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Arithmetic & Logic Variant
How to Play Killer Sudoku →

Daily Killer Sudoku: Play Free Expert Sum Puzzles Online

Looking for a challenge beyond classic 9x9 grids? Killer Sudoku (also known as Sum Sudoku) combines pure placement logic with mental arithmetic. Using dotted cages with target sum totals, you must solve the entire board while obeying both classic Sudoku rules and cage math constraints.

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The 3 Fundamental Rules of Killer Sudoku

Rule 1

Classic Sudoku Base

Every row, column, and 3x3 block must contain the numbers 1 to 9 with no repetitions.

Rule 2

Cage Sum Totals

The numbers inside each dotted cage line must add up precisely to the small number printed in its top-left corner.

Rule 3

No Cage Duplicates

No number may appear more than once inside the same cage, even if the cage crosses into another 3x3 box.

The Secret Weapon: The Rule of 45

The single most important mathematical strategy in Killer Sudoku is the Rule of 45. In every standard 9x9 Sudoku, the sum of digits 1 through 9 equals exactly 45:

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45

Because every row, column, and 3x3 block sums to 45, if a set of cages covers an entire 3x3 block except for one sticking-out cell, you can instantly calculate that cell's digit by subtracting the cage sums from 45! This technique (known as the "Innie/Outie" method) cracks open 90% of expert killer puzzles.

Killer Sudoku Unique Combinations Cheat Sheet

Always scan for cages that only have a single mathematical combination of digits:

Sum 3 (2 cells)[ 1, 2 ] strictly
Sum 4 (2 cells)[ 1, 3 ] strictly
Sum 16 (2 cells)[ 7, 9 ] strictly
Sum 17 (2 cells)[ 8, 9 ] strictly
Sum 6 (3 cells)[ 1, 2, 3 ] strictly
Sum 7 (3 cells)[ 1, 2, 4 ] strictly
Sum 23 (3 cells)[ 6, 8, 9 ] strictly
Sum 24 (3 cells)[ 7, 8, 9 ] strictly

Frequently Asked Questions (Killer Sudoku)

Is Killer Sudoku harder than regular Sudoku?

At first glance, yes—because opening boards usually have zero prefilled numbers! However, cage sums provide powerful mathematical clues. Once you memorize common combinations and apply the Rule of 45, Killer Sudoku is often more satisfying and intuitive than traditional puzzles.

Can a cage contain repeating numbers?

No. By official Killer Sudoku tournament rules, no digit may repeat inside the same dotted cage, even if the cage spans across multiple rows or 3x3 blocks.

How does iLoveSudoku Killer Sudoku compare to 247 Killer Sudoku and Sudoku.com?

Unlike 247 Killer Sudoku or Sudoku.com, iLoveSudoku offers a daily refreshed free Killer Sudoku puzzle with animated cage rendering, the Rule of 45 innie/outie solver highlight, unique combination detection, and full mobile dark mode — completely free without subscription paywalls or unskippable video ads.

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Killer Sudoku Online / Sumdoku

Play Killer Sudoku Online Free — Daily Killer Sudoku & Expert Sum Puzzles

Solve Free Killer Sudoku Online Puzzles Daily with Arithmetic Cages & Rule of 45 Math Logic

Play free Killer Sudoku online with zero intrusive video ads. Killer Sudoku (also called Sumdoku or Addoku) merges classic 9×9 Sudoku rules with dashed arithmetic cages, each displaying a target sum that must be matched with unique digits. Test your mental arithmetic and spatial deduction with new daily killer sudoku puzzles.

Play Killer Sudoku Online: How the Math Cages Work

Playing Killer Sudoku online combines classic logic placement with arithmetic problem solving. The board is divided into dashed groups of cells called "cages." Each cage has a small number in the top-left corner representing the sum of all digits inside that cage.

Rule 1: Standard Sudoku rules apply across every row, column, and 3×3 box (1 to 9 with no repeats).

Rule 2: Digits within a single cage must sum to the cage clue, with no duplicate digits allowed inside any single cage.

The Golden Rule of Daily Killer Sudoku: The "Rule of 45"

Because the digits 1 through 9 sum to exactly 45 (1+2+3+4+5+6+7+8+9 = 45), every complete row, column, and 3×3 box must total 45.

By summing all cages that completely cover a row or box except for one sticking-out cell, you can calculate that outlier cell’s value instantly: Outlier = Sum of Cages - 45!

Key Tactical Insights
  • Apply the Rule of 45 across full 3×3 boxes to reveal single-cell values early.
  • Remember: No digit can repeat inside a single cage, even if the cage spans multiple boxes.

Essential Killer Sudoku Cage Combinations (Cheat Sheet)

2-cell sum of 3: Must be [1, 2].

2-cell sum of 4: Must be [1, 3] (2+2 is illegal).

2-cell sum of 16: Must be [7, 9] (8+8 is illegal).

2-cell sum of 17: Must be [8, 9].

3-cell sum of 6: Must be [1, 2, 3].

3-cell sum of 24: Must be [7, 8, 9].

Killer Sudoku Expert Solving Strategies

For expert-level Killer Sudoku, combine arithmetic cage bounds with standard Sudoku pointing pairs and locked subsets. Look for cages that restrict numbers to specific boxes, and use overlapping cages to mathematically force single candidate solutions.

Frequently Asked Questions

Where can I play Killer Sudoku online for free?▼

You can play Killer Sudoku online for free on iLoveSudoku with zero banner popups, seamless mobile and desktop controls, custom difficulty tiers, and synchronized daily puzzles.

What is daily killer sudoku and how does it work?▼

Daily Killer Sudoku is a variant of Sudoku where you solve a fresh puzzle every day featuring cages with sum clues. Standard Sudoku rules apply (1–9 in each row, column, and 3×3 box), plus cells inside dashed cages must add up to the cage target number without repeating digits.

What are the official rules of Killer Sudoku?▼

Follow standard Sudoku rules (1–9 in each row, column, and 3×3 box). Additionally, cells inside dashed cages must sum to the small number in the cage corner without repeating digits.

Can numbers repeat inside a Killer Sudoku cage?▼

No, digits cannot repeat inside a cage, regardless of whether the cage extends across multiple rows or 3×3 boxes.

What is the Rule of 45 in Killer Sudoku?▼

The Rule of 45 leverages the mathematical fact that 1+2+...+9 = 45. Comparing cage totals in a unit to 45 reveals the value of cells that overlap outside.

Is there an expert mode for Killer Sudoku?▼

Yes! iLoveSudoku features multi-tier difficulty for Killer Sudoku ranging from beginner-friendly sum cages to diabolical expert grids.