Cousin Relationship Calculator
Calculate cousin relationships in your family tree.
Find out if someone is your first cousin once removed, second cousin, or third cousin.
The two-axis system
Cousin relationships have two axes that most people mix up:
- Degree (first, second, third, etc.) tells you how far back the common ancestor sits. First cousins share grandparents. Second cousins share great-grandparents. Third cousins share great-great-grandparents.
- Removed (once removed, twice removed) tells you how many generations apart the two cousins are from each other.
If both people are the same number of generations from the common ancestor, they are “plain” cousins (no removal). If one is a generation older or younger than the other, they are “removed” by that number of generations.
The math
If person A is gen_a generations from the common ancestor, and person B is gen_b generations:
- degree = min(gen_a, gen_b) − 1
- removed = |gen_a − gen_b|
A degree of 0 means the two are in sibling/uncle/aunt territory, not cousins. A degree of 1 makes them first cousins, 2 makes them second cousins, and so on.
Worked examples
| Person A | Person B | Common ancestor | Degree | Removed | Relationship |
|---|---|---|---|---|---|
| Grandchild (2) | Grandchild (2) | Grandparent | 1 | 0 | First cousins |
| Grandchild (2) | Great-grandchild (3) | Grandparent | 1 | 1 | First cousins once removed |
| Great-grandchild (3) | Great-grandchild (3) | Great-grandparent | 2 | 0 | Second cousins |
| Great-grandchild (3) | Great-great-grandchild (4) | Great-grandparent | 2 | 1 | Second cousins once removed |
| Great-great-grandchild (4) | Great-great-grandchild (4) | Great-great-grandparent | 3 | 0 | Third cousins |
The most common confusion
Your father’s first cousin is your first cousin once removed, not your second cousin. They share grandparents with your father, not with you, so the cousin-degree stays at “first” but you are a generation further down (once removed).
Your first cousin’s child is also your first cousin once removed (one generation removed from each other in the opposite direction).
Shared DNA falls off fast
Each step in degree roughly halves the expected shared DNA. First cousins share about 12.5%, seconds share 3.125%, thirds share about 0.78%. By fourth cousins, the expected shared DNA drops below 0.2% and many genuinely-related fourth cousins do not show up at all on a commercial DNA test. Family trees and paper records remain the most reliable tool past the third-cousin range.