The rules
- Rows, columns and boxes: like normal sudoku, every row, every column and every 3×3 box holds the digits 1 to 9 exactly once.
- Cages: the grid is split into cages drawn with dotted lines. The small number in the corner of a cage is the sum of the digits inside it.
- No repeats in a cage: a digit can appear only once inside a cage, even if the cage crosses a box line.
- Usually no starting digits: most killer sudoku grids begin empty. A cage with a single cell gives you that digit straight away.
Start with cages that have one answer
Some totals can only be made one way. A two-cell cage of 3 must be 1 and 2. A two-cell cage of 17 must be 8 and 9. You still don’t know which cell is which, but you know which digits are in the cage, and that rules them out for the rest of its row, column or box.
| Cells | Total | 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 | 11 | 1 2 3 5 |
| 4 | 29 | 5 7 8 9 |
| 4 | 30 | 6 7 8 9 |
The rule of 45
The digits 1 to 9 add up to 45, so every row, column and box totals 45. This is the most useful idea in killer sudoku.
- One cell left over: if the cages that sit completely inside a box add up to 38 and leave one cell uncovered, that cell must be 45 − 38 = 7.
- One cell sticking out: if a group of cages covers a whole box plus one extra cell outside it, and the cages total 52, the extra cell is 52 − 45 = 7.
- Bigger areas: it works for two or three rows together too. Three rows make 135, and the whole grid makes 405.
A good order to solve in
- Fill single-cell cages first.
- Mark the digits of cages with only one combination.
- Look for the rule of 45 in each box, then in each row and column.
- Use normal sudoku logic on what you have: singles, pairs and pointing digits.
Practise with hints that explain
Our free Killer Sudoku: No Ads, Offline app for iPhone and iPad has nine short lessons, a cheat sheet of cage combinations and a guided first puzzle. When you are stuck, a hint shows where to look and can explain the step, like the rule of 45 or a cage with only one combination. Every puzzle has exactly one solution reachable by logic, with no ads.