Rule 1 — the Sudoku rule
Every row, every column and every 3×3 box contains the digits 1 to 9 exactly once.
Killer Sudoku — rules and how to play, free
Killer Sudoku is Sudoku plus arithmetic: the grid is divided into dotted "cages", and the numbers in each cage must add up to the small sum printed in its corner — without repeating a digit inside the cage. Normal Sudoku rules still apply.
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- No sign-up needed
- Plays offline
Killer Sudoku is Sudoku plus arithmetic. The grid is divided into dotted outlines called cages; the digits inside each cage must add up to the small number printed in its top-left corner, and no digit may repeat inside a cage. All the normal Sudoku rules still apply.
The three rules
Rule 2 — the cage sum
The digits inside a cage must add up to the small number printed in its corner.
Rule 3 — no repeats in a cage
A digit cannot appear twice inside the same cage, even when the cage spans two boxes.
Five tips that make Killer click
Start with tiny cages
A two-cell cage summing to 3 can only be 1+2; summing to 17 it can only be 8+9. These forced pairs are free information.
Use the rule of 45
Each row, column and box totals 45. If the cages inside a box add up to 40, the cell left over must be 5.
Write the combinations, not the guesses
For each cage list every digit combination that reaches the sum, then delete the ones the grid forbids.
Watch cages that cross a box
A cage straddling two boxes carries information into both — those are usually the cells that unlock a stuck grid.
Let the hint teach you
When you are stuck, a hint in Skudo names the technique and shows why it applies here, so the next grid is easier on your own.
Cage sum lookup chart
Every cage sum, and the digit sets that can fill it — generated, so there is nothing to mistype.
Playing Killer in Skudo
Killer runs across all four difficulty levels, with hints that explain the technique behind the next move rather than just filling a cell. It plays fully offline in the downloaded app, and there is a gentler 6×6 Killer board when you want something shorter.
Frequently asked questions
What is Killer Sudoku?
Killer Sudoku is Sudoku plus arithmetic: the grid is divided into dotted "cages", and the numbers in each cage must add up to the small sum printed in its corner — without repeating a digit inside the cage. Normal Sudoku rules still apply.
Is Killer Sudoku harder than normal Sudoku?
It is different rather than strictly harder: an easy Killer grid is very approachable once you know the cage rule, because the sums give you information a plain Sudoku never would.
Can a digit repeat in a row if the cage allows it?
No. The normal Sudoku rules always win: a digit can never repeat in a row, column or box, no matter what the cage sum would allow.
Do I need to be good at maths?
No — the only arithmetic is adding digits from 1 to 9. Killer Sudoku is a logic puzzle; the sums are clues, not calculations.
Is Killer Sudoku free in Skudo?
Yes — every Killer difficulty is included for free, with no ads and no in-app purchases.
Play Killer Sudoku free
Free, ad-free Sudoku with a live AI rival. Classic & Killer. Plays fully offline.