THE SHORT VERSION
Sudoku: a 9×9 grid with some digits given. Fill the rest so that every row, column, and 3×3 box contains each digit 1–9 exactly once.
Killer Sudoku: the same 9×9 grid, the same three rules — but with no starting digits. Instead, the cells are grouped into dashed cages, each labelled with a target sum. The digits in each cage must add up to that target, and no digit can repeat within a cage.
If you can solve Sudoku, you have every tool Killer Sudoku needs. You just need to learn how to use the cage arithmetic to create your starting digits.
SIDE BY SIDE
Grid size
Sudoku: 9×9
Killer Sudoku: 9×9 (always)
Starting clues
Sudoku: some cells pre-filled with digits
Killer Sudoku: none — cage sums replace all starting digits
Row/column/box rules
Sudoku: rows + columns + 3×3 boxes
Killer Sudoku: rows + columns + 3×3 boxes (identical)
Extra constraint
Sudoku: none
Killer Sudoku: cage sums + no repeats within a cage
Operators
Sudoku: none
Killer Sudoku: addition only
WHAT STAYS THE SAME
Everything about the Sudoku grid is identical. Rows, columns, and 3×3 boxes all still require 1–9 exactly once. Every Sudoku technique — naked singles, hidden singles, pointing pairs, naked pairs — works exactly the same way.
The 9×9 structure means the grid sums to exactly 405 (nine rows of 1+2+…+9 = 45). This gives Killer Sudoku its most powerful tool: the 45-rule.
WHAT CHANGES — THE CAGE LAYER
The cage system replaces the starting-digit clues. Instead of a few cells pre-solved, you get arithmetic clues that constrain whole groups of cells at once.
A 2-cell cage summing to 3 can only contain {1, 2}. A 3-cell cage summing to 6 can only contain {1, 2, 3}. These unique-sum cages are Killer's equivalent of given digits — they lock the candidate set before you've done any scanning.
The 45-rule is new and powerful: any complete row, column, or box sums to 45. If you know all but one cell in a row, you can calculate the last cell directly — no guessing.
The extra constraint makes some Killer puzzles easier to start than their Sudoku equivalent. An advanced Sudoku with few given digits can be brutal; a well-constructed Killer of the same rating opens immediately from unique-sum cages.
WHICH IS HARDER?
It depends on where you are. For a complete beginner, Killer Sudoku looks harder because the starting position is an empty grid covered in dashed cages. For an experienced Sudoku solver, Killer is often easier at the equivalent difficulty tier — the cage arithmetic gives more logical footholds than a few scattered given digits.
The learning curve is different, not steeper. You need to learn unique-sum cages and the 45-rule before Killer's power becomes apparent. That's a few hours of practice, not a separate skill system.
STRATEGY THAT TRANSFERS
All standard Sudoku technique transfers directly: naked singles, hidden singles, pointing pairs, naked pairs, and everything harder. You apply them in exactly the same way once you have candidates.
The extra skill in Killer is generating those candidates from cage arithmetic rather than from a set of given digits. Once you have a working set of candidates, Killer Sudoku is Sudoku.
The reverse also holds: experienced Killer solvers often become better Sudoku solvers because arithmetic deduction makes candidate lists tighter and faster to work with.
WHEN TO MAKE THE SWITCH
If you play Sudoku: try Killer once you can reliably solve medium-difficulty puzzles using hidden singles and pointing pairs. You have enough technique that the cage layer becomes a fun extension, not a wall.
If you play Killer Sudoku: try regular Sudoku when you want a break from arithmetic. The pure logic of a well-constructed hard Sudoku is a completely different puzzle experience despite using the same grid.
Playing both regularly builds a wider technique library. The 45-rule and unique-sum thinking improve your candidate-management in standard Sudoku; standard Sudoku scanning clears Killer endgames faster.