Third Cousin

See how a third cousin is related to you, drawn as a family diagram with the shared ancestors at the top. Change the degree and the removal for any other cousin.

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A third cousin is a relative in your own generation who shares a 2x great-grandparent with you, which means both of you are four generations below the shared ancestors. The page opens on a third cousin and shows the relationship in words, as a family diagram and with the expected shared DNA. Press “Swap viewpoint” or a chip to compare it with a second cousin or a third cousin once removed. Change “Cousin degree” and “Times removed” to explain any other cousin the same way, or choose “One person (half cousins)” for a half third cousin. Everything is calculated in your browser, and the link keeps your choice.

How to find out who is your 3rd cousin

Read the heading “Your 3rd cousin” and the sentence under it. It names who the relative is in words.
Press a chip such as “3rd cousin once removed” to jump to another relationship.
Or set “Cousin degree” (1st to 10th) and “Times removed” (0 to 10) yourself.
Under “Who is younger?” choose whether they are younger than you or older than you. With 0 times removed, both of you are in the same generation.
Press “Swap viewpoint” to see the relationship from their side. The tiles “Seen from you” and “Seen from them” give the wording for each of you.
Under “Shared ancestors” choose “One person (half cousins)” if you share only one ancestor.
Look at the family diagram. It shows the common ancestors at the top, your line on the left and their line on the right.
Read “Generations from the common ancestor” and “Expected shared DNA” for the numbers behind the name.
Read the line under the diagram. It says how many generations you and they are below the shared ancestor and which person in the diagram is the cousin.
Press a neighbouring chip, for example the next degree or one more removal, and watch the two generation numbers and the DNA figure change.
Press “Copy link to this result” to share the exact relationship.

You and your third cousin are both 4 generations below the ancestors you share. Those ancestors are your 2x great-grandparents, so one of your great-grandparents and one of theirs were siblings. In the same way, the parents of first cousins are siblings, and the grandparents of second cousins are siblings.

What is a 1st, 2nd, 3rd and 4th cousin?

The number says how far back the shared ancestor is. 1st cousins share grandparents, 2nd cousins share great-grandparents, 3rd cousins share 2x great-grandparents and 4th cousins share 3x great-grandparents. Add one to the degree to get how many generations each of you is below the shared ancestors.

Cousin	Shared ancestors	Generations (each)	Expected shared DNA
1st cousin	grandparents	2	12.5% (1/8)
2nd cousin	great-grandparents	3	3.125% (1/32)
3rd cousin	2x great-grandparents	4	0.78125% (1/128)
4th cousin	3x great-grandparents	5	0.19531% (1/512)
5th cousin	4x great-grandparents	6	0.048828% (1/2,048)

On average 0.78125%, which is 1/128 of the genome, for full third cousins who share a couple of 2x great-grandparents. If only one ancestor is shared, the average is halved to 0.39063%. These are theoretical averages over many pairs. Real amounts vary, and at this distance some third cousins share no DNA from the common ancestor at all.

Is a third cousin the same as a second cousin once removed?

No. A 2nd cousin once removed shares a great-grandparent with one of you, with generation counts 3 and 4. A 3rd cousin shares a 2x great-grandparent with you and both of you are 4 generations below. The expected DNA is 1.5625% against 0.78125%.

Is a third cousin inbreeding?

The children of third cousins have an expected inbreeding coefficient of 1/256, or 0.39%, from the shared 2x great-grandparents. For the children of first cousins it is 1/16, or 6.25%. The inbreeding coefficient calculator works out the figure for other relationships. Whether a marriage is allowed is a matter of local law.

How do you find a third cousin in a family tree?

Go up four generations from yourself to a 2x great-grandparent. Then follow every child of that ancestor other than the one you descend from, and all their descendants down to your own generation. They are your cousins of that degree. People who descend from the same child as you are closer relatives: siblings, first cousins or second cousins. A descendant chart of the 2x great-grandparent lists everyone in these lines.

What is a third cousin once removed?

A third cousin once removed is the child of your third cousin, or the third cousin of your parent. The generation counts are 4 and 5, and the expected DNA is 0.39063% (1/256).

Frequently asked questions

You share 2x great-grandparents. Your great-grandparent and your third cousin’s great-grandparent were siblings, and you are in the same generation.

Is a third cousin considered family?

They are blood relatives who descend from the same 2x great-grandparents, so in genealogy they are family. Whether they feel like family depends on whether you know them.

Are third cousins and second cousins in the same generation?

Both are in your own generation, counted by the same line. The difference is how far back the shared ancestor is: great-grandparents for second cousins, 2x great-grandparents for third cousins.

What is the difference between a half cousin and a third cousin?

Half means only one ancestor is shared instead of a couple. Third says how far back the shared ancestors are. The two can combine, as in a half third cousin.

How many third cousins can I have?

It depends on how many children each ancestor had, so there is no standard number. The more siblings in each generation above you, the more cousins you have at every degree.

Does this upload my information?

No. Only the numbers you choose are used, and they stay in your browser.

Second Cousin Once Removed: the closer relationship, explained from both sides.
Half Cousin: what half cousins are and how much DNA they share.
First Cousin Once Removed: the most searched relationship, explained from both sides.
Cousin Calculator: work from the common ancestor to the name.
Cousin Chart: all cousin degrees and removals in one table.
Relationship Calculator: relationships between two people in a tree.
Relationship Chart: the chart of family relationships.
Inbreeding Coefficient Calculator: how related two parents are.
Kinship Terms: relationship words in other languages.
How Many Ancestors Calculator: ancestors per generation.
Gedcom Viewer: open a family tree file.