Killer Sudoku

How Killer Sudoku Works

Killer Sudoku keeps every rule of classic Sudoku — each row, each column and each 3x3 box holds the digits 1 through 9 exactly once — and adds one more: the grid is divided into dashed groups called cages, and the digits inside each cage must add up to the small number printed in its corner without repeating within that cage. Most Killer puzzles start with few or no given digits, because the cage sums carry the information the givens would otherwise supply.

The controls are the same as classic Sudoku. Select a cell, type a digit, and use Notes to track candidates. Auto Notes, Hints and Undo all work the same way. The interface highlights which cells belong to the same cage so you can see the sum you are working toward.

The key extra tool is the Rule of 45: because every row, column and box totals exactly 45 (the sum 1 + 2 + ... + 9), you can calculate missing cells when a cage overlaps a unit boundary. If a single row contains complete cages adding to 41, the remaining cells in that row must total 4. Combination tables are also helpful — for example, a two-cell cage summing to 4 can only hold {1, 3} since digits cannot repeat within a cage. Start with the smallest cages and the cages whose sum has only one possible combination, then work outward. Auto-Check, the three-mistake limit and browser-saved progress all behave the same as in classic Sudoku.