First cousins usually share around 12.5% of their DNA, but real results often range from about 7% to 14%.
Genetic tests now arrive with neat dashboards, match lists, and a line that says something like “first cousin, 12.7% shared DNA.” Then the questions start. Is that normal? Does it match what family stories say? Can this number rule out or confirm a guess about a parent or grandparent?
When people ask “how much dna do first cousins share?”, they want a clear number and an honest sense of the wiggle room around it. This guide walks through the textbook answer, the range seen in real data, and simple ways to use a first cousin match when you build or check a family tree.
How Much Dna Do First Cousins Share? Basics In Plain Language
On average, first cousins share one eighth of their DNA. In percentage terms, that works out to 12.5%. Many testing companies express the same thing as a centimorgan count, often around 850 to 900 cM. Studies of known first cousin pairs cluster near that level, with a long tail on both sides.
The Shared cM Project uses tens of thousands of reported matches and puts first cousins at about 874 cM on average, with most results a few hundred centimorgans either side of that level. Charts from major testing companies show the same band near 12.5% shared DNA.
| Relationship | Average Dna Shared (%) | Average Shared cM |
|---|---|---|
| Parent / Child | 50 | ~3400 |
| Full Siblings | 50 | ~2600 |
| Grandparent / Grandchild | 25 | ~1700 |
| Aunt / Uncle – Niece / Nephew | 25 | ~1700 |
| Half Siblings | 25 | ~1700 |
| First Cousins | 12.5 | ~874 |
| First Cousins Once Removed | 6.25 | ~439 |
| Second Cousins | 3.13 | ~220 |
Numbers in that table draw from tools such as the Shared cM Project and relationship charts used by genetic genealogy groups. Exact values vary a little from chart to chart, yet each major source places first cousins near 12.5% of shared DNA and around 850 to 900 cM on average.
Average Dna First Cousins Share By Percent And cM
To see why first cousin numbers look the way they do, think about how DNA passes down through generations. Each parent passes on half of their DNA at random to a child. Siblings do not receive the same half. One may share long stretches with you; another may share shorter, different segments. By the time you reach the next generation and meet your cousin, some pieces line up and others do not.
In a simple idealised model, first cousins should share exactly one eighth of their DNA, because they each receive half of their DNA from parents who are full siblings. Real life does not follow that tidy fraction. Random shuffling means some cousins share closer to 7% of their DNA while others reach 13% or a little more. A study quoted by 23andMe shows a range of around 7.3% to 13.8% for first cousins, while centimorgan based charts from the Shared cM Project show most first cousins between roughly 553 and 1225 cM.
These ranges matter because a first cousin who shares 550 cM and another who shares 1100 cM can both be genuine full first cousins. That spread by itself does not prove a different relationship. Interpretation depends on the pattern of segments, the way your family tree looks, and whether there are extra links such as double cousins or endogamy in your background. Those ranges help you stay calm when a match number looks odd at first glance. They show how DNA inheritance carries plenty of built in swing.
What Centimorgans Mean For First Cousins
Most testing companies express shared DNA for cousins as centimorgans, often shortened to cM. A centimorgan is a unit tied to the chance that a section of DNA recombines during meiosis. Longer shared segments correspond to more centimorgans, and many long segments usually point to a closer relationship.
First cousins often share several long segments and a handful of shorter ones. The sum of those segments gives the headline number that appears in your match list. Tools based on the Shared cM Project, such as the calculator hosted at DNA Painter, allow you to enter that number and see the range of relationships that match it, along with probabilities for each one.
First Cousin Dna Sharing In Real Life
The headline number for a first cousin match gives a clear starting point, yet real families rarely follow a neat textbook. One cousin may sit almost exactly on the average, while another rests at the high or low edge of the range. Both can match the known family tree once you read them in context.
When you see a first cousin match at around 850 to 900 cM, that result matches the classic expectation. A value near 700 cM or near 1100 cM might still fit first cousins, though it invites a closer check of other possibilities. Charts tied to the Shared cM Project and tools built on that data give probability curves that help you compare first cousin, half aunt or uncle, grandparent, or other options that sit in the same rough range.
The Shared cM Project tool lets you type in the centimorgans from your match report and view the list of possible relationships along with their likelihood. First cousin often comes out near the top when the number lies close to 850 to 900 cM, yet other relationships can sit within reach, especially when the value drifts toward extremes.
Half First Cousins And Once Removed Relationships
Half first cousins share one grandparent instead of two. Their parents are half siblings, not full siblings. That pattern means expected DNA sharing for half first cousins sits near 6.25%, about half the full cousin level, with centimorgan averages near the mid 400s.
First cousins once removed share a grandparent at different generational levels. One person is the child of your first cousin, or your parent’s cousin, and typical sharing again sits near 6.25% with a wide band that overlaps full first cousins and half aunts or uncles.
Second cousins sit lower again, near 3% of shared DNA, with shared cM totals in the low hundreds and below. More distant cousins begin to fall below detection for some testers, which is one reason first cousin matches carry strong weight for genetic genealogy work.
Reading First Cousin Dna Matches In Practice
Once you know the expected range for first cousins, the next step is turning a match into practical moves on your tree. That process often starts with checking which side of the family the match fits, then lining up their grandparents with yours.
If your match appears with a familiar surname and location, you may be able to place them quickly. Perhaps their grandparent matches one of yours. In other cases you may only see a username and a centimorgan number. In that case, shared matches, age information, and small clues from any public tree they provide can guide you toward the right branch.
Using Shared Matches To Pinpoint The Line
Shared matches show who appears in common between you and the first cousin in question. If the same group of relatives on your paternal side appears with this cousin, you can place the match on that side of your tree with reasonable confidence. The same approach works on the maternal side.
Once you know the side of the family, review grandparents on that side and check which ones fit the cousin’s location, surname, or known parents. Many first cousin puzzles resolve once both people build out grandparent level trees and compare names and places around the time their parents were born.
When The Numbers Seem Too High Or Too Low
Some first cousin matches sit near 1300 cM or even higher. Values in that band can point either to a full first cousin, to a half sibling, or to an aunt or uncle style relationship. In those cases, the match label inside the testing site, the age gap, and the way segments line up across chromosomes matter a lot.
On the low side, a match around 400 to 500 cM might still be a true first cousin, yet a first cousin once removed or half first cousin may fit better. Study the tree from both sides, test more close relatives where possible, and rely on paper records as well as DNA.
First Cousin Dna Match Scenarios
The table below lays out some simple first cousin match scenarios. These examples show how the same centimorgan value can point to more than one relationship. They are guides, not firm rules.
| Shared cM | Common Relationship Reads | Notes |
|---|---|---|
| 400 cM | First cousin, first cousin once removed | Often sits at low edge for full first cousins. |
| 600 cM | First cousin | Comfortable range, but still overlaps other close ties. |
| 850 cM | First cousin | Near classic average for full first cousins. |
| 1000 cM | First cousin, half aunt or uncle | High side of range; check ages and tree details. |
| 1200 cM | First cousin, aunt or uncle, half sibling | Could still be a cousin; needs strong tree evidence. |
Practical Steps When You See A First Cousin Match
Start by saving a snapshot of the match details: shared centimorgans, number of segments, any notes on longest segment, and the match label. Record which testing site the match comes from, since different companies use slightly different algorithms and may report slightly different totals for the same pair of people.
Next, add basic information about the match to your research notes. Include their displayed name, any public tree link, and hints about age or location. Draw a quick version of your own tree from grandparents down to yourself. Then begin pencilling in where this cousin could fit based on surnames, locations, and generations.
As you work, try to fit DNA numbers and tree evidence together instead of letting either one dominate. A value a little outside the ideal first cousin band can still fit once you notice double cousins, endogamy, or gaps in records on one side of the family. The goal is not to chase a perfect number, but to let DNA and records reinforce each other until the relationship makes sense.
In the end, the question “how much dna do first cousins share?” has two answers. On paper, the share sits at 12.5%. In real test results, true first cousins can land from the mid hundreds up toward 1200 cM, and that number, read alongside a solid tree, shows where this cousin belongs.