Killer Sudoku Rules: How to Play (With a Printable Example)

The rules of Killer Sudoku are the rules of Sudoku plus one idea: groups of cells called cages, each with a total. If you can play Sudoku, you already know two thirds of the game. This page lists the rules exactly as LogikLand's generator applies them, then checks them on a real, finished grid so you can see each rule at work.

The goal

Fill a 9 x 9 grid with the digits 1 to 9. No digit is given at the start: you find all of them from the sums written in the cages. Every grid made by LogikLand has exactly one solution.

The three rules

1. Each row contains the digits 1 to 9 exactly once.
2. Each column contains the digits 1 to 9 exactly once, and so does each of the nine 3 x 3 boxes.
3. The grid is divided into cages, drawn with dotted lines. The small number in the corner of a cage is the sum of its digits, and a digit cannot appear twice inside the same cage.

Rule 3 is the only new one. It also explains why Killer Sudoku needs no starting digits: a cage with only one cell shows its digit directly, and the other cages give you enough sums to build the grid.

Reading a cage

In the printed grid, a dotted line is drawn just inside the cells that belong to a cage, so that the thick lines of the boxes stay easy to see. The sum is written in the top left corner of the first cell of the cage, which is the cell highest in the grid and, when two are level, the one furthest to the left.

A cage of three cells with a sum of 21 can only be made of three different digits. There are three ways: 4 + 8 + 9, 5 + 7 + 9 or 6 + 7 + 8. Writing 7 + 7 + 7 is not allowed, even though it adds up to 21, because a digit cannot repeat in a cage. This is why two-cell and three-cell cages are so useful: the rule removes many possibilities at once.

Checking a finished grid

The grid below is an easy puzzle from the LogikLand generator, with its solution. Use it to check each rule: look at any cage in the solution and add its digits.

Three cages, three lessons

• A cage across two boxes. The cage in the top row with a sum of 14 covers R1C6, R1C7 and R2C7 (row, then column). R1C6 is in the top middle box and the other two are in the top right box. In the solution the digits are 1, 4 and 9: 1 + 4 + 9 = 14, all different.
• A bent cage. The cage with a sum of 11 covers R3C7, R3C8 and R4C7, an L shape. The solution has 1, 2 and 8, which add up to 11.
• A cage of one cell. The cage at R1C5 shows 9: the cell is a 9, and the solution agrees.

There is also a check on the whole grid. Each row, column and box adds up to 45, so the 81 cells add up to 405. In this example the 35 cage sums also add up to exactly 405, because the cages cover every cell once. If the sums of a grid you are given do not add up to 405, the grid is wrong.

The four levels

The Killer Sudoku assistant has four levels. They change the size and number of cages, not the rules.

Rules people mix up

• A cage is not a box. The thick lines show the nine boxes, the dotted lines show the cages, and both rules apply together.
• The sum does not tell you the order of the digits, only which digits are inside.
• Digits are not forbidden in neighbouring cages. Two touching cages may both contain a 5, if they are not in the same row, column or box.

Now try it: how to solve Killer Sudoku walks through the first moves of a grid. Books are made in the Killer Sudoku assistant after you create a free account; the Sudoku book generator page covers classic Sudoku. For the other variants in the family, see the rules of Sudoku variants.

Questions