Killer Sudoku: Rules & How to Solve
Cages with target sums turn classic sudoku into an arithmetic puzzle. Here's the rule and the techniques that crack it.
The rule
Digits inside a dotted cage must add up to the small number printed in its top-left corner, and a digit can't repeat within a single cage. Normal sudoku rules still apply: 1–9 once per row, column, and box.
How to solve killer sudoku
The 45 rule
Every row, column, and 3×3 box totals 45. If cages cover a box and spill out by one cell, you can solve that cell by subtracting the cage sums from 45.
Lock small cages first
A two-cell cage summing to 3 can only be {1,2}; a sum of 17 can only be {8,9}. These "killer pairs" eliminate candidates from the whole row, column, or box.
No repeats inside a cage
Even when several combinations fit a sum, the no-repeat rule often rules most of them out — a 3-cell cage of 7 is {1,2,4}, never {1,3,3}.
Frequently asked questions
- Do digits repeat inside a killer cage?
- No. A digit can appear at most once per cage, on top of the usual row, column, and box constraints.
- What is the 45 rule in killer sudoku?
- Each row, column, and box contains 1–9, which sums to 45. Comparing cage totals against 45 lets you deduce leftover cells.
- Is killer sudoku harder than normal sudoku?
- It adds an arithmetic layer, but the cage sums also give extra information — many solvers find well-set killers more satisfying than classic grids.
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