How to play killer sudoku
The usual sudoku rules apply: every row, column and 3×3 box must contain the digits 1 to 9 once each. On top of that, the grid is divided into cages, the groups outlined with dashed lines. The small number in the corner of a cage is its sum. The digits in a cage must add up to that sum, and no digit may repeat within a cage.
There are no starting digits. Everything comes from the cage sums, so your first moves are about arithmetic and combinations.
Controls
- Select a square with a click or tap, or the arrow keys. Type a digit or use the number pad.
- Notes (N key) adds small candidates. Placing a digit removes it from the notes in the same row, column and box.
- The cage helper under the buttons lists every combination for the selected cage.
- Hint fills one correct square, Undo has no limit, and Show mistakes marks wrong digits.
Levels
- Easy: small cages of one to four squares, mostly two or three, including some single squares that act like given digits. Most squares follow from simple combinations.
- Medium: cages of two to five squares and no single-square cages. You will need to combine cage sums with the row, column and box rules.
- Hard: larger cages of up to six squares, each with many possible combinations. Expect to use the 45 rule and careful notes.
Strategy tips
Learn the unique combinations
Some sums can only be made one way, and spotting them gives you a foothold.
| Cage size | Sum | Only possible digits |
|---|---|---|
| 2 squares | 3 / 4 | 1+2 / 1+3 |
| 2 squares | 16 / 17 | 7+9 / 8+9 |
| 3 squares | 6 / 7 | 1+2+3 / 1+2+4 |
| 3 squares | 23 / 24 | 6+8+9 / 7+8+9 |
| 4 squares | 10 / 11 | 1+2+3+4 / 1+2+3+5 |
| 4 squares | 29 / 30 | 5+7+8+9 / 6+7+8+9 |
Use the 45 rule
The digits of any row, column or box add up to 45. Add the cages that sit fully inside a box. If they cover all but one or two squares, the leftover squares must make up the difference. The same works across two or three neighboring rows (90 or 135).
Combine cages with the grid
If a two-square cage must be 1 and 2, and both squares sit in the same row, no other square in that row can hold 1 or 2. Treat it like a naked pair from regular sudoku.
Write notes early
On medium and hard puzzles, pencil in the possible digits for small cages first. As digits get placed, combinations drop out and the cage helper shows which sets are still alive.
Enjoy sums? Kakuro uses the same combination logic on a crossword-style grid.
Questions
Can a digit repeat inside a cage?
No. Digits inside a cage are always different, even when the cage spans two boxes. This rule is what makes the cage combinations so useful.
Why are there no starting numbers?
Classic killer sudoku gives you only the cages and their sums. The cage clues carry enough information for exactly one solution, which a solver checked for every puzzle on this page.
What does the cage helper show?
For the cage of the selected square it lists every set of different digits that adds up to the cage sum. Sets that cannot work with the digits you have already placed in that cage are crossed out.
What is the 45 rule?
Every row, column and box contains 1 to 9, which add up to 45. If the cages inside a box cover all but one square, that square equals 45 minus their total.