How to play sudoku: Rules and first steps
Learn classic sudoku rules, recognise rows, columns and boxes, and solve your first cells with worked examples.
One grid and three constraints
A classic sudoku has 81 cells arranged in nine rows, nine columns and nine boxes of 3 × 3 cells. Your aim is to fill each empty cell with a digit from 1 to 9. Every row, column and box must contain all nine digits exactly once. The digits supplied at the start are the givens, and they stay fixed. You do not need addition, multiplication or a numerical sequence. To decide where a digit belongs, look for a position that satisfies all three constraints at the same time.
How to refer to a cell
These lessons use r1c1 for the cell in row 1, column 1. Count rows from the top and columns from the left. For example, r4c7 is in the fourth row and the seventh column. We use the word unit for any row, column or box. Two cells see each other when they share at least one unit. This notation lets us explain a move without relying on the colour of a picture. It also gives you a precise way to describe a question when discussing a puzzle with another player.
Your first deduction
Imagine a row containing, from left to right: 5, 3, empty, 6, 7, 8, 9, 1, 2. Eight different digits are already present, so the missing digit is 4. The empty cell must therefore contain 4. Check its column and box as well. In a valid grid, neither will contain another 4 that rules out this placement. If you find one, review your previous entries or the way you copied the example. A contradiction is a reason to check your reasoning, rather than permission to ignore a constraint.
Pencil marks and final entries
A candidate is a digit you have not yet ruled out for a cell. Writing small 2 and 6 marks means both remain possible according to your checks. It does not mean they are equally likely, and it does not tell you to choose one at random. A final entry needs a reason. Begin with units that are nearly full, identify their missing digits and compare those digits with the crossing units. After placing an answer, revisit its row, column and box. That one placement may make several further deductions available to you.
Try a short exercise
Consider a different row: 2, empty, 6, 7, empty, 9, 1, 3, 4. The missing digits are 5 and 8. If the column through the first empty cell already contains an 8, that cell must be 5. The other empty cell will be 8, provided the remaining constraints of the grid are compatible. Say the reason for each entry before writing it. When you cannot justify a move, keep the candidates and inspect another part of the grid. A row without repetitions is progress; finishing the puzzle requires all three constraints to hold throughout the grid.
Which digit is missing from this row?
Row: 5 · 3 · ? · 6 · 7 · 8 · 9 · 1 · 2
Now try it on the grid.
Return to an easy puzzle and look for a chance to use what you’ve learned.
Practise with a Sudoku