How to play Sudoku

The rule

A Sudoku is a 9×9 grid, divided into nine 3×3 boxes. Some cells are filled in at the start. These are the clues. Your job is to fill every empty cell so that:

• each row contains the digits 1 to 9 once,
• each column contains the digits 1 to 9 once,
• each 3×3 box contains the digits 1 to 9 once.

That is the whole game. No digit may repeat in a row, a column or a box. The digits are only symbols. There is no adding or multiplying, and the puzzle would work just as well with nine letters or nine colours.

A proper Sudoku has exactly one solution, and you can reach it by logic alone. You never have to guess.

Words you will see

• Cell: one of the 81 small squares.
• Unit: any row, column or box. There are 27 of them.
• Clue or given: a digit printed at the start.
• Candidate: a digit that could still go in a cell.
• Notes or pencil marks: small candidates written in a cell to keep track.

Move one: find the only place for a digit

Pick a digit, for example 7, and look at a box that has no 7 yet. Every 7 elsewhere on the board rules out its whole row and its whole column. Cross those cells off in your head. If only one empty cell of the box is left, the 7 goes there.

This move is called a hidden single, and scanning for it is how most people start every puzzle. Go through the digits 1 to 9, and for each one sweep across the nine boxes.

Move two: fill the last gap

When a row, column or box has eight cells filled, the ninth must be the one digit that is missing. This is the last empty cell.

These two moves are enough to finish any puzzle rated easy or medium.

Move three: find the only digit for a cell

Sometimes the question works better the other way round. Pick an empty cell and look at its row, its column and its box. If eight different digits already appear there, the cell must be the ninth. This is a naked single, and hard puzzles need it.

Solve your first puzzle

Here is an easy puzzle. Tap a cell, then a digit. The Hint button shows the next move and explains why it is right.

A method that always works

1. Scan each digit from 1 to 9 through the boxes and fill every cell that has only one place.
2. Check rows and columns that are nearly full.
3. After each digit you place, look again at its row, column and box. New moves appear there first.
4. When you get stuck, look for a cell with only one possible digit.
5. On harder puzzles, write notes and use them to rule candidates out.

Mistakes beginners make

• Guessing. A guess that turns out wrong can stay hidden for twenty moves and then ruin the puzzle. If you are not sure, look elsewhere.
• Checking only the box. A digit has to be free in the row and the column too.
• Staying in one corner. When an area dries up, move on. The next move is often on the other side of the board.
• Messy notes. Notes that are not updated cause more errors than no notes at all.

Where to go next

Play a few easy puzzles until scanning feels natural, then step up to medium. When you meet a puzzle that stops giving you moves, the technique guides explain what to look for. And when you want company, Sudoku Clan lets you play with a friend on the same puzzle.