How Much Dna Do You Share With Your Cousin? | Simple Relatedness Guide

Wondering how much DNA sits in common between you and that cousin who shows up at reunions or on your test results? Genetic testing companies turn this into clear percentages that tell you how closely related you are. Once you understand the typical ranges, your match list starts to make a lot more sense.

How Much Dna Do You Share With Your Cousin? Basic Numbers

In everyday family talk, “cousin” usually means a first cousin. That person shares a pair of grandparents with you. On average, you and a first cousin share about 12.5% of your autosomal DNA. That average comes from simple inheritance math and has been confirmed by large testing databases from companies such as 23andMe and AncestryDNA.

The table below shows the typical shared DNA for several cousin relationships. These are rounded figures based on research and company statistics on shared DNA and centimorgans.

Relationship	Average DNA Shared (%)	Typical Range (%)
Parent / Child	50	Near 50
Full Sibling	50	Around 38–61
Half Sibling	25	Roughly 17–34
First Cousin	12.5	About 7–14
First Cousin Once Removed	6.25	About 3–9
Second Cousin	3.1	About 2–5
Third Cousin	0.8	Roughly 0.3–2

Genetic testing companies publish similar charts of average shared DNA by relationship, along with ranges that reflect real families in their databases. One example is the percent DNA shared by relationship table from 23andMe, which lines up well with these values.

How Much DNA You Share With Cousins By Type

Not all cousins sit at the same distance in your family tree. A first cousin shares grandparents with you, a second cousin shares great grandparents, and so on. Each extra generation cuts the expected shared DNA in half.

Here is a quick run through the most common cousin types and how much DNA they usually share with you.

First Cousins: Grandparents In Common

When people ask “how much dna do you share with your cousin?”, they usually mean a first cousin. You and a first cousin each get half your DNA from your parents, who are siblings. That shared pair of grandparents leads to an expected relatedness of one eighth, or about 12.5% shared DNA.

In centimorgan terms, studies such as the Shared cM Project and company match charts place first cousins around 680 to 1,200 centimorgans, with an average near 850 cM. Individual families can sit a bit higher or lower than the average, yet almost all first cousins fall somewhere in this band.

Second Cousins: Great Grandparents In Common

Second cousins trace back to a shared set of great grandparents. The extra generation in between you and the common ancestors cuts the expected shared DNA in half again. You and a second cousin average about 3% shared DNA, with a typical range around 2–5%.

On a match list, second cousins often sit around 200 to 400 centimorgans. They still share enough DNA with you that testing companies can label them as close relatives with high confidence.

Third And Fourth Cousins: Distant Yet Detectable

Third cousins share a pair of great great grandparents. The average shared DNA drops under 1%, often around 0.8%. In centimorgans that works out to roughly 70 to 150 cM, though there is a wide spread. Some third cousins share less DNA than this and may not appear as matches at every company.

Fourth cousins share even less DNA, around 0.2% on average. Testing services often report them in the 20 to 80 cM range. They may still help you connect family lines, yet their presence on a match list is much less predictable.

Why Shared DNA With Cousins Is A Range, Not A Single Number

The tidy percentages in cousin DNA charts come from simple probability. Each child gets half of each parent’s autosomal DNA, but the actual segments passed down are shuffled by recombination. That shuffle is random, so cousins in the same family can share more or less DNA than the textbook values.

Think of each grandparent’s DNA as a long deck of cards. When your parent forms eggs or sperm, the deck gets reshuffled, cut, and passed on in chunks. Your cousin’s parent goes through the same shuffle. Sometimes the two decks line up in many of the same places. Sometimes fewer segments match. The total shared DNA you see on a test result reflects how those random shuffles lined up.

Centimorgans Versus Percentages

DNA testing sites usually express shared DNA in centimorgans as well as in plain percentages. A centimorgan, often shortened to cM, is a unit that describes how likely it is for a recombination event to split a segment from one generation to the next. Longer shared segments equal more centimorgans and indicate a closer relationship.

Projects such as the Shared cM Project gather thousands of known relationships and the centimorgan totals between them. Those data help testers estimate the most likely relationship for any given match based on the cM amount and its range of possibilities.

How “Half” And “Removed” Cousins Change Shared DNA

Once you move beyond simple first and second cousins, terms such as “half cousin” and “once removed” start to appear on family trees and match lists. These labels describe either a missing grandparent in the shared line or an extra generation between the two of you. Each of those tweaks changes the expected percentage of shared DNA.

Half Cousins

Half cousins arise when the connecting relatives share only one grandparent instead of a pair. For instance, your parent and your uncle may be half siblings who share a father but not a mother. Their children still count as cousins, yet they are half cousins rather than full first cousins.

A half first cousin usually shares about half as much DNA as a full first cousin. That works out to an average around 6.25% shared DNA, with a wide range above and below that value. Similar logic applies for half second cousins and beyond, with each “half” connection cutting the expected shared DNA in half once again.

Removed Cousins

Removed cousins sit one or more generations apart. A first cousin once removed might be your parent’s first cousin or your cousin’s child. In both cases, the shared ancestors are the same grandparents, yet the extra generation means fewer shared segments.

A first cousin once removed averages about 6.25% shared DNA, or half that of a full first cousin. A first cousin twice removed averages about 3.1%, which matches a second cousin. When you drop more than one generation apart, DNA amounts overlap with other relationship types, so centimorgans alone cannot tell the full story without a family tree.

Reading Your Own Cousin DNA Matches

When your test results arrive, the match list sorts people by shared centimorgans. Cousins near the top of the list share more DNA and sit closer to you in the tree. Relatives lower on the list share less and sit further back in time.

If you see a match labeled as “first cousin,” check the centimorgan value and compare it with the expected ranges. A number between about 550 and 1,200 cM usually points to a first cousin, aunt or uncle, niece or nephew, or half sibling. To separate those options, look at age, known family ties, and tree data.

Relationship	Typical Shared cM Range	What That Often Means
First Cousin	550–1,200 cM	Shares grandparents with you
First Cousin Once Removed	200–650 cM	Shares grandparents, one generation apart
Second Cousin	75–360 cM	Shares great grandparents with you
Third Cousin	30–220 cM	Shares great great grandparents
Fourth Cousin	20–85 cM	Shares ancestors five generations back
Distant Cousin (5th+)	6–40 cM	Shared ancestor several generations back

These ranges overlap quite a bit. A match around 220 cM could fit as a close third cousin, a second cousin once removed, or another combination of shared ancestors. The percentage and centimorgan numbers give you a starting point, while records and family stories help narrow things down.

Why Some Cousins Share No Detectable DNA

As you move out to third cousins and beyond, the expected shared DNA gets small. By the fifth cousin level the average shared DNA sits well under 0.1%. At that point the random shuffle of recombination can easily erase all detectable shared segments between two cousins.

This is why a paper tree may show dozens of distant cousins while your match list only reports a handful. You do share ancestors, yet the DNA segments that came from those ancestors did not always pass down both lines. Genetic testing can only report segments that are still large enough and clear enough to stand out from background noise.

Using Cousin DNA Percentages In A Smart Way

Shared DNA with cousins is a handy clue, not a verdict. The percent and centimorgan amounts steer you toward the right slice of the tree. Names, dates, places, and records fill in the rest of the story.

When you wonder “how much dna do you share with your cousin?” you now have a ballpark number and a sense of how cousin types change that amount. You also have a feel for why real matches land across a range instead of one fixed value.