Home › Variants › Killer Sudoku
Killer Sudoku
Learn Killer Sudoku: classic rules combined with cages that sum to a target and never repeat a digit. The "rule of 45" explained.
Killer Sudoku blends Sudoku with simple arithmetic. The grid is divided into dashed "cages", each labelled with a target sum. The digits in a cage must add up to that sum and never repeat — and all the usual row, column and box rules still apply.
Most killer boards start with no given digits at all; the cage sums alone pin down a unique solution. The signature technique is the "rule of 45": every row, column and box totals 45, so comparing cage sums against 45 reveals hidden digits (innies and outies).
Killer is one of the most popular variants in print and apps, which makes it a high-value addition. Its solver needs cage-aware reasoning beyond the classic technique set — that work is on the roadmap.
Rules
- All classic Sudoku rules apply.
- Each cage’s digits add up to its printed sum.
- No digit repeats within a cage.
- Usually no starting givens — the sums do the work.
Frequently asked questions
What is the rule of 45 in Killer Sudoku?
Every row, column and 3×3 box contains 1–9, which always sums to 45. Comparing the cage sums inside a unit against 45 reveals the value of leftover cells.
Do Killer Sudoku cages allow repeated digits?
No. Within a single cage every digit must be different, even though the cage may span more than one box.
Play Sudoku now
Classic Sudoku is ready to play right now, with four difficulties and three grid sizes. Explore more variants or sharpen up with the strategy guides.
Play Sudoku free →