How to Play Killer Sudoku
Killer Sudoku is classic Sudoku with an arithmetic twist. The grid is divided into outlined groups of cells called cages, and the digits inside each cage must add up to the small number printed in its corner. You start with few or no given digits — the sums themselves are your clues.
Rule 1 — classic Sudoku still applies
Every row, column, and 3×3 box must contain the digits 1–9 exactly once, just like a normal puzzle. Whatever the cages say, a digit that would repeat in its row, column, or box is illegal. Solving is always a conversation between the classic constraints and the cage sums.
Rule 2 — every cage must hit its sum
The digits in a cage must add up to the cage's printed total. A two-cell cage marked 5 can only be 1+4 or 2+3. The fewer combinations a sum allows, the more the cage tells you — which is exactly what you'll exploit to get started.
Rule 3 — no repeats inside a cage
A digit can never appear twice in the same cage, even when the cage spans two boxes. This rule quietly does a lot of work: a two-cell cage marked 16 can't be 8+8, so it must be 7+9 — one of the "forced" cages that hand you your opening moves.
Your first strategy: forced cages and the 45 rule
Start with cages that allow exactly one combination — two-cell cages summing to 3, 4, 16, or 17, and three-cell cages summing to 6, 7, 23, or 24. Then remember that every row, column, and box totals 45: if a box is fully covered by cages summing to 47 with one cell poking out into the next box, that protruding cell holds a 2. These two ideas alone crack the opening of most killer puzzles.
Common beginner mistakes
Three errors account for most killer frustration. First, forgetting the no-repeat rule inside long cages — a five-cell cage summing to 35 has limited options precisely because repeats are banned. Second, treating cage math as the whole game and neglecting classic row-column-box scanning, which still does half the work. Third, filling a cage's digits before knowing their order: knowing a cage holds 7+9 doesn't tell you which cell holds the 7 — the classic constraints decide that. Slow down on exactly these three points and killer becomes far friendlier than it looks.