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Solving Tips for Killer Sudoku

Killer sudoku is one of the most popular sudoku variants. First, let's look at the rules of the standard 9x9 killer sudoku puzzle:

Rules of Killer Sudoku

  • Place the numbers 1 - 9 once in each region
  • The regions are the 9 columns, 9 rows and 9 3x3 bold-lined boxes
  • Ensure that the numbers you place in each cage (dotted-lined region) sum up to the number given at the top-left of that cage
And that's it for the rules. As you'll notice, there is one addition to standard sudoku: the caged regions, and the fact that the numbers are actually used as numbers by that additional rule, as opposed to standard sudoku where the numbers could be replaced by any symbol.

Killer Sudoku Solving Strategy

So, those are the rules. How do you go about solving killer sudoku? There is no doubt the grid can look quite intimidating at first, particularly as typically there will be no givens in the grid to get you started. There are two main sets of strategies: those that are generic to sudoku and those that are specific to killer sudoku.

Sudoku rules are those you will already be familiar with from standard sudoku: looking to see if there is only one place in a region where a value can go, looking at the possible values for each cell, looking to use region intersection rules and also looking at sets.

Since sudoku rules are already extremely well-documented and you will no doubt be familiar with them, we will instead look at the killer sudoku rules.

Killer sudoku solving rules start with the fact that you will have to get familiar with combinations, and those that sum to certain numbers. Since you won't want to go through the entire grid at the start and work out all possible candidates as that is dull and time consuming, it can help to look for either high or low value combinations for each length of cage. It will help you greatly to start to learn the most popular combinations so that you can write them straight in without working them out. For instance, if you have a cage of 24 from three squares, then you know those must contain the 7,8,9 in some order. If you have 10 from four squares, then those must contain 1,2,3,4 in some order.

If those squares are all inside a larger region, then of course you can eliminate them as possibilities from the other squares in the region. The interplay between known ranges plays a great part in whittling down the options and solving killer sudoku puzzles.

In fact, with many puzzles all you will need to solve them is a combination of the above rule - which will be familiar to anyone who has solved kakuro - and the sudoku rules.

There are some tougher killer sudoku strategies too. One of these is called the rule of 45, and is useful with harder killer sudoku puzzles. It is called the rule of 45 because the numbers from 1 - 9 sum to 45, and it reminds you that the total of each region - row, column and box - must be 45 in killer sudoku. We can use this fact to make deductions about possible values of squares in killer sudoku puzzles.

To find out how this works, let's look at an example: 45 rule example Here we have zoomed in to look at two box regions in a 9x9 killer sudoku puzzle that is completely unstarted. However, we can instantly place a number using the rule of 45. Look at the top left box. You know that the numbers there must sum up to 45. Now, we have three cages fully inside the box. They are the 24, 4 and 7. So we know those seven cells must sum to 24 + 4 + 7 = 35. So the other two cells must sum to 10. But look! Those two cells that sum to 10 are part of a region of 3 squares that sum to 18. This means that the square with the '18' in on the image must contain 8, since 18 - 10 = 8.

And there you go, using the rule of 45 you've instantly placed a number in the puzzle, how easy was that? If you get stuck when solving killer sudoku, or even right at the start, look for instances where you can apply the rule of 45 to make progress or possibly just narrow down the possible values for squares.

Good luck with solving killer sudoku - if you have any other solving tips, then we'd love to hear them so do feel free to contact us so we can add them here!

Killer Sudoku Book

Like to practice these strategies? Try our book of killer sudoku containing 100 fantastic puzzles! We also have Large Print Killer Sudoku available for those who prefer a larger print size.

Buy Book of Killer Sudoku from Amazon (UK) for £4.50

Buy Book of Killer Sudoku from Amazon (US) for $6.70


How to solve Hanjie Puzzles

In our new series of videos, you can learn about new puzzle types and take part in quizzes. Why not have a watch?

A fun logic puzzle, hanjies result in a simple piece of grid art being created. If you're new to this puzzle type but are intrigued and would like to give it a go, this video has a walkthrough of how to solve a simple hanjie puzzle to start you on your hanjie-solving adventures...




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