The rules of killer sudoku
Killer sudoku is sudoku with a twist: the 9×9 grid is split into irregular cages — dashed groups of cells — and the small number in each cage's corner tells you the sum of the digits inside it. Fill the grid so every row, column and 3×3 box contains 1 to 9 exactly once, and every cage's digits add up to its corner sum. Every puzzle has exactly one solution.
The five rules
- Fill every cell with a digit from 1 to 9.
- Every row, every column and every 3 × 3 box must contain 1 to 9 exactly once — normal sudoku rules.
- The digits inside each dashed cage must add up to the small number in the cage's corner.
- No digit may repeat inside a cage.
- There is exactly one solution, reachable by logic and simple arithmetic.
How to read the grid
The dashed lines mark the cages, and each cage shows its sum in one corner. A two-cell cage marked 3 must contain 1 and 2 in some order; a three-cell cage marked 6 must contain 1, 2 and 3. The sum is the only clue the cage gives you — the order of the digits is decided by the rows, columns and boxes around it.
One more thing: no repeats inside a cage
This is what separates killer sudoku from calcudoku. A cage is always a set of different digits — the cage's sum is the sum of distinct numbers. That makes the small cages wonderfully informative: a two-cell cage marked 3 can only be 1 + 2, and a two-cell cage marked 4 can only be 1 + 3.
The rule of 45
Because every row, column and box holds 1 to 9 exactly once, each of them sums to 45 (1 + 2 + … + 9). That one fact is the key to killer sudoku: if a row is covered by cages whose corner sums add up to, say, 51, the difference (6) must be hiding in the one cell that sticks out of the row. The rule of 45 turns whole lines into simple subtractions.