How to Play Killer Sudoku

Killer sudoku is classic sudoku with a twist: the grid is also divided into cages — small groups of cells outlined with dashed lines. The top-left corner of each cage shows a number: the sum its cells must add up to.

Cage combinations

Many cages have only one or two possible digit combinations. A 2-cell cage summing to 3 can only be 2. Summing to 17, only 9. Memorising these "innie" combos is the single fastest skill upgrade in killer sudoku.

The 45 rule

Each row, column, and 3×3 box contains the digits 1–9, which sum to 45. So the cages that fully cover a row (or column, or box) must add up to 45. If they don't quite cover it — say one cell is sticking out into the neighbouring row — you can solve for that single cell using arithmetic alone.

Example: a row's cages add to 47, with one cage spilling one cell into the row above. That spillover cell is 47 − 45 = 2. Done.

Where to start a new puzzle

1. Find any cages with only one possible combination (e.g. 2-cell sum of 3 or 17, 3-cell sum of 6 or 24)
2. Look for 45-rule opportunities along edges and corners
3. Mark candidates in pencil marks just like classic sudoku
4. Combine the two — a candidate is only valid if it also fits the cage's possible combos

When you get stuck

The most common breakthrough comes from a naked pair inside a cage: if two cells in the same cage can only be 4 or 7, you've simultaneously locked the cage's other cells and eliminated 4 and 7 from the rest of that row, column, or box.

Pick a difficulty and try one. Stuck? Check the strategy guide for the cage combo cheat sheet.