THE SHORT VERSION
Killer Sudoku is a Sudoku with sum cages laid on top. All Sudoku rules apply (rows, columns, AND 3×3 boxes) plus dashed cages whose digits must add to a target.
Calcudoku (also called KenKen) is a Latin square — rows + columns only, NO 3×3 box rule — divided into cages with any arithmetic operator (+, −, ×, ÷).
That single difference about the 3×3 box rule changes almost everything about how the puzzles feel.
SIDE BY SIDE
Grid size
Killer Sudoku: always 9×9
Calcudoku: 4×4 to 9×9 (often 6×6 for newspapers)
Latin square base
Killer Sudoku: rows, columns, AND 3×3 boxes
Calcudoku: rows and columns only
Cage operators
Killer Sudoku: sums only
Calcudoku: any of +, −, ×, ÷
No-repeat within cage
Killer Sudoku: yes (digits in a cage can't repeat)
Calcudoku: depends on grid size — repeats allowed when they don't violate the Latin-square rule
Starting clues
Killer Sudoku: usually none (the cages do all the seeding)
Calcudoku: usually none
WHY THE 3×3 BOX RULE CHANGES EVERYTHING
A Killer Sudoku is solvable with magic-sum thinking PLUS standard Sudoku scanning. Two-cell cages with target 3 must be {1, 2} — and then the 3×3 box those cells live in is also constrained.
A Calcudoku has no box. Every deduction lives entirely on row + column axes. Scanning has fewer dimensions, which sounds easier but actually means you rely much more on the arithmetic itself.
Calcudoku also lets cells repeat in non-Latin-square dimensions — a 3-cell L-shaped cage can have 2, 2, 4 if the two 2s are in different rows AND different columns. Killer Sudoku doesn't allow that.
OPERATOR FAMILY
Calcudoku's × and ÷ operators are where it gets interesting. A 2-cell cage with target 12 and × operator could be {2, 6}, {3, 4}, or {6, 2} — and the cell-ordering matters because you have to honour the Latin-square constraint.
Killer Sudoku's sum-only constraint is simpler (sums commute) but the no-repeat-in-cage rule plus the 3×3 box compensates by stacking constraints.
WHEN TO PICK WHICH
Pick Killer Sudoku if you already enjoy Sudoku and want arithmetic flavour on top. Your existing Sudoku scanning carries over directly.
Pick Calcudoku if you want pure arithmetic deduction without the 3×3 box layer. It's also the better entry point at smaller grid sizes — a 4×4 Calcudoku is genuinely friendly for kids learning times tables.
Solve both if you're building mental-arithmetic fluency. Killer Sudoku trains addition combinations; Calcudoku trains multiplication and division pattern recognition. The two together cover the whole operator space.
STRATEGY THAT TRANSFERS
Magic-sum thinking from Kakuro and Killer Sudoku transfers to Calcudoku's + and − cages. Latin-square scanning from Sudoku transfers to both. The skill that doesn't transfer is 3×3-box pattern recognition — you have to consciously turn it OFF when solving Calcudoku, because it'll make you miss the row + column axes that actually constrain the grid.