What is a third cousin?
Short answerYour third cousin is the child of your parent’s second cousin: you share a set of 2nd great-grandparents. It works the same both ways: you are their third cousin too. Full third cousins share about 0.78% of their DNA on average.
The family line, drawn out
Follow the highlighted line from you up to the shared ancestor and down to them.
How to work it out
- Find the nearest ancestor you share: your 2nd great-grandparent.
- Count the generations from each of you up to that ancestor: 4 for one, 4 for the other.
- Take the smaller number and subtract one: 4 − 1 = 3, so they are third cousins.
- The difference between the two numbers is how many times removed: 4 − 4 = 0 (not removed).
Cousins are only “removed” when they belong to different generations. Two people in the same generation are never removed, whatever their ages.
Their family and yours
Relationships shift by one step for each generation. Here is how the people around them relate to you:
| Their child | third cousin once removed |
| Their parent on the shared line | second cousin once removed |
| Your child sees them as your child’s | third cousin once removed |
| Your parent sees them as your parent’s | second cousin once removed |
| What they call you | third cousin |
Shared DNA
On average, full third cousins share about 0.78% of their DNA. That figure is the expected value: it halves with every generation of separation, and real amounts vary from person to person. At this distance, a DNA test may find no shared DNA at all even when the relationship is certain. A half relationship (through only one shared ancestor instead of a couple) shares about half as much: 0.39%.
Let Family Trees name it for you
In Family Trees, open anyone’s page to see how they are related to you in plain words, or choose any two people under How are they related?. The app counts the shortest blood line through birth and adoptive parents, names step, foster and in-law relations too, and uses exactly the words on this page. It’s free for a family of up to 100 people. You can also try the relationship calculator here.
Questions
What is a third cousin?
Your third cousin is the child of your parent’s second cousin: you share a set of 2nd great-grandparents. It works the same both ways: you are their third cousin too.
How much DNA do you share with a third cousin?
About 0.78% on average. Each generation of separation halves the expected amount, and individual results vary.
What is my third cousin’s child to me?
Your third cousin once removed.