Killer Sudoku rules, sums and solving tips

Killer Sudoku uses the normal row, column and box rules, then adds dotted cages with target sums. The arithmetic is small; the real skill is using each sum to eliminate combinations rather than trying numbers. Start with a small cage, record only the combinations that fit, and let the normal Sudoku rules narrow them further.

What are the rules of Killer Sudoku?

Every row, column and three-by-three box still contains the digits one to nine exactly once. In addition, the digits in each dotted cage add to the tiny number in its corner. A digit cannot repeat inside a cage, even when the cells are in different rows and columns. A two-cell cage totalling 3 is therefore always 1 and 2; a two-cell cage totalling 10 can never be 5 and 5.

How do cage sums and combinations help?

A cage sum does not tell you an order, but it limits the set of digits. For a two-cell cage totalling 15, the only non-repeating pair is 6 and 9. For larger cages, keep the possible sets rather than permuting every order. Then use the row, column and box to remove sets that conflict with digits already present. This is why Killer Sudoku is logic with light arithmetic, not a calculation race.

What is the rule of 45?

The digits 1 through 9 total 45, so every complete row, column and box totals 45. Add the cages wholly inside one unit and compare that total with 45: the difference belongs to the cells crossing its boundary. Two complete units total 90 and three total 135. This turns a collection of cage clues into a precise sum for the few cells that remain outside the group.

What are innies and outies?

An innie or outie is a cage part that crosses the edge of a row, column, box or a group of units. Add the cages inside the region, include or exclude the crossing cage as appropriate, and compare with the region total of 45, 90 or 135. The difference restricts the crossing cells. Begin with one-cell crossings; multi-cell versions are useful only when their remaining combinations are genuinely narrower.

How should a beginner start a Killer Sudoku?

Start with the smallest and most restrictive cages: single cells, two-cell totals such as 3, 4, 16 or 17, and cages whose sum has only one digit set. Keep candidate notes on harder puzzles and check each cage against its row, column and box before reaching for a named technique. When you stall, use a hint to see the highlighted cage proof, then open the linked lesson and practise that exact idea in a verified Killer position.

Turn the rules into a real deduction

Open an easy Killer puzzle to use the rules immediately, or follow the Killer learning path from cage combinations to advanced sum relations.

Killer Sudoku Learning path Play Easy

Frequently asked questions

Can a digit repeat in a Killer Sudoku cage?

No. Digits within one cage are all different. The normal Sudoku rules still apply too, so a digit also cannot repeat in its row, column or box.

Do I need to be good at maths for Killer Sudoku?

No. You mainly add small cage clues and compare a result with 45, 90 or 135. The important work is ruling out digit combinations logically.

Why does every row, column and box add up to 45?

Each one contains the digits 1 to 9 exactly once. Adding those digits gives 45, regardless of their order.

What is the easiest Killer Sudoku technique?

Start with cage combinations and the last empty cell of a cage. They need the least bookkeeping and create the candidates used by later sum techniques.

When should I use innies and outies?

Use them when cages almost cover a row, column, box or adjacent group of units, leaving only a few crossing cells. If many cages cross the boundary, another pattern is usually cheaper.

Is Killer Sudoku harder than regular Sudoku?

It adds another constraint, so the first puzzles feel different. Small cages often give very strong deductions, though, and easy Killer puzzles are a good way to learn the new arithmetic gradually.