How this killer solver works
Killer Sudoku adds one rule on top of regular sudoku: every cell sits inside a cage, and the digits in a cage must add up to the cage's printed sum without repeating. Building those cages is the part that slows people down most when typing a killer into a solver. The builder above is designed to make it quick: tap a cell to start a cage, tap an adjacent cell to extend it, type the sum, save. Repeat until every cell is in a cage, then press Solve.
Once the puzzle is solved, the replay walks back through the deductions. Killer Sudoku shares a lot of techniques with classic sudoku, but it has its own shortcuts that the replay names by their proper killer-specific labels.
Cage techniques you'll see
- Single-cell cage — a cage with exactly one cell. The digit is the cage's sum. Trivial, but easy to miss when you're reading a screenshot; the solver always grabs them first.
- Cage completion — when only one cell of a cage is still empty, its digit is the cage's sum minus the digits already filled in. This is what makes killer puzzles cascade once they get going.
- The 45-rule — every row, column, and 3×3 box must sum to 45. If a row is covered by cages with a total sum of 39 plus one stray cell, that cell must be 6. The rule comes in two shapes the solver will name for you: the "innie" pattern for single cells inside a unit and the "outie" pattern for cells just outside. Innies and outies tend to be the first real moves on harder killers, when nothing else is biting.
- Killer pair, triple, and quad — some cage shapes can only be filled one way. A two-cell cage summing to 17 must be 8 and 9. A three-cell cage summing to 7 must be 1, 2, and 4. When the cage's digits are locked, you can eliminate them from the rest of any row, column, or box those cells share with peers.
Classic techniques on a killer grid
Cage rules are extra constraints, not replacements. Every classic technique still works on a killer grid — naked singles, hidden singles, naked pairs, locked candidates, X-wing — and the replay will name them when they fire. On most easy and medium killers, the toughest move comes from a 45-rule innie that opens up a chain of plain naked singles afterwards.
Hard, master, and extreme killers
Killer puzzles get hard fast once the obvious cages are gone. Published expert and master killers sometimes need chains, advanced fish patterns, or careful trial-and-error that the solver doesn't teach yet. When that happens, the replay ends with a single "advanced deduction" step and skips to the solved grid. We're adding more techniques over time, so this fallback comes up less and less. In the meantime, our learn section is the best place to read up on what techniques like AIC and ALS look like.
If you want to practise the techniques you see in the replay, play a killer puzzle on the site. The play surface uses the same engine, so the cages and deductions you see during a game are the ones the solver would walk you through.