Meet Killer Sudoku

If you've solved a few classic Sudoku puzzles and you're curious what the variants look like, Killer Sudoku is the natural next step. It uses the same 9×9 grid, the same row, column, and box rules, and the same eventual goal — fill every cell with a digit from 1 to 9 with no repeats. What it adds is arithmetic.

Specifically: the puzzle starts with the grid divided into dashed-line cages, each labelled with a target sum in the top-left corner. The digits inside a cage have to add up to that sum, with no repeats inside the cage. The standard row, column, and box rules still apply on top. That's the whole rule set.

The result is a puzzle that feels different from classic in a way most classic-only solvers find surprising the first time they try one.

How the rules differ

The cage rule changes what the constraints on each cell look like. In classic Sudoku, a cell's candidates are the digits not already used in its row, column, or box. In Killer, those constraints still apply, plus the digit has to fit the cage's sum.

A two-cell cage labelled "3" can only be 1+2 — those are the only two distinct digits that sum to 3. A two-cell cage labelled "17" can only be 8+9. These small cages are huge information sources, because they constrain two cells to a known pair of digits before you've placed anything elsewhere.

Bigger cages have more flexibility but still inform the puzzle. A four-cell cage labelled "10" can only be 1+2+3+4 — the only four distinct digits summing to ten. A three-cell cage labelled "6" must be 1+2+3, the only triple summing to six. A two-cell cage summing to "11" could be 2+9, 3+8, 4+7, or 5+6 — looser, but still rules out 1.

Experienced Killer solvers keep a small mental library of these "unique combinations" and recognise them on sight. Beginners look them up the first few times and internalise them within a few puzzles.

How the experience differs

Most classic-only solvers describe their first Killer puzzle as feeling slower at the start and faster in the middle. The slow start is because Killer's opening doesn't have many givens to work from — most Killer puzzles start with no givens at all, and you have to build the grid out from cage constraints rather than from existing placements.

The faster middle is because once you've placed the first dozen digits, the cage constraints compound with the row/column/box constraints in ways that make the next placements come quickly. A digit you've placed eliminates options from its row, its column, its box, and its cage — four constraint dimensions instead of three.

The puzzle's particular pleasure is that arithmetic and logic interlock in a way classic doesn't offer. You're not just asking "what digit goes here?" but "what set of digits could go in this cage, given the rest of the grid?" — a question with more shape and more flavour than the classic version.

Where to start

Killer has the same four difficulty tiers as classic — easy, medium, hard, expert — but the difficulty curve is steeper. An easy Killer puzzle is roughly comparable to a medium classic in time and effort, because the cage rule adds a learning surface before the puzzle becomes natural.

If you're new to Killer, start on easy. Spend the first puzzle just getting comfortable with the cage notation — the dashed lines, the small sum labels — and don't expect to solve it quickly. The classic-Sudoku naked and hidden singles you've internalised still apply on every cell; the cage rule sits on top.

Your second puzzle will go faster. Your third will start to feel like a Sudoku again, with cages as a feature rather than a friction.

Once Killer is natural-feeling, hard and expert open up. Expert Killer puzzles in particular start with zero givens — the cage layout alone determines the unique solution. They're a different and rewarding cognitive shape.

Killer is one of those puzzles that some people prefer to classic from their first attempt and some people never quite warm to. Both reactions are reasonable. If your first easy puzzle clicks, you've found a new daily ritual; if it doesn't, classic is patient and will be here whenever you want to come back. If you'd like to try one, a Killer easy puzzle is the right starting point.