Play Killer Sudoku Online Free: Rules, History & Strategy

Take the elegant logic of classic Sudoku, sprinkle in dotted-outline “cages” with target sums, and remove almost every starting number. That is Killer Sudoku — a variant so cunning that newspapers reached for the word “killer” to describe it. It looks intimidating, but a single trick called the 45 rule cracks it wide open.

The history of Killer Sudoku

Killer Sudoku is what you get when you marry standard Sudoku to Kakuro, the number-crossword puzzle. The variant was already established in Japan by the mid-1990s, where it went by “Samunamupure” (サムナンプレ) — a Nipponised rendering of the English phrase “sum number place” (source).

The wider English-speaking world met it on August 31, 2005, when The Times of London began publishing it and coined the now-familiar name “Killer Sudoku.” The timing was perfect: it rode the global Sudoku boom of 2004–2005, when puzzles syndicated by the Japanese publisher Nikoli were spreading worldwide, and it quickly became a fixture of newspaper puzzle pages and dedicated puzzle books (source).

Mechanically, the leap from Sudoku is small but powerful. The grid is carved into cages — groups of cells marked with a dotted outline and a small target sum. The digits inside a cage must add to that total and may not repeat within the cage, all on top of the usual row, column, and 3×3 box rules. That single addition constraint is what lets setters strip away the givens and still guarantee a unique solution (source).

How to play Killer Sudoku

The rules build directly on Sudoku, and you can open a fresh grid on our free Killer Sudoku board in seconds.

1. Know the base rule. Like classic Sudoku, every row, every column, and each 3×3 box must contain the digits 1 to 9 exactly once.
2. Spot the cages. Dotted outlines group cells into cages. The small number in a cage’s corner is the target sum for all its cells combined.
3. Add to the target. The digits you place in a cage must add up exactly to that target sum.
4. No repeats in a cage. A digit may not appear twice inside the same cage — even if the cage spans more than one box.
5. Start with locked cages. Find cages whose sum and size allow only one combination, such as a two-cell cage of 3 (only 1+2) or 17 (only 8+9), and pencil in those candidates.
6. Use the 45 rule. Each house (row, column, or box) totals 45. Add the cage sums sitting inside a house and the leftover reveals a hidden cell’s value.
7. Finish and check. The puzzle is solved when all 81 cells are filled, every house holds 1–9 once, and every cage matches its sum.

What is the 45 rule in Killer Sudoku?

The 45 rule says that because every row, column, and 3×3 box contains the digits 1–9, each one always sums to exactly 45 — so by adding the cage totals inside a house you can deduce any leftover cell.

Strategy & tips

Killer Sudoku rewards arithmetic bookkeeping as much as logic. These tactics turn a blank-looking grid into a chain of deductions:

• Hunt locked combinations first: Sums with only one possible set of digits give you certain candidates immediately.
• Work the 45 rule on “innies” and “outies”: When cages spill just one cell outside a house, the difference from 45 pins that cell down.
• Track cage sums, not just digits: Note which numbers a cage can still legally hold as you eliminate options.
• Combine houses: Two stacked boxes total 90, three total 135 — handy when a cage straddles a boundary.
• Fall back on plain Sudoku logic: Once a few digits land, naked and hidden singles finish much of the grid.

Simon Anthony, co-founder of the Cracking The Cryptic YouTube channel and a former member of the UK’s World Sudoku and World Puzzle Championship teams, calls the 45 rule the foundation of the whole puzzle: since every house sums to 45, adding up the cage totals that fall inside it and comparing to 45 lets you deduce leftover “innie” and “outie” cells before you place a single digit (source). His co-founder Mark Goodliffe — a 14-time winner of The Times Crossword Championship and a UK Sudoku champion — advises starting with cages that have the fewest digit combinations, such as a two-cell cage summing to 3 (only 1+2) or 17 (only 8+9), because those locked combinations give certain candidates that propagate quickly through intersecting houses (source).

If this blend of logic and arithmetic clicks, try our sibling grids: the number-crossword Kakuro that inspired the cages, the operation-driven Calcudoku, or the diagonal twist of X-Sudoku.

Is Killer Sudoku harder than regular Sudoku?

Generally yes — Killer Sudoku usually starts with no given numbers and adds the cage-sum constraint, but that same arithmetic gives you extra footholds, so once you learn the 45 rule it can feel just as approachable.

Watch a video tutorial

Seeing the cages and the 45 rule in action makes everything click. A perfect starting point is “Killer Sudoku Made Easy: An expert solver explains” by Cracking The Cryptic, the most respected Sudoku channel on YouTube:

Want more angles? Cracking The Cryptic’s “Killer Sudoku Tutorial with Beginner Strategies” walks through gentler tactics, and puzzling.com’s “How to play Killer Sudoku puzzles” is a quick, clear primer on the rules.

Ready to solve?

The fastest way to learn Killer Sudoku is to find one locked cage and watch the 45 rule do the rest. Jump into a fresh grid and play Killer Sudoku free online — no download, no signup, just you and the cages. Hungry for more afterward? Test yourself with Nonograms or our rotating daily puzzles, and see how far your deduction can take you.