What is killer sudoku?
Killer sudoku mixes classic sudoku with a touch of arithmetic. Instead of given numbers, the grid is divided into cages outlined with dotted lines, and each cage shows the sum of its digits. You still place 1 to 9 in every row, column and box, but now the sums guide every move.
It is the natural next step once regular sudoku feels easy. Play killer sudoku online free here, with a new puzzle whenever you want one.
How killer sudoku works
Killer sudoku keeps every rule of normal sudoku, one of each digit in every row, column and box, but usually starts with few or no given digits. Instead, dotted cages cover the grid, each with a small number showing the sum of its cells. Digits may not repeat inside a cage. The puzzle became popular in British newspapers in the mid-2000s, soon after classic sudoku took off.
Unique cage combinations
Some sums can only be made one way, and learning them is the fastest way into any killer sudoku:
| Cage | Only possible digits |
|---|---|
| 2 cells, sum 3 | 1 and 2 |
| 2 cells, sum 4 | 1 and 3 |
| 2 cells, sum 16 | 7 and 9 |
| 2 cells, sum 17 | 8 and 9 |
| 3 cells, sum 6 | 1, 2 and 3 |
| 3 cells, sum 24 | 7, 8 and 9 |
The 45 rule
Every row, column and box contains the digits 1 to 9 exactly once, and they add up to 45. If the cages inside a box add up to 42 and one cell of the box is covered by a cage that sticks out, that cell must be 3. This “innie and outie” trick solves many hard spots.
How to play here
Tap a cell and type a digit, or use the number pad on a phone. Right-click for pencil marks on a computer, or use Mark mode on a phone. Once killer sudoku clicks, Calcudoku is a natural next step.
How to play Killer Sudoku
- Every row, column and 3×3 box holds the digits 1 to 9 exactly once, as in normal sudoku.
- The grid is split into dotted cages, each with a small number in its corner.
- The digits in a cage must add up to that number.
- A digit can never repeat inside the same cage.
Killer Sudoku tips
Learn the unique cages
Some sums can only be made one way. A two-cell cage of 3 is always 1 and 2; a two-cell 17 is always 8 and 9; a three-cell 6 is always 1, 2 and 3.
Use the 45 rule
Every row, column and box adds up to 45. Add the cages that fit inside one box, subtract from 45, and you know the total of the cells left over, often a single cell.
Combine sums with normal sudoku
Once a cage is narrowed to a few digits, switch to ordinary scanning. Cage limits and sudoku rules together clear candidates fast.
Frequently asked questions
Do killer sudoku puzzles have starting numbers?
Usually not. Most killer sudoku grids start completely empty, and the cage sums are your only clues.
Is killer sudoku harder than normal sudoku?
It asks for a little arithmetic, but the easy levels are gentle. Many players find it more satisfying, because every cage gives you something to work with.
What is the 45 rule in killer sudoku?
Each row, column and 3×3 box contains 1 to 9, which always adds up to 45. Comparing that total with the cages lets you work out hidden sums.
