What is Killer Sudoku?
Killer Sudoku is a Sudoku grid with an arithmetic layer laid over it. The 9×9 grid is divided into irregular groups of cells called cages, drawn with a dotted outline, and each cage prints the total its digits must add up to. Everything you know about Sudoku still applies: every row, every column and every 3×3 box holds the digits 1 to 9 exactly once. The cages add one more rule — no digit may repeat inside a cage — and take one thing away: the grid starts completely empty. Expert is the pure form; the easier levels place a few digits for you to start from.
What is the difference between Sudoku and Killer Sudoku?
Ordinary Sudoku hands you twenty-five or thirty digits and asks you to deduce the rest. Killer Sudoku hands you none. The cage totals carry the same information in a form you have to unpack first: a two-cell cage totalling 4 can only be 1 and 3, and a five-cell cage totalling 15 can only be 1, 2, 3, 4 and 5. Where a normal grid starts with a scan, a killer grid starts with arithmetic.
How to play Killer Sudoku
- Look for the cages with only one possible combination. A two-cell cage of 3 is 1+2; of 17 it is 8+9. Those are free.
- Use the rule of 45. Every row, column and 3×3 box totals 45, so if the cages inside a box add up to 40, the cells left over must hold 5.
- Write pencil marks for the combinations, not just the digits. A cage of 16 across three cells has several sets, but 1 appears in almost none of them.
- Remember the no-repeat rule inside a cage. It removes candidates that the row, column and box would otherwise allow.
The best Killer Sudoku tactic: the rule of 45
Each of the nine rows, nine columns and nine boxes contains 1 through 9 exactly once, so each totals 45. Add up the cages that sit entirely inside one of them and the difference is the total of whatever is left — often a single cell, which you can then simply write in. Solvers call the leftover cells inside a house innies, and the cells of an overlapping cage that stick out of it outies. Learning to spot them is most of what separates a fast killer solve from a slow one.
Are Killer Sudoku rules hard?
The rules are shorter than ordinary Sudoku’s: same grid, same digits, plus "cages add up to their total" and "no repeats in a cage". The arithmetic never goes beyond adding single digits. What makes a killer grid feel hard at first is unfamiliarity rather than difficulty — an easy killer puzzle is usually easier than a hard normal one, and the tactics transfer in both directions.
Why play Killer Sudoku?
Because it exercises a different half of the same skill. Normal Sudoku is pattern recognition; killer adds a small amount of arithmetic and a lot of combination logic, and that combination logic is what makes hard normal puzzles easier afterwards. It is also, for most people, more forgiving: with no given digits there is no wasted scanning, and there is always a cage somewhere with only one way to fill it.