Relationship calculator: how are we related?

How the calculator works

Every blood relationship leads back to a common ancestor. Count the generations from you up to that ancestor and from the other person up to the same ancestor, and the rest follows. You and your first cousin share grandparents: two generations up for you, two for them. If one of you is the other’s ancestor, you are related in the direct line, like a grandson and his grandmother.

Usually the common ancestor is a couple, a grandfather and a grandmother. If an ancestor had children with two partners, you may descend from only one person in common. Then tick the half-relationship box. The degree stays the same; the expected DNA share is halved.

Second cousin, once removed and other terms

The cousin number tells you how far back the common ancestor is. First cousins share grandparents, second cousins share great-grandparents, third cousins share 2nd great-grandparents. In both US and UK usage the number is one less than the generations to the shared ancestor.

“Removed” counts the generations between two cousins. Your parent’s first cousin is your first cousin once removed, and so is your first cousin’s child; the cousin number always follows the person closer to the common ancestor. Many families call a parent’s cousin “aunt” or “uncle” and that cousin’s children “second cousins”. That is fine at the dinner table, but in genealogy a second cousin means shared great-grandparents.

Degree of relationship in law

Civil-law systems count degrees by the number of births that connect two people: up from one person to the common ancestor and down to the other. Parent and child are related in the first degree, siblings in the second, uncle and nephew in the third, first cousins in the fourth. German, French and Spanish law all count this way, and many US states use the civil-law method for inheritance and marriage rules. The older canon-law method counts only the generations of the longer line, so first cousins are related in the second degree there. Rules differ between jurisdictions, so treat the number as orientation, not legal advice.

Shared DNA: an expected value, not a promise

Each child inherits half of their autosomal DNA from each parent, but which pieces is random. Parent and child therefore share very close to 50 %, while 50 % for full siblings is only the average. Through a common couple the expectation is 2 × (1/2) to the power of the total generations; through a single ancestor it is half of that: first cousins 12.5 %, second cousins about 3.1 %.

The more distant the relationship, the more real test results scatter. From about third cousins on, a DNA test may find no shared segments at all. Multiple relationships raise the share: when two brothers marry two sisters, their children are double first cousins and share 25 % on average. The calculator assumes a single relationship.