Number and logic puzzles
Sudoku
Fill the grid so every row, column and box holds the digits 1 to 9 once. Every puzzle has exactly one solution, and logic always finds it.
- Levels
- Easy, Medium, Hard, Expert
- Daily
- Medium
- Scored by
- Fastest time
- Today
- Daily puzzle
Setting up the board
Rules
- 1.The grid has 9 rows, 9 columns and 9 boxes of 3 by 3 cells.
- 2.Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once.
- 3.The given clues are fixed. Every Ribbia puzzle has exactly one solution, so you never need to guess.
How to play
| Input | Action |
|---|---|
| Click or tap a cell | Select it |
| 1 to 9, or the number pad | Enter a digit |
| N, or the Notes button | Switch between digits and pencil notes |
| Backspace or Delete | Clear the cell |
| Arrow keys | Move the selection |
| U or Z | Undo |
- Easy: 36 or more clues, singles only
- Medium: 27 to 33 clues, singles only
- Hard: Needs pointing pairs or naked and hidden pairs
- Expert: Needs techniques beyond pairs
Strategy
- Start with scanning: pick a digit and look for the one cell in a box, row or column where it can go (a hidden single).
- Look for cells with only one candidate left (a naked single). Pencil notes make these easy to spot.
- If a digit's candidates in a box all sit in one row or column, it cannot appear elsewhere in that row or column (a pointing pair).
- Two cells in a unit that share the same two candidates lock those digits in place (a naked pair); remove them from the rest of the unit.
- Use the highlight: selecting a digit lights up every copy of it, which makes gaps stand out.
Questions
- How are the difficulty levels graded?
- Each puzzle is solved by a program that uses human techniques in order of difficulty. Easy and Medium need only singles; Hard needs pointing pairs or naked and hidden pairs; Expert needs something stronger. The number of clues also sets the level.
- Does every puzzle have a unique solution?
- Yes. Every puzzle is checked by a solver that counts solutions, and it is only used if exactly one exists.
- What is the minimum number of clues?
- Seventeen. A 2012 exhaustive search proved that no puzzle with 16 clues has a unique solution.
- What does the mistake check do?
- When it is on, any digit that does not match the solution is marked in red. Turn it off for a pure solve.
More games
- 01Numbers
Minesweeper
3 levels / Time
- 02Numbers
2048
2 levels / Score
Quizzes to try
| No. | Quiz | Section | Format | Items | Added |
|---|---|---|---|---|---|
| 01 | Summer Olympics host cities Ten Summer Olympic Games, four cities each. Pick the city that hosted the Games in each year, from Athens 1896 to Paris 2024. Sports / Pick the answer / 10 questions | Sports | Pick the answer | 10 | 2026.09.30 |