Sudoku has one rule: fill the empty squares so that every row, every column and every three-by-three box contains each digit from 1 to 9 exactly once. You never need to add anything up (the digits are just nine different symbols), and you never need to guess. Every square can be worked out from the ones already filled in.
The best way to learn is with an easy puzzle in front of you: open Sudoku on Easy and follow along.
The grid
- 81 squares in nine rows and nine columns.
- Nine boxes of three by three, marked by the thicker lines.
- Some squares are filled in at the start. These are the givens; they are printed in bold and cannot be changed.
- Every row, column and box ends up with 1 to 9, each once.
Your first move
Pick the digit that appears most often among the givens. Take a box that does not have it yet. Every row and column that already contains that digit rules out the squares it passes through in your box. Often only one square is left, and that is where the digit goes.
That is called a hidden single, and it is most of what you need for an easy puzzle. Work through the same digit in every box, then move on to the next digit.
Your second move
Now look at it from the square's side. Choose an empty square in a busy row and column, and list the digits its row, column and box already contain. If eight of the nine digits are taken, the ninth is the answer.
Habits that help
- Place every digit you are sure of, then re-check the row, column and box it went into: one placement often opens the next.
- Do not write a digit in pen unless you are certain. If you are not, use Notes to pencil in the possibilities instead.
- The number pad shows how many of each digit are still to place. A digit with one or two left is quick to finish.
- If you get stuck, change direction: switch from scanning digits to scanning squares, or the other way round.
Mistakes and hints
In our game a wrong digit turns red and counts as a mistake, so you find out straight away rather than twenty moves later. A hint fills one square for you: the one you have selected, if it is empty or wrong, or otherwise the easiest square left. Both cost a few points, and undo is free.
Choosing a level
| Level | Good for |
|---|---|
| Easy | Learning the rules and the two basic techniques |
| Medium | The same techniques, with fewer places to start |
| Hard | Once singles feel easy: you will need pairs and pointing digits |
| Expert | For when hard stops being hard |
Ready for more? Read how to solve sudoku for the techniques that harder puzzles need, or go back to Sudoku.