How to Play Sudoku: Rules, Tips and Strategies for Beginners
Sudoku takes a minute to learn and a lifetime to master. This guide covers the rules, the words players use, and the handful of techniques that solve most easy and medium puzzles, without any guessing.
The rules of Sudoku
A Sudoku is a 9×9 grid split into nine 3×3 boxes. Some cells already hold a digit (the givens). Your job is to fill every empty cell with a digit from 1 to 9 so that:
- every row contains each digit from 1 to 9 exactly once,
- every column contains each digit from 1 to 9 exactly once, and
- every 3×3 box contains each digit from 1 to 9 exactly once.
That is all. There is no arithmetic: the digits are just nine different symbols. A well-made puzzle has exactly one solution and can be solved by logic, without guessing.
Words you will see
- Cell: one of the 81 squares.
- House: any row, column or box. Each house holds 1 to 9 once.
- Peers: the 20 other cells that share a house with a cell. A digit placed in a cell cannot appear in any of its peers.
- Candidates (pencil marks): the digits that could still go in an empty cell.
Beginner techniques, step by step
1. Scanning
Pick a digit, say 5. Look at every 5 already on the board and imagine lines through its row and column. In a box with no 5 yet, cross out every cell those lines pass through, plus the filled cells. If only one cell is left, the 5 goes there. Repeat for each digit from 1 to 9: on an easy puzzle, scanning alone gets you a long way.
2. Last free cell in a house
When a row, column or box has only one empty cell, the missing digit is obvious. In the row below, the only missing digit is 4:
3. Naked single
Look at one empty cell and list the digits that are already among its peers. If eight different digits are there, the ninth is the only candidate left, so it must be the answer for that cell.
4. Hidden single
Now turn it around: pick a house and a digit. If that digit can go in only one cell of the house (because every other cell is filled or sees the same digit elsewhere), it goes there, even if the cell itself still has other candidates.
5. Pencil marks
When scanning stops working, write the candidates into the empty cells of a box or row. Every time you place a digit, erase it from the candidates of all its peers. Clean pencil marks turn hidden information into something you can see.
6. Naked pairs
If two cells in the same house both have exactly the same two candidates, for example {3, 8}, those two digits must go in those two cells. You can erase 3 and 8 from every other cell of that house. The same idea works with three cells and three digits (a naked triple), even if each of the three cells holds only two of the digits.
7. Pointing pairs
If, inside one box, a digit can only go in cells that lie on the same row, then that digit must be in that row within this box. Erase it from the rest of the row outside the box. The same works for columns.
Common beginner mistakes
- Guessing. A guess that looks fine can break the puzzle twenty moves later. If you are not sure, look for another move.
- Forgetting to update pencil marks. Stale candidates hide the next move or lead to a wrong one.
- Only checking rows. Boxes and columns hide just as many moves. Scan all three.
Difficulty levels
Easy puzzles can be solved with scanning and singles. Medium puzzles need pencil marks. Hard puzzles add pairs and pointing pairs, and expert puzzles call for advanced patterns such as X-Wing. Move up a level once you finish the current one without help.
Frequently asked questions
Do you need to be good at math to play Sudoku?
No. Sudoku uses the digits 1 to 9 only as symbols; there is no adding or multiplying. You could play it with letters or colours. It is a logic puzzle.
Does every Sudoku have only one solution?
A well-made Sudoku has exactly one solution, and can be solved by logic, without guessing.
How long does a Sudoku take to solve?
Beginners usually finish an easy puzzle in 10 to 20 minutes. Medium puzzles take 20 to 40 minutes, and hard ones can take an hour while you are learning.
What should I do when I get stuck?
Write pencil marks in every empty cell of one box, row or column, then look for a cell with one candidate or a digit that fits in only one cell. If that fails, look for pairs.
Practise on your phone
Our free Sudoku app has four difficulty levels, a daily challenge and hints that explain which technique to use next, so you learn while you play. It works offline.