Killer Sudoku: The Rules and How to Solve It

What makes it Killer

A Killer Sudoku is a normal 9 by 9 board with the usual rule: each row, column and 3 by 3 box holds every value once. On top of that, the grid is divided into dotted regions called cages, and each cage shows a small target number. The values inside a cage must add up to that target, and no value may repeat within a cage.

Most Killer puzzles start with no given numbers at all. The cages and their sums are the only clues, which is what makes the puzzle feel fresh even to seasoned solvers.

The rule of 45

Here is the single most useful trick. Every row, column and box contains the values 1 through 9 exactly once, and those add up to 45. So if the cages inside a row almost fill it, you can subtract their sums from 45 to find the value of the leftover cell. This one idea unlocks a huge number of early moves.

Think in combinations

Small cages are your friends because their sums have few possibilities. A two-cell cage summing to 3 can only be 1 and 2. A two-cell cage summing to 17 can only be 8 and 9. A three-cell cage summing to 6 can only be 1, 2 and 3. Learning these locked combinations lets you pencil in candidates fast.

• Two cells summing to 3: only 1 and 2.
• Two cells summing to 4: only 1 and 3.
• Two cells summing to 16: only 7 and 9.
• Two cells summing to 17: only 8 and 9.
• Three cells summing to 6: only 1, 2 and 3.
• Three cells summing to 24: only 7, 8 and 9.

Then it is just Sudoku

Once the cage math gives you a foothold, the classic techniques take over. Singles, pairs and scanning all work exactly as they do in normal Sudoku. Killer simply gives you an extra source of clues at the start, then hands the puzzle back to your usual logic.

Where to practice

Killer is one of many twists on the classic game. If you enjoy it, explore the other variants too, and keep your core skills sharp with a standard board whenever you want a pure logic workout.