The goal
Fill the grid so that every row, every column and every box contains each digit exactly once. On a classic board that means the digits 1 to 9. The numbers printed at the start are the clues, and they never change.
Rows, columns and boxes
Tap any cell to see the three groups it belongs to. A digit placed in the cell can’t appear again in any of them.
Row 5, column 5. No digit can repeat in its row, column, box.
How rows work
A row runs left to right. Each of the nine rows holds 1 to 9 once, so if a row already has a 7, no other cell in that row can be 7.
How columns work
A column runs top to bottom, with exactly the same rule. Checking rows and columns together often leaves a single possibility for a cell.
How the 3 × 3 boxes work
The thicker lines split the board into nine 3 × 3 boxes. Each box also holds 1 to 9 once. Beginners often forget the boxes, but they are where most early progress comes from.
Candidate notes
When you can’t place a digit yet, write down the digits that are still possible. These small numbers are called candidates.
- Centre notes list every candidate in the middle of the cell. They are the most common style.
- Corner notes mark where a digit could go within a box, which helps you spot patterns between boxes.
- Whenever you place a digit, remove it from the notes in the same row, column and box. SUDORI can do this for you.
Beginner techniques
Full House
When a row, column or box has one empty cell left, it takes the one missing digit.
Hidden Single
Pick a digit and a box. If the digit can only go in one cell of that box, it goes there. Do the same for rows and columns.
Naked Single
Look at one empty cell. If its row, column and box already use every digit but one, that one digit is the answer.
Scan the busiest groups first
Rows, columns and boxes with many digits have fewer possibilities, so they give answers sooner.
Use notes when you’re stuck
Writing candidates turns guesswork into something you can see. Patterns such as pairs appear on their own.
Common mistakes
- Guessing
- A well-made Sudoku never needs a guess. If you’re stuck, a deduction is waiting somewhere on the board.
- Forgetting the boxes
- Checking only rows and columns hides half of the information.
- Stale notes
- Notes that weren’t updated after a placement lead you astray. Clean them as you go.
- Rushing the last cells
- Most errors happen near the end. Check each final digit against its row, column and box.
Ready to try?
Start with a gentle 4 × 4 puzzle right here. The rules are the same, just smaller.