How to play Sudoku
Sudoku is a logic puzzle played on a 9×9 grid. Some cells already contain digits (the clues). Your job is to fill every empty cell with a digit from 1 to 9 so that each row, each column and each of the nine 3×3 boxes contains every digit exactly once. You never need to guess or do any arithmetic: the digits are just nine different symbols.
The grid
A Sudoku grid has 81 cells arranged in 9 rows and 9 columns. Thick lines divide it into nine 3×3 boxes. Rows, columns and boxes are together called units or houses; every cell belongs to exactly one row, one column and one box.
The digits printed at the start are the givens. They cannot be changed. A properly made Sudoku has exactly one solution, and it needs at least 17 givens: mathematicians proved in 2012 that no 16-clue Sudoku with a unique solution exists.
The three rules
- Each row must contain the digits 1 to 9, each exactly once.
- Each column must contain the digits 1 to 9, each exactly once.
- Each 3×3 box must contain the digits 1 to 9, each exactly once.
That is all. Because a digit may appear only once in every unit, each digit you place removes that digit as a possibility from 20 other cells: the rest of its row, its column and its box.
How to start a puzzle
Start by scanning. Pick a digit, say 5, and look at one box that does not contain a 5 yet. Every row and column that already has a 5 elsewhere is blocked for that box. If only one empty cell in the box is left unblocked, the 5 must go there. This move is called a hidden single, and it solves most of an easy puzzle.
When scanning stops working, write candidates (pencil marks): the small digits that are still possible in each empty cell. A cell with only one candidate left is a naked single and can be filled in. Keep your candidates up to date every time you place a digit; most mistakes come from a forgotten candidate.
A worked example
Imagine the top-left box is missing a 7. The 7 already appears in row 2 and row 3 in other boxes, so the 7 of the top-left box must be in row 1. Row 1 of that box has two empty cells, but one of them is in column 3, which already contains a 7 further down. Only one cell is left, so the 7 goes there. Three short checks, no guessing: this is exactly how every Sudoku is solved, just with harder patterns as the difficulty rises.
Common beginner mistakes
- Guessing a digit and hoping it works. A fair Sudoku never requires it; if you are stuck, look for a pattern you have not checked.
- Checking the row and column but forgetting the box.
- Not erasing a candidate after placing the same digit in its row, column or box.
Why some puzzles are harder
Difficulty does not depend on how many clues a puzzle has, but on the hardest technique you must use. Easy puzzles need only singles; medium ones need pairs and locked candidates; hard ones need patterns such as X-Wing and Swordfish, and expert puzzles need chains. Ozerlyn rates every puzzle with the Sudoku Explainer (SE) system, and you can rate any puzzle yourself with our free SE Rating calculator.
Learn the techniques
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