CageDokuKiller Sudoku rules
Killer Sudoku (also called Sum Sudoku or Sudoku Cages) combines Sudoku with the arithmetic of a sum puzzle. Here is everything you need to start solving.
The three rules
- Sudoku: every row, every column and every 3×3 box contains the digits 1 to 9 exactly once.
- Cages: the grid is split into groups of cells outlined by dotted lines. The small number in the corner of a cage is the sum of all digits in it.
- No repeats in a cage: a digit may be used only once inside a single cage, even if the cage bends around a corner.
A tiny example
Imagine a two-cell cage with the sum 3. The only two different digits that add to 3 are 1 and 2, so the cage holds {1, 2} in some order. A two-cell cage with the sum 17 can only be {8, 9}. These "unique combinations" are where almost every Killer Sudoku begins — see the full cage combinations table.
Why 45 matters
A row of nine cells holds 1+2+…+9 = 45. The whole grid therefore sums to 405. When cages fill most of a row, column or box, the cells that stick out of that region ("outies") or the cells of the region not covered by the cages ("innies") must add up to the difference. Learn this "rule of 45" and Killer Sudoku opens up — details in the solving tips.
Difficulty levels
Gentle puzzles fall to unique cage combinations and hidden singles. Moderate and Tough puzzles need the 45 rule and combination elimination. Diabolical and Extreme puzzles ask for locked candidates, pairs, triples and sometimes fish patterns. In CageDoku every puzzle is rated by the hardest human technique it requires and none needs guessing.
Frequently asked questions
What is Killer Sudoku?
Killer Sudoku is a Sudoku variant. Besides the usual rule that every row, column and 3×3 box contains the digits 1–9 once, the grid is divided into dotted "cages" and each cage shows a sum. The digits in a cage must add up to that sum and cannot repeat inside the cage.
Are there givens in Killer Sudoku?
Usually none or very few. The cage sums replace the starting digits, which is why Killer puzzles are solved with combination logic rather than scanning for singles.
What is the 45 rule?
Each row, column and box contains 1–9, so it sums to 45. If the cages inside a region add up to less or more than 45, the leftover cells (outies or innies) can be worked out directly.
Can a digit repeat in a cage?
No. Digits inside one cage must all be different, which is what makes combination tables so useful.