It suits classic sudoku players who find the standard puzzle has become routine. The scanning you already know still matters, but most of your progress now comes from adding up, and from knowing which sets of digits can make a given total.
The rules
- Place the digits 1 to 9 once each in every row, every column and every 3×3 box, exactly as in classic sudoku.
- The digits in each cage must add up to the number printed in its corner.
- A digit can't appear twice inside the same cage, even when the cage spans two boxes.
- Every cell belongs to exactly one cage. Our cages are between one and five cells, and no digits are given at the start.
First moves for a new solver
Start with the cages that can only be filled one way. A 3 in two cells is always 1 and 2. A 4 in two cells is 1 and 3, a 16 is 7 and 9, a 17 is 8 and 9. In three cells, 6 is 1, 2 and 3, 7 is 1, 2 and 4, 23 is 6, 8 and 9, and 24 is 7, 8 and 9. Pencil those pairs and triples in first; you won't know the order yet, but you know the digits.
Next, use the fact that every row, column and box adds up to 45. If the cages lying wholly inside one box total 38 and a single cell of the box is left over, that cell is 7. The same works in reverse: when a cage sticks one cell out of a box, the amount by which the cages exceed 45 is the value of that outside cell. Solvers call these innies and outies, and the trick is known as the rule of 45.
Once a few digits are fixed, ordinary sudoku scanning takes over again. Keep switching between the two: arithmetic gives you candidates, and the rows, columns and boxes trim them.
What changes from easy to expert
Our grader solves every puzzle step by step, cheapest technique first, and the level is the hardest step it needed. Easy puzzles fall to cage combinations, naked and hidden singles and single-cell innies and outies. You will rarely need more than one cage and one house in view at a time.
Medium adds pointing and claiming, naked pairs, innie/outie pairs (two unknown cells left over in a house, whose sum you then know) and cage/house interaction, where a digit forced into one cage clears it from every cell that sees all of that cage's possible spots.
Hard asks for longer arithmetic: rule of 45 sums over three or more cells, innies and outies across two or three neighboring rows or two boxes, plus hidden pairs and naked triples. Expert keeps all of that and adds the classic chain patterns: hidden triples, naked quads, XY-wing, XYZ-wing, swordfish and simple coloring. An expert grid can look stuck for a while before one of those patterns opens it.
Guides, tools and printables
If you are new to cages, the how to play killer sudoku guide walks through a whole puzzle. Keep the killer sudoku cheat sheet open beside the board, or use the cage combinations calculator to list every set of digits for a total and cage size, with digits you've ruled out removed.
Prefer paper? The printable killer sudoku packs come as PDFs with the answers on the last pages, and a large print edition uses bigger cells and type. For one new medium puzzle each day, the same for everyone, try the daily killer sudoku.