How to play killer sudoku: a complete beginner's guide

Killer sudoku looks intimidating — an empty grid, no starting digits, just dashed boxes with little numbers. Ten minutes with this guide and you'll wonder why you ever found it scary.

What is killer sudoku?

Killer sudoku is a cross between sudoku and kakuro. The grid is standard 9×9 sudoku, but instead of given digits, the grid is divided into outlined regions called cages. The small number in each cage's corner tells you what the digits inside it must add up to.

Despite the name (a translation of the Japanese satan namba era of fiendish variants), most killer sudokus are no harder than a decent newspaper sudoku — the sums give you information a normal sudoku doesn't have.

The three rules

1. Sudoku rules apply. Every row, every column, and every 3×3 box contains the digits 1–9 exactly once.
2. Cage sums must match. The digits inside each dashed cage add up to the number printed in its corner.
3. No repeats in a cage. A digit can't appear twice inside the same cage, even when the cage snakes across boxes and a repeat wouldn't break rule 1.

Your first moves: a walkthrough

Facing an empty grid, here's the exact order to attack it:

1. Hunt the forced cages

Some cage sums can only be made one way. A 2-cell cage summing 3 must be 1+2. Summing 17? That's 8+9, no other option. A 3-cell cage totalling 24 is always 7+8+9. Scan the whole grid for these before doing anything else and pencil-mark the digits. (The full cheat sheet of forced combinations lists every one.)

2. Fill the single-cell cages

Easier puzzles include cages of just one cell — the sum is the digit. Free squares. Take them.

3. Let sudoku decide the order

Knowing a cage holds 8 and 9 is half the battle; normal sudoku logic settles which cell gets which. If one of the cage's cells shares a column with an existing 9, the 9 goes in the other cell.

4. Add the cages up against 45

Here's the trick that unlocks everything: the digits 1–9 sum to 45, so every row, column and 3×3 box totals exactly 45. If cages covering a box add up to 43 and one cell of the box belongs to a cage poking outside, arithmetic hands you that cell's relationship on a plate. This deserves its own page — see the rule of 45.

Pencil marks: not optional

In regular sudoku, strong players can hold candidates in their head. In killer sudoku, even experts pencil-mark, because cage combinations create candidate lists that shift as sums resolve. On Zaney Sudoku, tap the ✏️ Notes button (or press N) — and when you place a digit, your notes in that row, column, box and cage tidy themselves automatically.

Five beginner mistakes to avoid

• Forgetting the no-repeat rule in cages that span two boxes. The cage looks legal by row and column but repeats a digit inside itself.
• Only checking a cage when it's full. A partially filled cage already constrains its remaining cells: if a 3-cell cage summing 12 contains a 7, the other two cells total 5 — so 1+4 or 2+3.
• Ignoring what a sum excludes. A 2-cell cage summing 5 will never contain a digit above 4. Exclusions are eliminations.
• Guessing. Every puzzle on this site is verified to have exactly one solution reachable by logic. If you're guessing, there's a deduction you haven't spotted — usually a 45 trick.
• Starting with the big cages. A 5-cell cage summing 26 has too many possibilities to be useful early. Work the extremes first, and the big cages collapse on their own.

Which difficulty should you start on?

Easy puzzles here use small cages and reveal a few starting digits — ideal for your first dozen games. Medium removes the training wheels. Hard and Expert use larger, more ambiguous cages where the rule of 45 and combination analysis do the heavy lifting.