How to Play Killer Sudoku

The rules, the combinations worth memorizing, and the rule of 45.

Killer Sudoku rules

Fill the grid so that every row, column and 3x3 box contains 1 to 9 exactly once, just like Sudoku.
The grid is split into cages (dotted outlines). The small number in each cage is the sum of its cells.
A number can't repeat inside a cage.
There are no given numbers. Everything comes from the sums.

Cages with only one combination

Memorize these. They are the easiest way into any Killer Sudoku.

Cells	Sum	Must be
2	3	1 + 2
2	4	1 + 3
2	16	7 + 9
2	17	8 + 9
3	6	1 + 2 + 3
3	7	1 + 2 + 4
3	23	6 + 8 + 9
3	24	7 + 8 + 9
4	10	1 + 2 + 3 + 4
4	11	1 + 2 + 3 + 5
4	29	5 + 7 + 8 + 9
4	30	6 + 7 + 8 + 9
5	15	1 + 2 + 3 + 4 + 5
5	16	1 + 2 + 3 + 4 + 6
5	34	4 + 6 + 7 + 8 + 9
5	35	5 + 6 + 7 + 8 + 9
6	21	1 + 2 + 3 + 4 + 5 + 6
6	22	1 + 2 + 3 + 4 + 5 + 7
6	38	3 + 5 + 6 + 7 + 8 + 9
6	39	4 + 5 + 6 + 7 + 8 + 9
7	28	1 + 2 + 3 + 4 + 5 + 6 + 7
7	29	1 + 2 + 3 + 4 + 5 + 6 + 8
7	41	2 + 4 + 5 + 6 + 7 + 8 + 9
7	42	3 + 4 + 5 + 6 + 7 + 8 + 9

The rule of 45 (innies and outies)

Every row, column and 3x3 box contains 1 to 9 once, so it always adds up to 45.

Innies: add up the cages that sit completely inside a box. Subtract that from 45, and you get the sum of the box's remaining cells.

Outies: add up every cage that touches the box, including the ones that stick out, then subtract 45. The result is the sum of the cells outside the box.

When only one cell is left over, you know its number outright. It works for rows and columns too, and for groups of them (two rows add up to 90).

Example from a real puzzle: in box 9, the shaded cages sit completely inside the box and add up to 21 + 3 + 14 = 38. Only one cell is left (?), so it must be 45 − 38 = 7.

How to start a Killer Sudoku

Look for cages with only one possible set of numbers. A 2-cell cage of 3 must be 1 and 2; a 2-cell cage of 17 must be 8 and 9.

Use the rule of 45: every row, column and box adds up to 45. If the cages inside a box add up to 38 and one cell is left over, that cell is 7.

Mix in normal Sudoku: once a 1 and 2 are locked into a cage, no other cell in that row or box can be 1 or 2 if the cage sits in it.

Play Killer Sudoku →

Levels

Level	What to expect
Easy	Cages up to 4 cells. Solvable without guessing
Medium	Cages up to 5 cells. Solvable without guessing
Hard	Cages up to 6 cells. Needs deeper reasoning
Expert	Cages up to 7 cells. The hardest level

FAQ

Is Killer Sudoku harder than Sudoku?

It starts with an empty grid, so it can feel harder at first. With the unique combinations and the rule of 45, Easy and Medium puzzles are very approachable.

Does every puzzle have one solution?

Yes. Each puzzle is checked by a program to have exactly one solution, twice: when it's made and again before it's published.

Do I need to guess?

Not on Easy and Medium: every puzzle there is checked to be solvable without guessing, using cage combinations, the rule of 45 and basic Sudoku logic. Hard and Expert need deeper reasoning.

Is it free?

Yes. Every level is free and unlimited. No sign-up.