What are the rules of Sudoku?
Fill every empty cell of the 9 by 9 grid with a number from 1 to 9 so that no number repeats in any row, any column or any of the nine 3 by 3 boxes. There is no math involved. The digits could just as well be nine different symbols.
Some numbers, called givens or clues, are already placed when the puzzle starts, and they cannot be changed. Every puzzle on this site is generated fresh by the computer, and each one is checked to have exactly one solution. That means you never need to guess: logic alone always gets you to the end.
How do you start solving a Sudoku?
Pick a number that already appears many times and look for a box where it has only one possible cell. On easy puzzles, repeating this scan is enough to finish the whole grid.
- Scan one number at a time. If there are six 7s on the board, go to each box without a 7 and rule out every cell whose row or column already has a 7. If one cell is left, that is the 7.
- Look at nearly full rows, columns and boxes. A row with eight numbers just needs the missing one. A row with seven usually sorts itself out quickly too.
- Use notes when you get stuck. Pencil in the candidates for each cell, and the patterns in the table below start to jump out.
- Recheck after every number you place. One new digit often removes candidates elsewhere and sets off a run of easy cells.
What are the main Sudoku solving techniques?
From easiest to hardest, the core techniques are naked singles, hidden singles, locked candidates, pairs and the X-Wing. The first two alone solve every Easy puzzle here.
| Technique | In plain words | Needed from |
|---|---|---|
| Naked single | A cell has only one possible number left, so that number goes in. | Easy |
| Hidden single | In a row, column or box, a number fits in only one cell. It goes there, even if that cell has other candidates. | Easy |
| Locked candidates (pointing, claiming) | If a number's candidates inside a box all sit in one row, remove that number from the rest of the row outside the box. It also works the other way, from a row or column into a box. | Normal |
| Naked pair | Two cells in the same unit both have exactly the same two candidates. Those two numbers belong to those two cells, so remove them from the rest of the unit. | Normal |
| Hidden pair, triples | Two numbers can only go in the same two cells of a unit, so every other candidate in those cells can go. Triples extend the idea to three numbers and three cells. | Hard |
| X-Wing | A number fits in exactly the same two columns in two different rows, forming a rectangle. It can be removed from every other cell in those two columns. | Expert |
What is the difference between the difficulty levels?
Levels here are set by which techniques a puzzle actually requires, not just by how many clues it has. The computer solves each puzzle the way a person would, always trying the easiest technique first, and the hardest technique it needed becomes the level.
| Level | Clues | What it takes |
|---|---|---|
| Easy | 38 to 44 | Naked and hidden singles only. |
| Normal | 30 to 35 | Needs locked candidates or a naked pair at least once. |
| Hard | 26 to 29 | Needs hidden pairs or triples. |
| Expert | 22 to 26 | Gets stuck with all of the above. Needs X-Wing, Swordfish, XY-Wing or deeper reasoning. |
How do Sudoku notes (pencil marks) work?
Turn on Notes (or press N) and tap numbers to write them as small candidates inside the selected cell. Tap the same number again to remove it.
When you place a real number, it is automatically removed from the notes in the same row, column and box. So once you have pencilled in the candidates, they keep shrinking as you go, and cells down to a single candidate become obvious. You can get through Easy without notes, but from Hard upward they make solving much faster.
With "Show mistakes" on, any number that does not match the solution turns red right away and adds to your mistake count. Turn it off if you would rather check your own work. Even with it off, two equal numbers in the same row, column or box are still shown in red.
Controls
- Entering numbers: tap a cell, then tap a button from 1 to 9 below the grid. The small number under each button shows how many of that digit are still missing, and the button fades once all nine are placed.
- Keyboard: type digits to fill, use the arrow keys to move, and Backspace or Delete to erase. N toggles notes and Ctrl+Z undoes.
- Undo and Erase: Undo takes back your last action. Erase clears the number and notes in the selected cell.
- Hint: reveals the selected cell if it is empty or wrong, or otherwise the easiest cell to solve right now. The number of hints you used is shown at the end.
- Resume: your grid and time are saved in your browser, so you can reload or come back later and pick up where you left off. Each level keeps its own game, and the clock only runs while the page is on screen.
Where does Sudoku come from?
The modern puzzle first appeared in an American puzzle magazine in 1979 under the name "Number Place". In the 1980s the Japanese puzzle company Nikoli introduced it in Japan as "Sudoku", and the name stuck.
Sudoku is a shortened Japanese phrase meaning roughly "the digits must be single". The puzzle became a worldwide craze in the mid 2000s after British newspapers started printing it, and today it is one of the most common number puzzles in newspapers and on phones.