Killer Sudoku Rules: How to Play Killer Sudoku
Killer sudoku is sudoku with dotted outlines called cages. As in sudoku, every row, column, and 3×3 box contains the digits 1 to 9 once each. On top of that, the numbers in each cage add up to the small total in its corner, and the same number never appears twice in a cage.
The grid and the cages
The grid has 81 cells: a horizontal line is a row, a vertical line is a column, and each 3×3 block outlined in bold is a box.
Dotted outlines called cages cover the grid, and the small number in a cage's top-left corner is the total of the numbers inside it. Most killer sudokus start with few or no numbers filled in, so the cage totals are your clues.
The rules
1. Each row contains the digits 1–9 exactly once.
2. Each column contains the digits 1–9 exactly once.
3. Each box contains the digits 1–9 exactly once.
4. The numbers in a cage add up to the cage's total, and no number repeats inside a cage.
How to start: cages with one combination
Some cages can only be filled one way. Two cells totaling 3 must be 1 and 2, two cells totaling 17 must be 8 and 9, and three cells totaling 6 must be 1, 2, and 3.
Pencil those numbers into the cage, then place them using the row, column, and box rules, just like in sudoku.
→ Killer Sudoku Cage Combinations Chart (2–9 Cells, All 502)
The Rule of 45
The numbers in any row, column, or box always add up to 45.
Subtract the totals of the cages that fit completely inside a box from 45, and you get the total of the remaining cells — the parts of cages poking in from outside. If only one cell remains, its number is decided. It's the most-used technique in killer sudoku.
When you get stuck
Look again for cages with very small or very large totals (they have few combinations), and for rows, columns, or boxes where the cages almost fit.
If that doesn't help, pencil in the candidates and use sudoku techniques too, like pairs and pointing. The hints in the app and the browser version show you where to look next in three steps.
Frequently asked questions
Does a killer sudoku have only one solution?
A properly made killer sudoku has exactly one solution. The puzzles here are all checked to have a single solution that can be found by logic.
Can a number repeat inside a cage?
No. Two cells totaling 4 can only be 1+3, never 2+2.
Is killer sudoku harder than regular sudoku?
The empty starting grid can feel strange at first. Once you know the common combinations and the Rule of 45, you'll make steady progress just like in regular sudoku.
Where should I start solving?
Start with cages that have only one combination (2 cells: 3, 4, 16, 17; 3 cells: 6, 7, 23, 24) and with boxes that the cages almost fill.