Play Killer Sudoku Pro Online: The Ultimate Math & Logic Puzzle

If you have already mastered standard Sudoku and are looking for a brand new way to test your brain, you have found it. Welcome to one of the toughest challenges on Puzzrs.com: Killer Sudoku Pro.

Also known as Sumdoku or Mathdoku, this addictive game combines the spatial logic of standard Sudoku with the deductive addition of Kakuro. There are no starting numbers given to you—instead, you must use math and logic to crack the puzzle completely from scratch.

Choose your difficulty below, flex your math muscles, and start calculating!

How to Play Killer Sudoku (Rules for Beginners)

If you look at the board, you will see it is entirely blank, but divided into specific “cages” with dashed lines. To win the game, you must follow the standard rules of Sudoku, plus one major mathematical twist:

1. Standard Sudoku Rules: You must fill the 9×9 grid so that every row, every column, and every 3×3 block contains the numbers 1 through 9 exactly once.
2. The “Cage” Sums: The board is grouped into cages marked by dashed borders. The tiny number in the top-left corner of each cage tells you the exact sum of the numbers hidden inside that cage.
3. No Repeats in a Cage: Just like Kakuro, you cannot repeat a number within a single dashed cage. (For example, to make a sum of 4 in a 2-cell cage, you must use 1 and 3. You cannot use 2 and 2!)

(To play the game above, click an empty square to select it, then use the on-screen number pad or your keyboard to type your guess. Use your Arrow Keys to quickly navigate the board!)

Winning Strategies: How to Solve Killer Sudoku

Because you start with zero given numbers, the opening moves in Killer Sudoku require pure logic. Here are the three best strategies to help you break through the Hard difficulty:

1. The “Rule of 45”

This is the most important rule in Killer Sudoku! Because every row, column, and 3×3 box must contain the numbers 1 through 9 exactly once, the sum of every row, column, and 3×3 box is always exactly 45 (1+2+3+4+5+6+7+8+9 = 45). If you find a 3×3 box that is completely filled by cages except for one single square jutting out, you can add up the cages inside the box, subtract from 45, and instantly figure out the exact number of that outer square!

2. Look for Unique Cage Sums

Just like Kakuro, certain sums can only be created with one specific combination of numbers. Memorizing these “magic blocks” gives you a massive head start:

3-Cell Cage = 24: Must be 7, 8, and 9.

2-Cell Cage = 3: Must be 1 and 2.

2-Cell Cage = 4: Must be 1 and 3.

2-Cell Cage = 16: Must be 7 and 9.

2-Cell Cage = 17: Must be 8 and 9.

3-Cell Cage = 6: Must be 1, 2, and 3.

3. Cross-Hatching Constraints

Once you solve a 2-cell cage of 17 (which has to be 8 and 9), you don’t know which order they go in yet. But you do know that the rest of that row, column, and 3×3 box cannot contain another 8 or 9! Use this knowledge to narrow down the possible numbers in neighboring cages.

Looking for More Logic Puzzles?

If the math in this puzzle is a bit too intense, you can always brush up on your pure logic skills with our classic Sudoku Pro. Or, if you want to try math without the 9×9 grid restrictions, give Kakuro Pro a try!