How to play sudoku
Sudoku takes one rule to learn and a lifetime to master. Here are the rules, plus the three techniques that solve most easy and medium grids.
The rules of sudoku
The grid is a 9×9 square divided into nine 3×3 boxes. Some digits are already given; your job is to fill every empty cell.
Each row must contain the digits 1 to 9 exactly once. Each column must contain the digits 1 to 9 exactly once. Each 3×3 box must contain the digits 1 to 9 exactly once.
That is the whole rulebook. No arithmetic is involved: you place digits by logic, and every well-made sudoku has exactly one solution.
Getting started: scan the grid
Don't start in the top-left corner and grind through empty cells one by one. Pick a digit that already appears often (say five 7s are on the board) and trace its rows and columns across the grid. Every cell in those rows and columns is ruled out for a 7.
If a 3×3 box does not yet contain a 7, but eight of its nine cells are blocked by rows or columns that already hold one, the last open cell must be the 7. Players call this cross-hatching, and it alone solves most easy grids.
Three techniques that solve most grids
1. Scanning
Take one digit at a time and rule out every cell in a row, column or box that already contains it. Whatever is still possible in exactly one place is certain.
2. Single candidate
Pick one empty cell and list the digits already in its row, column and box. If eight different digits are ruled out, the ninth is the answer: write it in.
3. Pencil marks
When nothing is certain yet, write the two or three possible digits in small type inside the cell. In Free Sudoku, the Notes button switches pencil marks on and off, and they clear automatically when you place a digit.
Mistakes are part of learning
Every sudoku player guesses wrong now and then. Free Sudoku gives you three mistakes per game and an undo button. Hints explain which cell to look at and why, so a stuck moment becomes a lesson instead of a dead end.
Ready to try? The easy level is built for exactly this: plenty of clues, quick wins, and the same logic as the harder grids.
Learn it by playing it
Reading takes two minutes; the habit takes a grid or two. Free Sudoku is available on Google Play at no cost.