How much DNA do cousins share?
Every generation, a parent passes on about half their DNA. Walk up to your shared ancestor and back down to your cousin, halving at each step, and you have the expected share.
See the halving happen
first cousin
12.5%
≈ 869 cM expected (theoretical average)
Why full cousins get a ×2
Full cousins descend from a couple: two shared ancestors, each contributing their own stream of DNA down both lines. Half-cousins share just one grandparent, so they get one stream and about half as much. That is why a half first cousin and a first cousin once removed land on the same 6.25%.
Shared DNA chart by relationship
| Relationship | Expected share | Average cM | Observed range (cM) |
|---|---|---|---|
| Parent / child | 50% | 3,485 | 2,376–3,720 |
| Full sibling | 50% | 2,613 | 1,613–3,488 |
| Grandparent, aunt/uncle, half-sibling | 25% | ≈1,750 | ≈1,150–2,300 |
| First cousin | 12.5% | 866 | 396–1,397 |
| Great-aunt / great-uncle | 12.5% | ≈850 | ≈330–1,470 |
| Half first cousin | 6.25% | 449 | 156–979 |
| First cousin once removed | 6.25% | 433 | 102–980 |
| Second cousin | 3.13% | 229 | 41–592 |
| First cousin twice removed | 3.13% | 221 | 43–531 |
| Second cousin once removed | 1.56% | 122 | 0–316 |
| Third cousin | 0.78% | 73 | 0–234 |
| Second cousin twice removed | 0.78% | 71 | 0–261 |
| Third cousin once removed | 0.39% | 48 | 0–192 |
| Fourth cousin | 0.20% | 35 | 0–139 |
Centimorgan figures: the Shared cM Project (Blaine Bettinger, version 4), rounded; "≈" rows combine several relationships with similar values. Expected share is the theoretical average.
Reading a DNA match
A match's shared cM usually fits several relationships. 850 cM could be a first cousin, a great-aunt or a half-aunt. Combine the number with ages, the family tree and which of your other matches you share, then use the calculator to name the candidates once you find the common ancestor.
Questions people ask
How much DNA do second cousins share?
About 3.1% on average, roughly 230 centimorgans according to the Shared cM Project, with a real range from about 40 to over 500 cM.
Why do the real numbers vary so much?
Each parent passes on a random half of their DNA, in large chunks. Over several generations those random choices add up, so two relatives of the same type can share noticeably different amounts.
Can a third cousin share no DNA with me?
Yes. Roughly one in ten third cousins shares no detectable DNA, and the chance rises for fourth cousins and beyond, even when the paper trail is correct.
What is a centimorgan?
A unit of genetic distance that DNA testing companies use to measure how much DNA two people share. A person has roughly 6,800–7,000 cM in total, depending on the company.