The relationship calculator names how two relatives are related once you know their closest shared ancestor. Enter how many generations each person is from that ancestor and it tells you the relationship, such as "2nd cousin once removed", and how much DNA the two are expected to share on average.
How to use it
- Find the most recent ancestor (or ancestral couple) the two people have in common.
- Count the generations from you up to that ancestor: 1 for a parent, 2 for a grandparent, 3 for a great-grandparent, and so on.
- Count the same for the other person.
- Tick "Half relationship" if the two lines come from one shared ancestor rather than a couple, for example through a grandfather's second marriage.
The answer describes the other person relative to you. The chart under it draws both lines down from the shared ancestor.
The cousin chart
Rows are generations from you to the shared ancestor; columns are generations from the other person.
| 1 | 2 | 3 | 4 | 5 | 6 | |
|---|---|---|---|---|---|---|
| 1 | Sibling | Niece or nephew | Grandniece or grandnephew | Great-grandniece | 2nd great-grandniece | 3rd great-grandniece |
| 2 | Aunt or uncle | 1st cousin | 1st cousin once removed | 1st cousin twice removed | 1st cousin 3 times removed | 1st cousin 4 times removed |
| 3 | Great-aunt or great-uncle | 1st cousin once removed | 2nd cousin | 2nd cousin once removed | 2nd cousin twice removed | 2nd cousin 3 times removed |
| 4 | 2nd great-aunt | 1st cousin twice removed | 2nd cousin once removed | 3rd cousin | 3rd cousin once removed | 3rd cousin twice removed |
| 5 | 3rd great-aunt | 1st cousin 3 times removed | 2nd cousin twice removed | 3rd cousin once removed | 4th cousin | 4th cousin once removed |
| 6 | 4th great-aunt | 1st cousin 4 times removed | 2nd cousin 3 times removed | 3rd cousin twice removed | 4th cousin once removed | 5th cousin |
The rule behind it: the cousin number is the smaller of the two counts minus one, and "removed" is the difference between the two counts.
How the DNA figure is worked out
Each generation passes on half of a person's DNA. Two full relatives who share an ancestral couple are expected to share one half to the power of (the two generation counts added together, minus one). Half relatives share one ancestor instead of two, so they share half of that. The calculator turns the share into centimorgans using 6,800 cM for a whole genome, the usual convention, under which a parent and child share 3,400 cM.
So first cousins (2 and 2) expect one half to the power of 3, which is 12.5%, about 850 cM. Second cousins (3 and 3) expect 3.1%, about 213 cM.
Limits
These are averages, not predictions. Real DNA is inherited in random chunks, so actual sharing between cousins varies a lot, and from about third cousins onward some genuine cousins share no detectable DNA at all. If two people are related in more than one way, for example double cousins, add the figures for each relationship.
Frequently asked questions
- What does "once removed" mean?
- Removed means the two people are from different generations. Your first cousin's child is your first cousin once removed, and so is your parent's first cousin. Twice removed means two generations apart.
- What is the difference between a second cousin and a first cousin once removed?
- Second cousins are the same generation and share great-grandparents. A first cousin once removed is one generation away from you and shares your grandparents (or you share theirs). They are often confused because both feel like "distant cousins".
- What is a half cousin?
- Half cousins descend from one shared ancestor instead of a shared couple, usually because a grandparent had children with two different partners. Half relatives share about half as much DNA as full relatives of the same kind.
- How much DNA do second cousins share?
- On average about 3.1% of their DNA, roughly 213 centimorgans (cM). Real pairs vary widely around that average because DNA is passed down in random chunks, so two second cousins can share noticeably more or less.
- How do I count the generations?
- Find the most recent ancestor the two people have in common. Count the steps from each person up to that ancestor - one for a parent, two for a grandparent, three for a great-grandparent - and enter the two numbers.
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