes milliseconds even for Expert grids. The removal stops at the level’s clue target or earlier when no clue can be dropped without creating a second solution, which is why Expert puzzles can land a few clues above 24. A mathematical result by McGuire, Tugemann and Civario (2012) shows that no uniquely solvable Sudoku has fewer than 17 clues; shared links with fewer clues are rejected.
There is no points system: the score is the elapsed time, measured from the first display of the puzzle, with paused time excluded. The tab becoming hidden pauses the game automatically. A mistake is any entry that differs from the solution, counted when it is typed (undo does not reduce the count). Best times are kept per level under calcpark:sudoku:best:v1 in localStorage and only games without hints qualify. The puzzle itself is encoded in the page link as 81 digits (#d=hard&p=…), so copying the link shares the exact same grid.
Sources: Wikipedia – Sudoku, McGuire, Tugemann, Civario – There is no 16-clue Sudoku (2012), Peter Norvig – Solving Every Sudoku Puzzle.
Questions
- Is every puzzle solvable?
- Yes, and uniquely so: a clue is only removed during generation if the solver still finds exactly one solution. Every puzzle can be completed by logic alone, although Expert grids may need advanced techniques such as X-Wings or chains.
- What do the red digits mean?
- A red digit is a rule conflict visible on the board: the same digit already sits in its row, column or box. It is shown immediately and costs nothing. Entries that are wrong but not yet in conflict are only revealed by Check.
- Why does the clock stop when I switch tabs?
- The game pauses itself whenever the tab is hidden, so interruptions do not inflate your time. Press Resume (or P) to continue; the board is hidden while paused.
- Can I play the same puzzle as a friend?
- Yes. Use "Copy link to this puzzle" – the 81-digit grid is in the link after #, and opening it loads exactly that puzzle on any device.