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Sudoku24x7

How to play sudoku

Sudoku is a logic puzzle played on a 9x9 grid. No arithmetic, no reading, no guessing required — every empty cell can be worked out from three rules and the digits already on the board. This covers the rules, the notation used throughout this site, and the controls, in about ten minutes.

The three rules

A finished grid is valid when:

  1. Every row contains the digits 1-9, each exactly once.
  2. Every column contains the digits 1-9, each exactly once.
  3. Every 3x3 box contains the digits 1-9, each exactly once.

That's the entire rule set. A puzzle starts with some cells already filled in (the "givens") and asks you to fill in the rest so all three rules hold everywhere, at once. A properly made puzzle — every puzzle on this site — has exactly one way to do that.

Reading the board: row, column, box

Rows are numbered 1-9 top to bottom, columns 1-9 left to right. Throughout this site — in technique pages and in-game hints — a specific cell is written as r4c7: row 4, column 7, row number first. The grid below has its given digits colored the same way the real game board does, so you can see which cells are starting clues and which are filled in by a solver:

6
5
1
2
9
7
8
3
4
7
9
3
8
4
6
1
2
5
4
2
8
5
3
1
9
6
7
2
1
9
3
7
4
5
8
6
3
4
6
9
8
5
7
1
2
8
7
5
6
1
2
4
9
3
1
8
7
4
6
3
2
5
9
9
3
2
7
5
8
6
4
1
5
6
4
1
2
9
3
7
8

A finished grid — every row, column, and 3x3 box contains 1-9 exactly once.

Your first moves: naked singles

The easiest thing to look for is a cell where every other digit has already been ruled out by its row, column, and box, leaving exactly one possibility — a naked single. This board is a real example, mined from an actual generated puzzle, at the exact point our solver found one:

13789
2
134789
38
137
178
347
6
5
137
1347
5
9
1237
6
2347
237
8
3789
378
6
238
5
4
237
2379
1
1578
14578
1478
48
6
3
9
1278
247
3678
9
3478
1
47
2
34678
5
3467
13678
13478
2
5
9
78
34678
1378
3467
4
35
39
7
8
5place 5
1
239
2369
2
1378
1378
346
134
9
5
378
367
135789
6
13789
23
123
15
2378
4
2379

r7c6 has only one candidate left: 5.

Most puzzles graded Easy on this site can be finished using nothing but naked singles and their close cousin, the hidden single — see that page for the difference. Once those stop finding anything, the techniques hub covers everything else, roughly in the order you'll need it.

Tracking possibilities: pencil marks

Once singles stop appearing, most techniques require knowing which digits are still possible in a cell — its candidates. Writing those down as small pencil marks is covered fully on how to use pencil marks; the short version is don't mark every cell immediately, only where you're actually stuck.

Playing on this site

Click or tap a cell to select it, then enter a digit two ways:

Full keyboard controls, if you'd rather not reach for the mouse:

Mistakes don't end the game — there's no forced game-over after a fixed number of errors. Settings let you choose whether conflicting digits get highlighted, checked against the actual solution, or left alone entirely. If you get stuck, the hint button explains the next logical step and names the technique, rather than just dropping in a digit.

Ready to play

Start with an Easy puzzle if this is new, or jump straight to the home page to pick a level. The techniques hub and the solver are both there once you want to go further than singles.