Ready to go deeper? Medium Killer Sudoku introduces the 45 Rule and multi-cage interactions. You'll need to look across rows and columns to crack the hidden values.
Every row, column, and 3x3 box sums to 45. If all cages in a row are known except one cell, the missing value = 45 - (sum of all other cage totals in that row). This is the single most powerful Killer technique.
Innies and Outies
When a cage partially straddles a row/column/box boundary, the cells inside the boundary are "innies" and those outside are "outies." Their sum difference reveals exact values using the 45 rule, often resolving cells without any candidates.
Medium tip -
After placing forced cages, apply the 45 rule row-by-row and column-by-column. Look for innies and outies whenever a cage crosses a box boundary - They often reveal a cell value without needing candidates at all.
Frequently Asked Questions
Can digits repeat in a cage?
No - Every cage follows the no-repeat rule. Each digit appears at most once within any single cage.
What is the 45 rule?
Every row, column, and 3x3 box contains digits 1-9, summing to 45. Subtracting known cage sums within any unit reveals leftover cell values.
Is Killer Sudoku harder than Classic?
Generally yes - There are no given digits, so you rely entirely on cage arithmetic and elimination. Easy Killer is roughly equivalent to Medium Classic.