How to play Sudoku
Sudoku is a placement puzzle. You are given a nine-by-nine grid with some of its digits already filled in, and your job is to complete it so that three things are true at once.
Every row must contain one of each digit from one to nine. Every column must contain one of each digit from one to nine. And every one of the nine three-by-three boxes must contain one of each digit from one to nine. That is the whole of the rules. Nothing is added at higher difficulties, and no arithmetic is ever involved.
Your first move, every time
The way into almost any grid is to find somewhere that has only one possibility left. Two kinds of place are worth looking at first, and both are easy to spot before you know anything else about the puzzle.
The first kind is a cell like that one, where everything else around it has been decided. The second is the same argument run the other way: pick a digit, look at one box, and cross off every cell in it that already sees that digit in its row or its column. If exactly one cell survives, the digit goes there, even if that cell has five other digits it could otherwise have taken.
Those two moves alone finish an Easy board. They are also what you fall back on when a hard one finally cracks open.
When nothing is obvious
On a harder grid you will reach a point where no cell has one candidate and no digit has one home. What you are looking for then is a pair: two cells in the same row, column or box that can only hold the same two digits between them. You do not know which is which, and you do not need to. Those two digits are used up by those two cells, so they can be struck out of every other cell in that line — and it is usually one of those eliminations that opens the next move.
Harder puzzles run the same idea further: three cells sharing three candidates, or a digit whose possible positions across several rows line up in a way that rules it out elsewhere. The reasoning is longer. It is never a different kind of reasoning.
What the four difficulties mean
The difficulties describe technique, not patience. Easy falls to the two moves above. Medium wants pairs and locked candidates. Hard adds triples, X-wings and Y-wings. Master is swordfish territory, where a single argument runs across three rows and three columns at once.
What a harder tier never means is more guessing. Every grid on this site has exactly one solution and a path to it made only of deductions. When you are stuck, there is something on the board you have not seen yet — which is a different feeling from being asked to pick a branch and find out whether it was right.
Playing here
Click a cell and type a digit, or use the number pad — beside the grid on a wide screen, below it on a phone. Pencil marks are for the candidates you want to remember; switch to them when you start ruling things out rather than filling things in. Puzzles are generated the moment you ask for one, so there is no library to exhaust and no daily limit, and nothing is behind an account.