Sudoku rules

The row rule

A row is one of the nine horizontal lines of cells, running the full width of the grid. Between its nine cells, every digit from 1 to 9 has to turn up exactly once. Put the same digit in a row twice, no matter how far apart the two cells sit, and the puzzle is no longer valid.

The column rule

A column runs top to bottom instead of side to side, but the rule reads the same. Nine cells, the digits 1 to 9, no repeats.

The box rule

The third unit is the box, one of the nine 3x3 blocks marked off by thicker lines. A box ignores which row or column a cell sits in and just needs its own nine cells to hold 1 to 9 once each. Every cell in the grid belongs to one row, one column and one box at the same time, and it has to satisfy all three at once. Because every cell answers to all three, you can often pin down a digit just by looking at what already sits around it.

Parts of the grid

A few words come up on every page of this site. Here is what each one means.

• Cell: one of the 81 small squares that make up the grid.
• Row: one of the nine lines of cells running left to right.
• Column: one of the nine lines of cells running top to bottom.
• Box: one of the nine 3x3 blocks marked off by the thicker grid lines.
• Given: a digit printed on the grid before you start playing. Givens cannot be changed or removed.

What isn't a rule

A few common assumptions are not real rules. There is no adding or subtracting anywhere on the grid. The digits are just nine symbols, and letters would work exactly the same way. A properly made puzzle never needs guessing. Every puzzle on this site has exactly one solution. Classic Sudoku has no diagonal rule, so the two long diagonals can repeat digits freely, unless you are playing a variant built to add that constraint on purpose. There is also no required order for filling cells. Some solvers work box by box, others chase one digit across the whole grid, and neither way is wrong.

Rules for other versions

Killer Sudoku keeps the same three rules and adds cages, dashed outlines drawn over two or more cells. The digits inside a cage have to add up to the small number printed in its corner, and the same digit cannot repeat inside one cage. See Killer Sudoku.

Diagonal Sudoku, also called X-Sudoku, keeps every standard rule and adds one more. Both main diagonals, running corner to corner across the whole grid, also need the digits 1 to 9 with no repeats. See Diagonal Sudoku.

Jigsaw Sudoku swaps the nine 3x3 boxes for nine irregular shaped regions. Rows and columns work as normal. Only the shape of the regions changes. See Jigsaw Sudoku.

Mini Sudoku shrinks the grid to six rows, six columns and six 2x3 boxes, with the digits 1 to 6 instead of 1 to 9. See Mini Sudoku.

Samurai Sudoku joins five overlapping 9x9 grids in a cross shape. Each of the four outer grids shares one 3x3 box with the grid in the middle, so placing a digit near that shared box can affect both grids at once. See Samurai Sudoku.

Try it

If you would rather see the rules in action than read about them, the guided first puzzle walks you through a real easy grid step by step and takes about ten minutes. Once you are ready to play on your own, try an easy puzzle on the homepage, or see every technique this site teaches on the technique lessons hub.