How to play Sudoku
Sudoku has one rule and no arithmetic. If you can tell nine things apart, you can solve one. This is everything you need for your first grid, in the order you will need it.
The goal
You are given a 9×9 grid with some squares already filled in. Fill every empty square with a digit from 1 to 9 until the grid is complete. That is the whole objective. Every puzzle here has exactly one solution, which matters more than it sounds: it means there is always a next square you can prove, and you never have to guess.
The one rule
The grid is divided three ways at once — into nine rows, nine columns, and nine 3×3 boxes. The rule applies to all three:
- Each row holds 1–9, each digit exactly once.
- Each column holds 1–9, each digit exactly once.
- Each 3×3 box holds 1–9, each digit exactly once.
Three ways of grouping the same 81 squares. Every square belongs to one of each, and every group must hold 1–9 once.
Finding your first move
Do not start by staring at an empty square and wondering what goes in it. Start with a digit instead.
- Pick a digit that already appears several times on the board — a 5, say.
- Find a 3×3 box that has no 5 in it yet.
- Look along the rows and columns that pass through that box. Any row or column that already contains a 5 cannot hold another, so every empty square in that box on that line is ruled out.
- If exactly one square in the box survives, that is where the 5 goes.
That is called scanning, and on an easy grid it will carry you most of the way. When you place a digit, the board here lights up its row, its column, its box and every other square holding the same number — so your next scan is half done for you.
Using notes
Sooner or later scanning runs out and you meet a square with two or three possibilities and no way to choose yet. That is what notes are for: turn Notes on and the number keys pencil candidates into the square instead of committing an answer.
Two things make them worth the effort:
- Writing a real digit into a square automatically clears that digit from the notes of every square it can see, so the marks stay honest as you go.
- Once a unit's notes are filled in, patterns become visible that were invisible before — a digit that only fits in one square, or a pair of squares that between them own two digits and lock everyone else out.
You do not have to pencil in the whole grid. One box at a time is usually enough, and it is much less work.
Hints, mistakes and the clock
Hints reveal the correct digit for the square you have selected, or the most useful square on the board if you have not selected one. They are limited per game and the allowance shrinks as the grade rises, from five on Easy to three on Expert.
Mistakes cost you. A wrong digit stays on the board in red and uses one of your three tries; run out and the run ends. You can always restart the same puzzle with the same clues.
The clock runs while you play and stops when you pause. Your best time is kept per grade, so a fast easy grid never flatters itself into being your expert record.
Habits worth building
- Sweep by digit, not by square. Nine passes over the grid, one per digit, finds more than staring at one hole.
- Re-scan after every placement. A digit you just placed changes three units at once; the next move is often in one of them.
- Trust the uniqueness. If you are about to guess, there is a deduction you have not found. Turn on notes and look again.
- Do not race at first. The clock is there to beat later. Accuracy costs three tries; speed costs nothing.
Where to go next
Ready to play? Easy Sudoku is where the habits above are enough. When they stop being enough, the strategy guide picks up with locked candidates, pairs, triples and the X-wing, and the rules page covers the corner cases — why a valid grid always has one answer, and the mistakes that quietly break one.