What Is Killer Sudoku
Killer Sudoku combines standard Sudoku rules with the sum-based logic of Kakuro. The grid is 9x9 and the standard rules apply: no repeating digits in any row, column, or 3x3 box. But instead of given numbers, the grid has dotted cages. Each cage shows a target sum. The digits in each cage must add up to that sum and cannot repeat within the cage.
The Key Difference
Standard Sudoku gives you some filled-in cells. Killer Sudoku gives you no filled cells. Every cell starts blank. Your only clues are the cage sums. This makes the opening moves different. You start by figuring out which combinations of digits can produce each sum.
Essential Combinations
Memorize the forced combinations. A two-cell cage with a sum of 3 must be 1 and 2. A two-cell cage summing to 17 must be 8 and 9. A three-cell cage summing to 6 must be 1, 2, and 3. Knowing these by heart speeds up your solving significantly.
Strategy: Start With Forced Cages
Look for cages that have only one possible combination. A two-cell cage summing to 3 or 4 has very few options. These give you starting candidates. From there, apply standard Sudoku elimination to narrow down placements.
Strategy: Use Row and Column Sums
Every row and column in a Sudoku grid sums to 45 (because 1+2+3+4+5+6+7+8+9 = 45). If you know the sum of most cages in a row, you can calculate the missing value. This is called the 45 rule and it is one of the most powerful Killer Sudoku techniques.
Strategy: Innies and Outies
When cages overlap with rows, columns, or boxes, you can find single cells whose value is determined by the difference between the cage total and the row or box total. These are called innies (cells inside a region but not covered by known cages) and outies (the reverse). They let you solve cells that seem impossible at first glance.
Where to Find Killer Sudoku
Many puzzle apps and websites offer Killer Sudoku. Look for it in puzzle magazines and specialty books. It is a great next step after you master standard Sudoku because it adds arithmetic challenge without changing the core placement logic.