Every row, column, and box sums to 45 — the leftover “innie” or “outie” cells have a known total.
Each house (row, column, box) totals 45. Add up the cages that sit fully inside a house; the cells that spill in (innies) or out (outies) must make up the difference. That hidden total constrains those cells like a mini-cage.
Killer Sudoku keeps every rule of Classic Sudoku — fill the 9×9 grid so each row, column, and 3×3 box holds the digits 1 to 9 exactly once — and adds cages: dashed groups of cells with a small target sum, where no digit may repeat inside a cage. Puzzles usually start with zero given digits, so every deduction begins from the cage sums. That single extra rule unlocks a whole family of arithmetic techniques.
Practise this in Killer mode on Sudoku Challenge, where the in-game Next Step button walks the exact deduction taught in this lesson.
Explore every Classic and Killer technique in the complete Learning Hub, or read how to play Sudoku to get started.