How to play sudoku
One rule, three directions. Every row, every column and every box contains each number exactly once.
The rules
A classic sudoku is a grid of 9 by 9 cells, split into nine boxes of 3 by 3.
Some cells are already filled in. Your job is to fill in the rest with the numbers 1 to 9.
Each number appears once in every row, once in every column and once in every box. That is all.
A good puzzle has exactly one solution, and you can reach it by logic alone. You never have to guess.
How to start
Scan for a number that appears often, say 7. Look at a box without a 7 and see which cells are blocked by the 7s in nearby rows and columns. If only one cell is left, that is where the 7 goes.
Then look at single cells. Count which numbers are already used in its row, column and box. If eight are taken, the ninth is the answer.
Repeat. Each number you place makes the next one easier to find.
When to use notes
On medium puzzles and harder, write the possible numbers in small print in each empty cell. These are called candidates.
When you place a number, cross it out of the candidates in the same row, column and box. Patterns will appear that you would never see in your head.
Ready for more?
The techniques page walks you through every pattern our puzzles use, from hidden singles to the Swordfish.