Killer Sudoku looks like a regular sudoku that someone forgot to fill in. There are few given numbers or none at all. Instead the grid is covered in dashed outlines called cages, each with a small number in its corner. That number, and a little arithmetic, is what you solve from.
If you already know classic sudoku, you’re most of the way there. If you don’t, read How to Play Sudoku first; Killer keeps every one of those rules.
The rules
- Classic rules still apply. Every row, every column and every 3×3 box contains the digits 1 to 9 exactly once.
- Each cage adds up to its sum. The digits inside a dashed cage must total the small number in its corner.
- No digit repeats inside a cage. A two-cell cage of 4 can only be 1 and 3, never 2 and 2.
That’s the full rule set. Every Killer puzzle with a single solution can be solved from it without guessing.
Your first moves: the easy cages
Some cages can only be made one way. Find those first.
- A two-cell cage of 3 is always 1 and 2. A cage of 17 is always 8 and 9.
- A three-cell cage of 6 is always 1, 2 and 3. A cage of 24 is always 7, 8 and 9.
- A one-cell cage is simply a given: its sum is the digit.
These cages tell you which digits are inside them but not in which order. Pencil both or all three in as candidates; the rows, columns and boxes around them will sort the order out.
The Killer Sudoku combinations chart lists every cage size and sum, with the single-answer cages marked.
The rule of 45
This is the technique that makes Killer its own game.
The digits 1 to 9 add up to 45. So every row, every column and every box adds up to 45. If you know the total of most of a box, you know the rest.
Example. A box is covered by three cages that sit entirely inside it, adding up to 38, plus one cell that belongs to a cage poking in from outside. That lone cell must be 45 − 38 = 7.
It works in two directions:
- Innies. Cages fill a row, column or box except for a few cells that belong to cages sticking out. Those leftover cells add up to 45 minus the cages that fit inside.
- Outies. A cage pokes out of a box by a cell or two. Add up every cage that touches the box and subtract 45; what’s left is the total of the cells sticking out.
Once you know two cells add up to, say, 4, you’re back in familiar territory: they’re 1 and 3.
The rule scales up, too. Two neighbouring rows add up to 90, and three add up to 135. Harder puzzles ask you to use those.
A routine that works
- Mark every single-answer cage.
- Run the rule of 45 over each box, then each row and column, looking for one or two leftover cells.
- Use ordinary sudoku logic on what you’ve placed: singles, then pairs.
- When a cage still has several combinations, look for a digit that’s in every one of them. That digit is definitely in the cage, so it can’t appear anywhere else in a row, column or box the cage sits entirely inside.
- Go back to step 2. Each digit you place makes another 45 count possible.
Killer in One Sudoku
The app has three Killer levels, and each one asks for a specific set of ideas:
| Level | What you start with | What it asks of you |
|---|---|---|
| Easy | Small cages of 2 to 3 cells, plus a few filled-in digits | Cage sums and singles |
| Medium | Sums only, cages of 2 to 4 cells | Adds the rule of 45 on a row, column or box |
| Hard | Sums only, cages of 3 to 5 cells | Adds the rule of 45 over two rows or columns, naked pairs and pointing |
Every board is checked by a solver that works the way a person does, so there is always a logical next step and you never have to guess. If you’ve played Classic up to Hard, start Killer on Easy: the arithmetic takes a few games to become automatic, and after that the levels go quickly.
