A cage's digits must be different and add up to its total. For small and extreme totals that leaves very few options. Highlighted sets are the only possibility for that total, the best place to start any Killer puzzle.
The rule of 45, step by step
Step 1 of 4
This is one 3×3 box. Its nine digits are 1–9, so together they always add up to 45.
Handy facts to remember
- A 9-cell cage always totals 45; an 8-cell cage is 45 minus the missing digit.
- Two-cell cages totalling 10 can never contain 5 (5+5 repeats).
- In a two-cell cage, an odd total means one odd and one even digit; an even total means both odd or both even.
- The minimum total for n cells is 1+2+…+n; the maximum is 9+8+…, counted down n digits.
Put it to work on a fresh puzzle: play Killer Sudoku. The in-game cage helper uses the same table.
Ask the puzzle coach
Stuck on a rule or a tactic? Ask in plain words. The coach only talks puzzles, and the answer is written for you on the spot.
Questions players ask
Which Killer Sudoku cages have only one combination?
Two cells: 3 (12), 4 (13), 16 (79), 17 (89). Three cells: 6 (123), 7 (124), 23 (689), 24 (789). Four cells: 10 (1234), 11 (1235), 29 (5789), 30 (6789). Five cells: 15 (12345), 16 (12346), 34 (46789), 35 (56789). Larger cages follow the same pattern at the extremes.
What is the rule of 45 in Killer Sudoku?
Each row, column and box contains 1–9 once, so it sums to 45. Subtract the cages that lie fully inside a unit from 45 and you get the total of the cells that remain, which often pins down a single cell.
Can a cage repeat a digit?
No, never, even when the cage crosses a box boundary.
How do I use the digit filter?
Tap a digit once to require it (green), twice to exclude it (red), a third time to clear. The table then shows only sets that fit, which is how you use what the rest of the grid already tells you.