What killer sudoku is
Killer sudoku is a variant of the classic number puzzle that layers a second challenge on top of the usual grid. You still fill a nine-by-nine board with the digits 1 to 9, and you still obey the three classic rules — no repeated digit in any row, column, or three-by-three box. What changes is that the puzzle gives you almost no starting numbers. Instead it draws dotted outlines called cages across the board, and each cage carries a small number in its top-left corner: the target sum of every digit inside it. To crack the puzzle you have to reason from arithmetic as much as from position.
That single addition turns sudoku from a game of pure placement into a game of placement plus sums. It is the variant that Sudoku by WizusLabs ships alongside the classic mode, and it is the one most people find addictive once the arithmetic starts to click.
How killer sudoku differs from classic sudoku
A classic sudoku hands you a scatter of given digits and asks you to complete the rest. A killer sudoku usually gives you no givens at all. Every piece of information is encoded in the cages and their sums, so the puzzle looks intimidatingly blank at first glance. The trade is fair: the cages tell you far more than they seem to, because a target sum sharply limits which digits can live in a group of cells.
If you already know classic technique, none of it goes to waste. Naked singles, hidden singles, pointing pairs, and the rest all still apply. Killer sudoku simply gives you a second, parallel source of deductions — one built from addition — that you interleave with the classic logic. If you are new to the placement side, read the how to play sudoku guide first, then come back here for the arithmetic layer.
The rules in full
The three classic constraints
Every row contains the digits 1 to 9 exactly once. Every column contains 1 to 9 exactly once. Every three-by-three box contains 1 to 9 exactly once. Nothing about killer sudoku relaxes these; they are the floor you build on.
The cage rule
The board is partitioned into cages — groups of one or more orthogonally connected cells marked by a dotted border. The small number printed in a cage's top-left corner is the sum its digits must add up to. A two-cell cage summing to 4, for example, must hold two digits that total 4. Cages come in all shapes; they can bend, straddle box lines, and span rows and columns.
No repeats inside a cage
This is the rule newcomers forget most often. A digit may not repeat within a single cage, even when the classic row, column, and box rules would otherwise allow it. So a two-cell cage summing to 4 cannot be 2 + 2; it must be 1 + 3. That no-repeat constraint is what makes cage sums so powerful, because it collapses many sums into a small set of possible digit combinations.
Core strategies
Cage sum combinations
The single most valuable habit in killer sudoku is recognising which digits a cage can hold. Because digits never repeat in a cage, many small cages have only one possible combination. A three-cell cage summing to 6 can only be 1 + 2 + 3. A two-cell cage summing to 3 can only be 1 + 2. A two-cell cage summing to 17 can only be 8 + 9. These locked combinations are gold: you may not know the order yet, but you know exactly which three or two digits occupy those cells, which immediately rules those digits out of the rest of the row, column, or box.
A few worth memorising for two-cell cages: 3 is {1,2}; 4 is {1,3}; 16 is {7,9}; 17 is {8,9}. For three-cell cages: 6 is {1,2,3}; 7 is {1,2,4}; 23 is {6,8,9}; 24 is {7,8,9}. The extreme sums, high and low, are always the most constrained, so scan for them first.
The rule of 45
Every row, every column, and every three-by-three box holds the digits 1 through 9, which add up to 45. This is the backbone technique of killer sudoku. If a region is covered almost entirely by cages whose sums you know, you can subtract those sums from 45 to find the value of the leftover cell or cells. Suppose a box is covered by cages totalling 38, with one cell sticking out into another region: that stray cell must be 45 − 38 = 7. The rule of 45 turns known cage sums into the value of the cells they do not fully cover.
Innies and outies
Innies and outies extend the rule of 45 across region boundaries. An "innie" is a single cell inside a region that a cage pokes into from outside; an "outie" is a single cell of a region that a cage carries beyond its edge. By adding the sums of the cages that lie mostly inside a region and comparing the total to 45, you can pin down the value of that one intruding or escaping cell. Applied to two adjacent boxes or a pair of rows at once — treating them as a block worth 90, or three as 135 — this technique routinely cracks a puzzle open when single-region logic stalls.
Pencil-marking for cages
Pencil marks — small candidate digits noted in a cell's corner — matter even more in killer sudoku than in the classic game. Write the possible combinations for each cage as candidates, then prune them with the classic constraints. If a cage summing to 5 across two cells is {1,4} or {2,3}, and the 4 is already used elsewhere in that row, the cage collapses to {2,3}. Good pencil-marking discipline lets you carry many partial deductions at once and see where two constraints intersect. In Sudoku by WizusLabs, candidate notes are built in, so you can mark up cages without keeping a mental ledger.
A worked example
Picture the top-left three-by-three box. A two-cell cage in its top row is labelled 17, and a three-cell L-shaped cage below it is labelled 6. Start with the combinations. The 17 cage must be {8,9}, because that is the only pair of distinct digits summing to 17. The 6 cage must be {1,2,3}, the only triple of distinct digits summing to 6. So five of the nine cells in this box are already committed to the digit sets {8,9} and {1,2,3} — you do not yet know their order, but you know they are spoken for.
Now apply the rule of 45. The box must total 45. The 17 cage and the 6 cage account for 23 of that, leaving 22 spread across the four remaining cells, and those four cells can only hold digits from {4,5,6,7} — because 1, 2, 3, 8, and 9 are already claimed by the two cages. The digits 4 + 5 + 6 + 7 sum to exactly 22, which confirms the arithmetic and tells you the last four cells are precisely {4,5,6,7} in some order. With that, the whole box is reduced to three tidy digit sets, and a single crossing constraint from a neighbouring row — say a 9 already placed in the first column — is enough to start fixing exact positions. That is killer sudoku in miniature: combinations narrow the candidates, the rule of 45 confirms them, and classic logic lands the final digits.
Where to play
Sudoku by WizusLabs ships both classic and killer puzzles, generates them on device, and works offline with progress saved locally. The daily challenge is free for everyone, candidate notes are built in, and every generated puzzle has exactly one solution — which means the deductions above will always resolve without guessing.
Keep reading: classic sudoku techniques · free offline games (no account needed)