IF YOU CAN PLAY SUDOKU, YOU'RE 90% THERE
Killer Sudoku is normal Sudoku with one extra rule bolted on. So the Sudoku part is exactly what you'd expect:
- every row has 1 to 9, each once, no repeats;
- every column has 1 to 9, each once, no repeats;
- every 3×3 box has 1 to 9, each once, no repeats.
The twist is the cages — those dotted-outline shapes dotted around the grid. Each cage has a little number in its top-left corner, and that's the sum: the squares inside the cage have to add up to it. One more rule comes with them: no number repeats inside a cage either.
That's the whole game. And here's the good news nobody tells beginners: those cage sums aren't there to make it harder — they hand you starting numbers Sudoku never would.
LET'S ACTUALLY SOLVE ONE — STEP BY STEP
Here's a real Killer Sudoku. The dotted shapes are the cages; the small corner numbers are their sums. Don't read the whole grid at once — we only ever look at one cage at a time.
The starting grid — dotted cages, each with a sum to hit.
The move that cracks Killer Sudoku is cage completion: a cage's squares add up to its sum, so the moment only one square in a cage is empty, you don't guess — you subtract. Let's do it for real, easiest first.
Step 1 — the freebie: a cage with just one square.
See the little cage on its own with a 7 in the corner? One square that has to add up to 7… can only be a 7. That's it placed, and you've not done a scrap of Sudoku yet. Single-square cages are pure gifts — always fill them in first.
Step 2 — a two-square cage: now you subtract.
This cage on the bottom row holds two squares and sums to 15. One of them is already a 9 (it came as a clue). Two numbers adding to 15, one of them 9 — the other has to be 15 − 9 = 6. No scanning, no Sudoku logic at all, just one subtraction. That's the move you'll make most.
Step 3 — and it works no matter how big the cage is.
Here's a four-square cage that sums to 22. Three of its squares are already filled — a 6, an 8 and a 3 (that's 17). One square left, and it has to make up the difference: 22 − 17 = 5. Same move every time — when a cage has one gap, the gap is just the sum minus what's already in it.
That's the Killer-specific bit, and it's honestly most of the job early on: hunt for cages with a single empty square and subtract. Each number you drop in fills another cage's last gap, or gives ordinary Sudoku scanning something to bite on. From here it's the two games taking turns — fill a cage, that unlocks a row, that fills another cage — and the grid quietly empties out:
Over halfway — cages and scanning feeding each other.
Done. Every row, column and box has 1 to 9 — and every cage adds up. 👻
WHEN NO CAGE HAS A GAP — TWO MORE TRICKS
Early on, plenty of cages have one empty square and you just subtract. When that dries up, here are the two moves that get you moving again — both still no guessing.
1. Read what a sum can't be.
A cage's sum quietly limits its numbers. A two-square cage adding to 3 can only be a 1 and a 2 (no repeats allowed, and nothing else makes 3). A two-square cage of 17 is forced to be 8 and 9. You won't always know which square gets which yet — but knowing a square is "an 8 or a 9, nothing else" is often the clue that cracks its row or box. Tiny cages with extreme sums are the most generous.
2. Fall back on plain Sudoku scanning.
The cage hints sit on top of a normal Sudoku, so every Sudoku move still works. Pick a number, pick a box, and see if its rows and columns pin it to a single square — exactly the cross-hatching you already know. Cages get you started where Sudoku can't; Sudoku finishes the squares the cages don't reach.
So the whole loop is: clear the single-square cages, subtract the last gap out of every cage you can, read the tight sums for "it must be one of these," and lean on ordinary scanning between times. Never a guess — every square has a reason.
THAT'S IT — GO DO ONE
Find a cage with one empty square, subtract, repeat — and let plain Sudoku mop up the rest. The first time a big cage falls to a single tidy subtraction it feels a bit like cheating, in the best way. There's a fresh one waiting below — no words, no ads in the middle of your puzzle, and a ghost who's quietly chuffed when you finish.