Killer Sudoku Online: Master the Art of Sums and Logic

Killer Sudoku is a number puzzle that combines the deductive logic of classic Sudoku with the arithmetic of Kakuro. You fill a 9×9 grid so every row, column, and 3×3 box contains the digits 1 through 9 exactly once — but with one twist: the grid is also divided into dashed-line groups called cages, and the digits inside each cage must add up to the small target number printed in its corner. No digit may repeat inside a single cage. Play unlimited Killer Sudoku puzzles for free on SudokuPro, with no registration required, across every difficulty from Easy to Evil.

Characteristics of Killer Sudoku

Killer Sudoku — sometimes called Sumdoku, Sum Sudoku, or Samunamupure — keeps the familiar 9×9 board divided into nine 3×3 regions, but it usually starts with very few (or zero) given numbers. Instead of clue digits, the solver gets cages: irregular dashed-line groups of one or more cells, each labeled with a target sum in the top-left corner.

The defining attributes of every Killer Sudoku puzzle are:

Cages are non-overlapping groups of orthogonally connected cells outlined by a dashed border.
The target sum in each cage's corner equals the total of the digits placed inside it.
No digit repeats inside a cage, even when the standard Sudoku rules would otherwise allow it.
Standard Sudoku constraints still apply: each digit 1–9 appears exactly once per row, column, and 3×3 box.

These extra arithmetic constraints make logic flow differently than in classic Sudoku. Instead of scanning rows and columns first, solvers begin by analyzing cage sums and the possible digit combinations that produce them.

Solving Strategies for Killer Sudoku

Killer Sudoku rewards a hybrid of arithmetic reasoning and traditional Sudoku scanning. These three techniques form the foundation of every level on SudokuPro:

The Rule of 45 — Every row, column, and 3×3 box contains the digits 1 through 9, so their sum is always 45. If a row is covered almost entirely by cages totaling 41, the leftover cell in that row must contain a 4. This single principle unlocks more deductions than any other technique.
Unique Sum Combinations — Many small cages have only one possible digit set. A 2-cell cage with sum 3 must be {1, 2}; a 2-cell cage with sum 17 must be {8, 9}; a 3-cell cage with sum 6 must be {1, 2, 3}. Memorizing these "killer pairs and triples" turns cages into near-given clues.
Cage Elimination — Once you know the candidate set for a cage, you can eliminate those digits from other cells in the same row, column, or box. This is the same logic as Sudoku's naked pair, applied through arithmetic rather than direct clues.

Next Steps and Progression

If you are new to the puzzle, start at the Easy Killer Sudoku section to get comfortable with cage notation and the Rule of 45 before tackling tougher boards. As your speed grows you can climb through Medium, Hard, and Expert, and eventually try the Extreme and Evil levels where overlapping logic and large cages dominate. For a refresher on the underlying mechanics of classic Sudoku, our how-to-play guide walks through every rule step by step, and you can return to the SudokuPro homepage to explore other variants any time.