Blog · How to play
How Killer Sudoku works
No starting digits, only cages and their sums. Here is how to read a Killer board, and three moves that solve most of it.
The rules
Killer Sudoku keeps every rule of Classic Sudoku: each row, each column and each 3×3 box holds the digits 1 to 9 once. It adds one more. The board is divided into cages, groups of squares outlined with a dashed line, and the small number in the corner of each cage is the sum of the digits inside it. A digit can’t repeat inside a cage.
There are no starting digits. Everything you place, you work out from the sums.
Move 1: look for cages with only one answer
Some sums can only be made one way. A two-square cage that adds up to 3 must hold 1 and 2, because no other pair of different digits makes 3. You don’t yet know which square gets the 1, but you know both digits, and that alone rules them out of every other square in the same row, column or box.
| Squares in the cage | Sum | Digits |
|---|---|---|
| 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 | 30 | 6, 7, 8, 9 |
Move 2: the 45 rule
The digits 1 to 9 add up to 45, so every row, every column and every box adds up to 45 too. If the cages inside a box cover all but one of its squares, that last square is 45 minus their sums. Say a box’s cages cover eight squares and add up to 38: the ninth square must be 7. The same trick works across two rows, a row and a box, or any area whose total you know.
Move 3: split what is left
Once a few digits are in, return to the cages with more than one answer. A two-square cage of 10 could be 1 and 9, 2 and 8, 3 and 7 or 4 and 6. If the row already holds a 9 and an 8, the first two pairs are out. Each placed digit narrows the cages around it, and the puzzle starts to fall into place.
Try it in Sudoku+
Sudoku+ has a daily Killer puzzle and a free pack of 50 to work through, alongside Classic, Butterfly and Mini. Pencil notes are the Killer player’s best friend: note the possible digits for a cage and cross them off as the board fills in. While the app is in review on the App Store, you can play a sample Classic here to get a feel for the controls.
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