How to play sudoku

The rules take three lines. What takes longer is learning to see, and that is the part this page is actually about.

The rules

1. The grid is nine rows, nine columns and nine boxes of three by three.
2. Every row, every column and every box holds each digit from 1 to 9 exactly once.
3. Some digits are given. You work out the rest. There is no arithmetic and no guessing: every grid here has exactly one solution and can be reasoned to.

The first two things to look for

Almost all of an easy grid falls to one move: pick a digit, pick a box, and ask where it can still go. If every cell but one is ruled out by that digit already appearing in the cell's row or column, you have found its home. That is a hidden single, and it is the first technique everybody learns.

The mirror image is a naked single: pick an empty CELL and list the digits still legal in it. If only one survives, that is the answer. It sounds like the same move and it is not — one starts from a digit and finds a cell, the other starts from a cell and finds a digit, and grids that resist the first often fall to the second.

When neither works, you start writing candidates into cells (the pencil button on our board does this) and looking for patterns among them: naked pair, pointing pair, box/line reduction, naked triple. Each has a page here with a worked example you can check yourself.

How our board treats a wrong digit

Most sudoku apps compare every digit you place against the finished grid they are holding, and take a life when it differs. On the board in front of you the move was legal, so you are being punished for not guessing the answer. Here the only mistake is one you could have seen: a digit that repeats another in the same row, column or box. That one is refused, the cells that prove it light up, and it costs a life.

Anything else goes down exactly as it would in a newspaper, and the game says nothing, because the board does not know anything. If it turns out to be wrong you find out the way you would on paper: some cell runs out of digits, and the board says which one. The undo key takes back whatever you put down.

Asking for help without being handed the answer

A hint here is a ladder and you choose how far down it to go. The first step names the technique and the row, column or box to use it in, and it is free. The second explains the reasoning in the words solvers use. The third points at the cell. Only the fourth gives you the digit. Walking the whole ladder costs exactly what a plain reveal costs, so learning is never the expensive option.

What a difficulty level means here

Difficulty on this site is a property of the SOLVING, not of how many digits you start with. A grid is built by taking clues away only while it can still be solved with the techniques its level allows, and it is thrown away and rebuilt if it never ends up needing the hardest one it promises. So a hard grid really does need a pair or a locked digit, and really never needs anything beyond a triple.

Every level also carries a floor on its starting digits, and that number is there for looks rather than difficulty. It is worth saying so: a grid dug down to 23 clues does not read as hard, it reads as unfinished. The Guardian runs 32, 33, 28 and 30 clues from easy to expert, so their expert carries more clues than a naive "fewer clues is harder" rule would ever allow. The technique is the difficulty; the clue count is what a solver expects to see.

Where to start

• Easy Scan the boxes. No pencil marks needed.
• Medium Pencil marks start to pay off.
• Hard Needs a pair or a locked digit.
• Expert Goes past the standard techniques. Take your time.

There is also a daily grid that everybody gets on the same day, with a leaderboard that ranks how CLEANLY you solved it rather than how fast. Or just start on the main sudoku page and switch levels as you go.