How to play Sudoku
Sudoku has one rule. Fill the grid so that every row, every column and every box contains the numbers 1 to 9 exactly once.
That is all there is to it. You do not need to be good at maths, because you never add, subtract or multiply. The numbers could just as well be letters or colours. What the puzzle tests is your ability to notice where something can and cannot go.
The parts of the grid
It helps to know the words people use, because every guide and every technique relies on them.
| Square | One of the 81 small spaces. Some guides call it a cell. |
|---|---|
| Row | Nine squares running across the grid, left to right. |
| Column | Nine squares running down the grid, top to bottom. |
| Box | One of the nine 3 by 3 blocks marked by the thicker lines. Also called a block or a region. |
| Given | A number that is printed at the start. Givens are shown in a darker colour and cannot be changed. |
| Candidate | A number that could still go in a square. The small numbers you write as notes are candidates. |
Rows, columns and boxes are sometimes grouped together and called units. Every square belongs to exactly three units: one row, one column and one box. The rule applies to all 27 units at once.
Solving your first puzzle, step by step
- Find a busy row, column or box. Start where most squares are already filled. A box with seven numbers only has two left to place, so it is the quickest place to make progress.
- Pick one missing number. Choose a number that is missing from that box, for example a 4. Look along every row and column that runs through the box and see where a 4 already sits.
- Rule out squares. Each 4 you find rules out every square in its row and column. If only one empty square in the box is left untouched, the 4 must go there.
- Repeat with the next number. Every number you place gives you new information. Go back to the busiest part of the board and try the next missing number.
- Use notes when the easy moves run out. When no square has an obvious answer, write the possible numbers into each empty square as notes. Then look for squares with a single note left, or a number that fits in only one square of a row, column or box.
A good Sudoku never needs a guess. If you find yourself thinking "it is probably a 6", stop and look for a square or a number with only one option. There is always one somewhere.
Scanning: the move you will use most
Scanning means taking one number and checking where it can go in a single box. Say you are looking at the top left box and it has no 7. Look at the three rows that pass through the box. If two of them already have a 7 somewhere else on the board, the 7 in this box must be in the third row. Now look at the three columns. If two of those have a 7 too, only one square is left, and that is your answer.
Experienced solvers scan the whole board for 1s, then for 2s, and so on. On Easy and Medium puzzles, scanning alone will usually take you to the end.
When scanning stops working
Harder puzzles are built so that scanning runs dry after a while. This is when notes come in. Write every possible number into every empty square, then look for these two patterns:
- A square with one note. If only one number fits a square, it goes there. This is called a naked single.
- A number with one home. If a number appears in the notes of only one square in a row, column or box, it goes there, even if that square has other notes too. This is called a hidden single.
Our strategy guide covers the next steps, such as pairs, pointing pairs and the X-Wing.
Using this site
Click or tap a square, then choose a number from the pad. The row, column and box of the square you picked are shaded, and every copy of the same number lights up, which makes scanning much faster.
- Notes switches the pad between answers and small pencil marks. The button shows ON or OFF so you always know which you are writing.
- Erase clears the square you picked. Correct answers and givens stay.
- Undo takes back your last move, including notes.
- Hint fills in one square for you and costs 15 points.
- Pause hides the board and stops the clock.
A wrong number turns red and the squares it clashes with are marked. After three mistakes the game ends, and you can try the same puzzle again from the start. Your progress is saved after every move, so you can close the page and come back later.
Keyboard shortcuts
| Move between squares | ← ↑ → ↓ |
|---|---|
| Enter a number | 1 to 9 |
| Clear a square | Backspace, Delete or 0 |
| Notes on or off | N |
| Undo | Z, U or Ctrl + Z |
| Hint | H |
| Pause and resume | P |
Which level should I start on?
If you have never played, start on Easy, or try Mini Sudoku, which uses a 6 by 6 grid and the numbers 1 to 6. Move up a level once you can finish without hints. Most people settle on Medium or Hard for everyday play and keep Expert and above for when they have time to sit with a puzzle.