How to play NUMBACK and the competition rules
NUMBACK is a daily competitive sudoku where everyone solves the same puzzle on the same day. A new worldwide puzzle drops every day at midnight (JST) and your solve time places you on the global leaderboard.
Basic sudoku rules
- Fill the 9×9 grid with the digits 1 to 9.
- No digit may repeat within a row.
- No digit may repeat within a column.
- No digit may repeat within a bold-bordered 3×3 box.
- Every puzzle has exactly one solution. NUMBACK verifies uniqueness when generating each puzzle.
How timing works
- The clock starts on the server the moment you tap Start.
- Server time is used rather than device time, so changing your clock cannot help you.
- The clock keeps running even if you pause or close the app. It is real elapsed time.
Leaderboard and tie-breaks
- Players are ranked by solve time, fastest first.
- On identical times, whoever submitted earlier ranks higher — the same tie-break used in competitive sudoku events such as the WPF Sudoku GP.
- Top players' solves are published as replays, so you can step through their move order and learn from it.
Mistakes and penalties
- Wrong entries carry no time penalty.
- They still cost you time indirectly, since the grid will not complete until they are fixed.
- Implausibly fast solves or solves with insufficient input logs are flagged automatically and kept as reference records.
What is free
- The daily puzzle is always free and unlimited.
- Free Play offers unlimited puzzles across five difficulty levels.
- Two weeks of archive and one leader replay per day are free.
- There are no ads anywhere in the app.
Play NUMBACK (free)