- Killer Sudoku: standard Sudoku rules apply, plus cells are grouped into "cages" with a sum target
- No digit can repeat within a cage (even if the cage spans multiple rows/columns)
- Key fact: any row, column, or box always sums to 45 — this enables innies-and-outies logic
- Learn the 45-rule: if a cage partially spans a unit, the "leftovers" are calculable
- Expert Killer uses small cages with forced combinations and large cages with many possibilities
The Core Killer Rules
Killer Sudoku uses a standard 9×9 grid with the standard uniqueness rule (1–9 once per row, column, and box). Additionally, dotted-line cages cover groups of cells, and each cage has a target sum. The sum of all digits in the cage must equal that target. No digit may repeat within a cage (regardless of standard Sudoku constraints).
The 45 Rule
Because every row, column, and box contains digits 1–9 once, each unit sums to exactly 45 (1+2+3+4+5+6+7+8+9). This is the foundation of the most powerful Killer technique: innies and outies.
If all cages within a unit except one are entirely inside that unit, the "innie" (cells inside the unit not covered by complete cages) or "outie" (cells outside the unit belonging to cages that mostly inside) can be calculated from the difference between 45 and the sum of the complete cages.
Innies and Outies in Practice
Example: A row contains three cages entirely within it (sums: 12, 14, 8 = 34 total), plus one cell that belongs to a cage mostly outside the row. That single cell must be 45 − 34 = 11... but wait, 11 is not a single digit. This means the analysis is telling you the cage extends beyond the row, so the "outie" calculation gives you the extra cell's contribution to the cage sum.
Working these calculations for multiple units systematically is the defining skill of expert Killer solving.
Cage Combination Analysis
| Cage Size | Sum = 3 | Sum = 4 | Sum = 17 | Sum = 16 |
|---|---|---|---|---|
| 2-cell | {1,2} only | {1,3} only | {8,9} only | {7,9} only |
Certain small-cage sums have only one possible combination. A 2-cell cage summing to 3 must be {1,2}. A 3-cell cage summing to 6 must be {1,2,3}. These "forced combination" cages allow immediate candidate elimination. Build a mental library of forced combinations for 2-cell cages (sums 3, 4, 16, 17) and 3-cell cages (sums 6, 7, 23, 24).
Getting Started with Killer
Killer Sudoku is available in the Killer Sudoku section of the site. Start at the Easy difficulty to build intuition for cage combinations before attempting hard Killer puzzles. The standard techniques (Hidden Singles, Locked Candidates) still apply — cage math gives you extra deductions on top of the standard toolkit.