Killer Sudoku FAQ

Despite the fearsome name, Easy Killer Sudoku can actually be easier than regular Sudoku - Small cages with only one valid combination give you free digit information that classic Sudoku clues don't. At the harder end, though, Killer Sudoku removes all given digits entirely. With only cage sums to go on, Expert Killer is significantly harder than Expert classic. The difficulty scales sharply with cage complexity and the absence of givens.

The 45 rule is the single most important Killer Sudoku technique. Every row, column, and 3×3 box must contain digits 1-9, which sum to exactly 45. Add up all cage sums within a unit and subtract from 45. Any difference equals the sum of cells not fully contained in that unit - These are your innies and outies. If there's only one such cell, you know its exact value immediately.

An innie is a cell that sits inside a row, column, or box but belongs to a cage that extends outside it. An outie is the reverse - A cell outside the unit whose cage crosses into it. Using the 45 rule, you can calculate the exact digit in a single-cell innie or outie without knowing any surrounding values. In harder puzzles, innies/outies with 2-3 cells give you a restricted digit sum, which narrows candidates dramatically.

Forced cage combinations are the quickest wins in Killer Sudoku:

2-cell cages: Sum 3 → {1,2}  |  Sum 4 → {1,3}  |  Sum 16 → {7,9}  |  Sum 17 → {8,9}
3-cell cages: Sum 6 → {1,2,3}  |  Sum 7 → {1,2,4}  |  Sum 23 → {6,8,9}  |  Sum 24 → {7,8,9}
4-cell cages: Sum 10 → {1,2,3,4}

Spot these first - They give you free placements that cascade through the rest of the puzzle.

Only basic addition. You add cage sums (always under 45) and occasionally subtract from 45 for the 45 rule. There is no multiplication, fractions, or advanced calculation involved. The main mental skill is remembering which digit combinations are possible for a cage of a given size and total - Something that becomes second nature with practice.

No. Just as digits cannot repeat within a row, column, or box, digits cannot repeat within a single cage. A 3-cell cage summing to 9 cannot use {3,3,3} - Each digit must be unique. This uniqueness constraint is what makes cage arithmetic so restrictive and why forced combinations are possible.

1. Identify all forced cages (one possible combination) and note their digits.
2. Apply the 45 rule to every row, column, and box to find single-cell innies/outies - Free digits.
3. Use cage overlap with rows/columns/boxes to eliminate candidates.
4. Switch to standard Sudoku techniques (naked singles, hidden singles, pairs) to fill remaining cells.

In classic Sudoku, some cells have pre-filled digits. In Killer Sudoku, no cells have given digits - Instead, groups of cells (cages) are given a target sum. The standard Sudoku rules (1-9 in every row, column, and box) still apply. Killer adds the constraint that each cage's digits must sum to its target, with no repeats within a cage.

Yes. Our online Killer Sudoku includes a notes mode, hint system, and undo button. The notes mode is especially valuable in Killer because tracking cage candidates is complex. You can also use pencil marks in the interface the same way you would on paper.