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How to play Killer Sudoku

Killer sudoku is sudoku with addition. If you can finish a classic puzzle and you can add single digits, you can start. The grid does not give you many numbers. It gives you groups.

1. Draw the grid in your head

A killer board is a normal 9×9 sudoku grid. Rows, columns, and the nine 3×3 boxes each contain the digits 1 to 9 exactly once.

2. Read the cages

Dashed lines group cells into cages. The small number at the top-left of a cage is the sum of its digits. Those digits are all different.

3. Use sums before guesses

Some sums can be made in only one way. Others become unique after a single digit is placed nearby. The 45 rule — each house totals 45 — finds cells the cages do not cover cleanly.

A legal cage

Every cell belongs to exactly one cage. The cage is an orthogonally connected shape: you can walk from any cell in it to any other without jumping a gap, moving only up, down, left, or right. Diagonal touches do not count as connection. Digits inside the cage do not repeat, which is why 2+2 is never the way to make 4. The sum in the corner equals the digits in the solution, and that sum is always possible for the size of the cage. A two-cell cage cannot sum to 1 or to 18.

What you are allowed to do

You may pencil every candidate a cell can still hold. You may erase. You may leave a cell blank until you know it. You may not place a digit that repeats in a row, column, box, or cage, and you may not invent a digit to “see what happens” if your aim is the solution we rated. Every published puzzle can be finished by the techniques on the strategy pages, in an order that starts with the simplest step available.

A first puzzle

Open an easy board. Mark the unique cages using the combination table. Fill any cage that is down to one empty cell. Then scan for a digit that fits in only one cell of a box. That loop is the entire beginner game. When it stops being enough, the 45 rule is the next page, not a harder puzzle.