What Is Killer Sudoku?
Killer Sudoku is a hybrid puzzle that combines the logic of classic Sudoku with elements of Kakuro (sum puzzles). It was popularized in Japanese puzzle magazines and has grown into one of the most beloved Sudoku variants worldwide. While the core rules of the 9×9 grid still apply, Killer Sudoku introduces cages — groups of cells that must sum to a given total.
The Rules of Killer Sudoku
Killer Sudoku follows all standard Sudoku rules plus one additional constraint:
- Each row, column, and 3×3 box must contain the digits 1–9 with no repetition (standard Sudoku rules).
- The grid is divided into cages — outlined groups of 2 to 5 cells — each labeled with a small target sum.
- The digits within each cage must add up to the cage's target sum.
- Within a single cage, no digit may repeat.
Notably, Killer Sudoku grids typically have no pre-filled "given" numbers. All information comes from the cage sums and standard Sudoku constraints.
How Cages Work: The Key Mechanic
Cages are the heart of Killer Sudoku. Here's how to use them:
Small Cages Are Your Best Friends
A 2-cell cage with a sum of 3 can only be filled one way: 1 + 2. A 2-cell cage summing to 17 must be 8 + 9. These "forced" cages give you free information right from the start.
Useful Cage Sum Reference
| Cage Size | Min Possible Sum | Max Possible Sum |
|---|---|---|
| 2 cells | 3 (1+2) | 17 (8+9) |
| 3 cells | 6 (1+2+3) | 24 (7+8+9) |
| 4 cells | 10 (1+2+3+4) | 30 (6+7+8+9) |
| 5 cells | 15 (1+2+3+4+5) | 35 (5+6+7+8+9) |
The 45 Rule
One of the most powerful Killer Sudoku techniques is the 45 Rule: since every row, column, and box must contain digits 1–9, the sum of all cells in any row, column, or box is always 45. If the cages within a row sum to 38 and only one cell is unaccounted for, that cell must equal 45 − 38 = 7.
Killer Sudoku vs. Classic Sudoku: Key Differences
- No givens: Classic Sudoku gives you pre-filled numbers; Killer Sudoku gives you none — only cage sums.
- Arithmetic involved: You need to think about sums and combinations, not just placement.
- Extra constraint layer: The no-repeat rule inside cages adds a second elimination tool on top of standard row/column/box logic.
- Difficulty range: Killer Sudoku can range from simple to extraordinarily complex depending on cage sizes and layouts.
Tips for Solving Your First Killer Sudoku
- Start by listing all possible digit combinations for each cage, especially small ones.
- Apply the 45 Rule to rows, columns, and boxes where most cages are fully contained within them.
- Look for "innies" (cells inside a house but outside all listed cages) and "outies" (cells extending beyond a house boundary) — they often yield precise values.
- Use standard Sudoku scanning as soon as you've placed a few digits.
Where to Try Killer Sudoku
Many free Killer Sudoku puzzles are available online and in puzzle books. Start with Easy or Gentle rated Killer puzzles before moving to Moderate or Tough. Once you understand cage logic, you may find Killer Sudoku even more satisfying than the classic version!