Killer sudoku

How to play killer sudoku

You don’t get any digits to start with. Everything comes from the cages, so your first job is to find a cage that can only be made one way.

Take the two cages below. Two different digits that add up to 3 can only be 1 and 2. Two that add up to 4 can only be 1 and 3, because 2 + 2 would repeat a digit. The 3 cage already holds the row’s 1, so the top cell of the 4 cage can’t be 1. It’s 3, and the cell under it is 1.

From there it’s like a classic sudoku: each digit you place rules that digit out of its row, column and box.

Beginner techniques

Cages with one combination

Some totals can only be made one way. A 2-cell cage of 17 is always 8 and 9, and a 3-cell cage of 6 is always 1, 2 and 3. Pencil those digits in first. You won’t know the order yet, but you know which digits that cage uses up.

These are the ones worth knowing by heart on a 9×9 grid:

Cells	Small total	Its digits	Big total	Its digits
2	3	1 + 2	16	7 + 9
2	4	1 + 3	17	8 + 9
3	6	1 + 2 + 3	23	6 + 8 + 9
3	7	1 + 2 + 4	24	7 + 8 + 9
4	10	1 + 2 + 3 + 4	29	5 + 7 + 8 + 9
4	11	1 + 2 + 3 + 5	30	6 + 7 + 8 + 9
5	15	1 + 2 + 3 + 4 + 5	35	5 + 6 + 7 + 8 + 9

On the 6×6 grid the small totals work the same way. The big ones change, because the highest digit is 6: 10 in two cells is 4 + 6, and 11 is 5 + 6.

Use the 45 rule

The digits 1 to 9 add up to 45, so every row, column and 3×3 box adds up to 45. On the 6×6 grid it’s 21. When the cages inside a box cover every cell but one, add up their totals and take that from 45. What’s left is the missing cell. It works on rows and columns too.

Let cages rule out digits

Once a cage owns some digits, nothing else in its box can use them. The same goes for a row or column that holds the whole cage. That often cuts a cage with several combinations down to one, so check the cages next to each one you pencil in.

For the rule of 45 and the cage combinations worth knowing, read how to solve killer sudoku.

Difficulty and grid sizes

Every Owlpuzzles killer sudoku has exactly one solution. The week starts small and gets harder:

Days	Level	Grid	Cages
Monday, Tuesday	Easy	6×6, digits 1 to 6, boxes 3 wide and 2 tall	Mostly 2 cells, with the odd single cell
Wednesday, Thursday	Medium	9×9	Mostly 2 to 4 cells
Friday to Sunday	Hard	9×9	Up to 5 cells

Bigger cages have more ways to make their total, so on hard days you’ll use the 45 rule a lot more. Easy grids fall to cage totals and singles. Midweek grids need a step further, like the 45 rule. Hard grids need harder steps, like the 45 rule across several rows.

Where killer sudoku comes from

Killer sudoku started in Japan, where it’s called samunamupure, a Japanese rendering of “sum number place”. (Number Place was sudoku’s original name.) The Times in London was the first paper outside Japan to print it, in 2005, under the name Killer Su Doku. You’ll also see it called sumdoku, sum sudoku or addoku. Its cages work like the clues in kakuro, the cross-sum puzzle, so it plays like a mix of sudoku and kakuro.

Killer sudoku questions

What are the rules for killer sudoku?

Fill every row, column and box with each digit once. The digits in a cage add up to the small total in its corner, and no digit repeats inside a cage.

How difficult is killer sudoku?

It’s harder to start than a classic sudoku, because there are no digits to begin with. The cages with one combination and the 45 rule give you a way in, and the 6×6 grids on Mondays and Tuesdays are a good place to practise them.

What’s the trick to killer sudoku?

Two things do most of the work: cages that can only be made one way, and the 45 rule. Start with those, then use each digit you place to cut down the cages around it.

Can numbers repeat in a killer sudoku cage?

No. Early puzzles didn’t always say so, but no repeats in a cage is the standard rule now, and Owlpuzzles follows it.

Is there a new killer sudoku every day?

Yes. You can play a new one free online every day: easy on Mondays and Tuesdays, medium on Wednesdays and Thursdays, and hard from Friday to Sunday.

Puzzles like killer sudoku

If you like the counting, try shikaku, where every number is the area of a rectangle. For logic with no adding up at all, try star battle or a nonogram.