Fill the empty squares with the numbers 1 to 9. Every row, every column and every 3 × 3 box must contain each number exactly once. The starting numbers stay in place. You do not need arithmetic: Sudoku is a puzzle about elimination.
The three rules
- Rows: each horizontal line needs the numbers 1 to 9, with no repeats.
- Columns: each vertical line needs the same nine numbers, with no repeats.
- Boxes: each of the nine 3 × 3 boxes also needs the numbers 1 to 9.
A number is only valid when it fits all three rules at once. A blank square is not an invitation to guess.
Find your first move
Start with a row, column or box that already has many numbers. List what is missing. For each empty square, look along its row, down its column and inside its box. Remove every number that already appears in any of those places.
Example: a row contains 1, 2, 3, 4, 6, 7, 8 and 9. Its only empty square must be 5. Check the column and box as well; if either already contains 5, revisit an earlier entry.
If two or more numbers remain possible, leave the square open or add pencil marks. Move to another part of the board.
Build a solving routine
After placing a number, check the affected row, column and box again. Your new entry may remove an option from a nearby square. Work through the board in a repeatable order so you do not overlook an area.
Another useful approach is to pick one number, such as 7. In a box that does not yet contain 7, rule out squares whose row or column already has one. If only one square remains, 7 belongs there.
When you get stuck
- Check that every entry has a reason behind it.
- Use notes or pencil marks for the remaining possibilities.
- Look for a square with only one candidate, then for a number with only one possible place in a row, column or box.
- Take a break. Returning to a puzzle can make a missed move easier to see.
Start with an easier puzzle while learning. Speed comes later; a correct, understood move is more useful than a fast guess.
Try it on Android
Sudoku – Classic Puzzle provides a readable board, notes and hints. The same rules apply whether you play on a phone or on paper.