How Much Dna Do You Share With Your First Cousin? | Kinship Percentages Explained

If you have taken a DNA test, the first question that pops up once you see a cousin match is simple: what does this number mean? The phrase “first cousin” is clear in family talk, yet the amount of DNA shared behind that label can look puzzling on a test report.

This guide breaks down how much DNA first cousins usually share, why the number is not the same for every pair, and how that compares with other relatives on your family tree. You will also see how testing companies turn those centimorgan figures into relationship hints you can use in real life.

How Much Dna Do You Share With Your First Cousin On Average?

Geneticists describe the strength of a family tie using the coefficient of relationship. For first cousins, that value is 0.125, which means you and a first cousin share about 12.5% of your autosomal DNA on average. In practical testing terms, that usually works out to roughly 396–1397 centimorgans (cM) of shared DNA across many segments, with an average near 866 cM.

Commercial testing companies report similar values. For instance, 23andMe lists first cousins as sharing an average of 12.5% of their DNA, with most first cousin pairs falling between about 7.3% and 13.8%. This wide span explains why two cousins in the same family can show noticeably different match strengths.

Average Shared Dna By Close Relationship

To see where first cousins sit among other relatives, check the shared DNA figures in the table below. These values are averages, not fixed rules.

Relationship	Average Dna Shared	Typical Range
Parent / Child	50%	Narrow, near 50%
Full Sibling	50%	About 38–61%
Grandparent / Grandchild	25%	Varies by pair
Aunt / Uncle With Niece / Nephew	25%	Varies by pair
Half Sibling	25%	Roughly 17–34%
First Cousin	12.5%	About 7–14%
Second Cousin	3.125%	About 2–5%

So when a report says you share about one eighth of your DNA with a first cousin, that lines up well with simple probability and with real test data from large databases.

Why First Cousins Share About One Eighth Of Their Dna

To understand why the answer to “how much dna do you share with your first cousin?” sits around 12.5%, you need a quick look at how DNA passes from one generation to the next.

How Dna Passes From Grandparents To Cousins

You inherit half of your autosomal DNA from each parent. Each parent in turn received half of their DNA from each of their parents, your four grandparents. When you and your first cousin share the same pair of grandparents, the math works like this:

• Your parent and your cousin’s parent are siblings, sharing about 50% of their DNA.
• You receive about 50% of your DNA from your parent, and your cousin receives about 50% from theirs.
• Multiplying those links (0.5 × 0.5 × 0.5) gives 0.125, or 12.5% shared DNA between you and your first cousin.

This neat fraction follows the same pattern as other relatives. Grandparents sit at 25%, second cousins at about 3.125%, and so on.

Recombination And Why Numbers Vary

The 12.5% figure is an average, not a rigid rule. Each time a child is conceived, the chromosomes passed on from each parent are shuffled. This process, called recombination, mixes DNA segments before they move to the next generation.

Because of this shuffling, one pair of first cousins might share closer to 7% of their DNA, while another pair from the same extended family might land near 14%. Testing companies measure not just the total amount of shared DNA, but also how many segments match and how long those segments are.

Centimorgans, Percentages, And What Testing Reports Show

Consumer DNA tests rarely show “12.5%” next to a cousin’s name. Instead they show centimorgans and segments, sometimes along with an estimated percentage. Understanding that language helps you read your cousin match list with more confidence.

What Centimorgans Measure

A centimorgan is a unit that reflects how likely it is that a stretch of DNA will be split by recombination from one generation to the next. Longer shared segments in centimorgans point to a closer relative, while shorter segments or very small totals hint at a distant cousin.

For first cousins, the shared centimorgan project and several major testing companies report a typical range around 396–1397 cM, with an average near 866 cM. That spread still fits well within the expected 12.5% share.

Percent Match Versus Relationship Labels

Most testing dashboards show both a relationship label and a range of possibilities. For example, a match that shares about 900 cM with you might be a first cousin, a half aunt or uncle, or a great aunt or uncle. The site uses age, tree data, and segment patterns to narrow that list.

This is why your report might tag a match as “first cousin – great aunt/uncle – half aunt/uncle” all at once. The amount of DNA shared fits several possible spots on the family chart that all sit close to the same 12.5% level.

Comparing First Cousins With Other Cousin Types

Once you understand how much dna you share with your first cousin, it helps to check other cousin relationships that appear on match lists. The numbers drop by half with each extra step away from the common ancestor.

Half First Cousins

Half first cousins share only one grandparent instead of a pair. In that case, the expected shared DNA falls to about 6.25%, or roughly half of what full first cousins share. On reports, that often shows up near 449 cM, with a broad range that overlaps first cousins once removed.

First Cousins Once Removed

A first cousin once removed is the child of your first cousin, or the first cousin of your parent. With one extra generation between you and the shared grandparent pair, the expected share is again about 6.25%. Testing projects report a range near 102–980 cM for this relationship.

Second Cousins And Beyond

Second cousins share great grandparents. On average that works out to about 3.125% shared DNA, with many pairs closer to the low end of the range. By the time you reach third cousins, the average shared DNA drops below 1%, and many such relatives will not appear as matches at all.

Using Cousin Matches In Genetic Genealogy

Knowing how much DNA a first cousin shares with you gives more than a fun trivia fact. It helps you sort through match lists, spot likely relationships, and build or confirm parts of your family tree.

Why First Cousin Matches Matter

First cousin matches often anchor your match list. They sit close enough in your tree that you can usually identify them and share family information. Once you know exactly how you connect, you can work outward together, linking second and third cousin matches to specific ancestral couples.

Many guides from testing companies show how to group your matches by shared segments and by which close cousin they match. For instance, 23andMe’s DNA relatives help page explains how percent shared DNA links to typical relationships and how the site makes those predictions.

Building Clusters Around First Cousins

Once you have confirmed a first cousin match, you can check the list of people who share DNA with both of you. Anyone who matches you and that cousin likely sits on the same side of your family. That cuts your search field in half when you try to place new matches.

When Numbers Do Not Match The Paper Tree

Sometimes your shared DNA level with a known first cousin does not land near 12.5%. The figure might be lower than you expected, or closer to the range for a half cousin. Recombination can explain part of this, yet in some families a mismatch points to a hidden parentage event or a mis-recorded relationship in records.

In those cases, many genealogists compare multiple tests from different relatives and consult detailed charts such as the coefficient of relationship table to see what other relationships fit the shared centimorgan total. The goal is to match both the DNA figures and the paper trail.

Quick Reference: Shared Dna Levels For Cousin Relationships

This summary pulls together the main cousin types and their expected shared DNA levels. Values are rounded for simplicity, and real results can fall above or below these numbers.

Relationship Type	Approximate Percent Shared	Typical Centimorgan Range
Full First Cousin	12.5%	About 396–1397 cM
Double First Cousin	25%	Often near full sibling range
Half First Cousin	6.25%	About 156–979 cM
First Cousin Once Removed	6.25%	About 102–980 cM
Second Cousin	3.125%	About 46–515 cM
Second Cousin Once Removed	1.5%	Roughly 30–215 cM
Third Cousin	0.78%	Often 0–217 cM

Putting Your First Cousin Match In Context

So, how much dna do you share with your first cousin in real life? A simple guide is that full first cousins share around one eighth of their autosomal DNA, usually between 7% and 14% or roughly 400–1400 cM.

When a new cousin appears in your match list, check three things: the total shared centimorgans, the number and size of shared segments, and the age and tree information that each of you can provide. Together, these details help you decide whether that cousin is a likely first cousin, a half cousin, or a different close relative who just happens to land near the 12.5% mark.