What is the 45 rule in Sudoku?

The 45 rule is essential to understand in order to complete sudoku puzzles.

The rule of 45 is based on the rules of Sudoku which state that each row, column and nonet can only use the numbers from 1 to 9 once. Since each number can only be used once, once a row, column or nonet is filled in all of the numbers will add to a total of 45.

This rule is especially useful in Killer Sudoku where the digit sums are important in determining the solution.

Let’s take a look at an example to further understand the rule of 45.

We can use the 45 rule to check if a sudoku problem has been completed correctly. Take a look at each row, column and nonet. All of these should sum to 45 if completed correctly. If the value is higher or lower, that means that the puzzle was filled out incorrectly and the row, column or nonet needs to be corrected.