KarHow to play Sudoku
Sudoku asks you to fill a 9×9 grid with digits 1–9, without repetition in any row, column or 3×3 box. Play alone with notes and hints, or race a rival on the same grid.
Start a game
- Choose a solo difficulty or a duel.
- Select a cell and enter a digit using the keypad.
- Use notes to record candidate digits where available.
- Complete every row, column and box without duplicates.
- In ranked play, submit a correct full board before your opponent.
The basic constraint
Every row, column and 3×3 box must contain each digit from 1 to 9 exactly once. The given digits form the starting puzzle and are not choices to change.
Sudoku is about placement rather than arithmetic. Check all three groups belonging to a cell before deciding which digits remain possible.
Solo tools and difficulty
Solo mode offers Easy, Medium, Hard and Expert, along with notes and hints. Notes record possibilities rather than committing a final digit. The timer lets you compare your own solving pace.
A useful move is a single candidate in a cell, or a digit with only one possible location in a row, column or box. Candidate pairs and pointing pairs can remove additional possibilities when direct singles run out.
Ranked duels
Both players receive the same medium grid with 46 empty cells. You see your opponent’s progress, not their digits. The server checks a full board rather than revealing mistakes as each digit is entered.
An incorrect full submission causes a 30-second input lock, after which you must find the error. A ranked match lasts at most 15 minutes; if nobody finishes, the number of correctly completed cells decides the result under the game’s ending rules.
Example: a hidden single
A 3×3 box still needs a 5. Check each empty cell against its row and column. If all but one location already share a row or column with a 5, the remaining location must contain 5, even if that cell appeared to allow other digits initially.
Repeat this scan for digits 1–9 across every box. When no placement follows, record candidates instead of inserting an unsupported guess that may be expensive to untangle later.
How duel grids are prepared
The server starts from a valid full solution and removes digits while checking that exactly one solution remains. A removed digit is restored when uniqueness would be lost.
Identical starting grids make a duel a comparison of reasoning and pace. Accuracy matters: a fast but incorrect full board can cost more than checking a doubtful candidate first.
Strategy and useful habits
- Scan both cells with one candidate and digits with one possible location.
- Keep notes consistent after each confirmed placement.
- Recheck uncertain regions before a full ranked submission.
- Use solo hints for learning, without expecting them in ranked play.
Common mistakes
- Checking a row but forgetting the cell’s column or box.
- Treating notes as confirmed digits.
- Guessing rapidly in ranked play and triggering the lock.
- Assuming the server highlights each wrong digit immediately.
Questions and answers
Are ranked mistakes highlighted as I type?
No. A full board is checked; an incorrect submission causes a 30-second lock.
How long can a ranked match last?
Up to 15 minutes.
Do both players get the same puzzle?
Yes. The ranked duel uses the same grid for both players.