How Much Dna Do 2Nd Cousins Share? | Dna Match Guide

If you have a new match on a testing site, one of the first questions that pops up is how much DNA you should share with that person. When the site labels someone as a second cousin, the numbers on the screen can feel abstract. Understanding what those centimorgans mean helps you decide whether that second cousin label fits your family story.

This guide walks through what second cousins are on a family tree, how much DNA they usually share, what a normal range looks like, and why real results bounce around that range. You will also see how testing companies group matches and how to use your shared DNA with a second cousin to push your research in the right direction.

How Much Dna Do 2Nd Cousins Share? Core Numbers First

On paper, second cousins share great-grandparents, so each person sits three generations away from the shared couple. With that setup, genetic theory says that second cousins share on average 3.125% of their DNA. In practice, testing companies usually report a match between about 100 and 350 centimorgans (cM), with an average near 200 to 230 cM and a wider possible span from roughly 40 up to 600 cM in rare cases.

To see where second cousins sit compared with other relatives, it helps to place them in a small comparison chart.

Relationship	Average Dna Shared (%)	Typical Shared Range (cM)
Parent / Child	50%	2,300–3,700
Full Sibling	50%	2,200–3,400
Grandparent / Grandchild	25%	1,400–2,100
Aunt / Uncle – Niece / Nephew	25%	1,300–2,300
First Cousin	12.5%	400–1,400
Second Cousin	3.125%	40–600
Third Cousin	0.78%	0–230
Second Cousin Once Removed	About 1.5%	30–400

Even at a glance, you can see that second cousins are several steps out from the close family circle but still share enough DNA to show up as clear matches. Testing projects based on large sets of real data show that true second cousins match nearly every time, even though the number of shared centimorgans can slide up or down within that wide range.

What Makes Someone A Second Cousin?

Before going deeper into numbers, it helps to be clear on the relationship itself. You and a second cousin share a pair of great-grandparents. Your grandparents are siblings, your parents are first cousins, and you sit in the same generation as your second cousin.

A simple way to sketch it is to start with the shared great-grandparents at the top. They have two children who grow up to be your grandparent and your cousin’s grandparent. Each of those grandparents then has a child, who become first cousins. The next generation, the children of those first cousins, are second cousins. That path through the tree explains why the shared DNA drops compared with first cousins, who share grandparents instead.

The number of centimorgans that shows on your test reflects those layers of separation. Every time you step down a generation from a shared ancestor, the expected share of DNA roughly halves, but the random way DNA is shuffled in each generation adds a lot of variation as well.

Average Dna Shared By 2Nd Cousins Across Families

When people ask how much dna do 2nd cousins share, they usually want more than a textbook percentage. They want to know whether a specific match looks normal. Large data sets gathered by genetic genealogists give a practical answer.

The Shared cM Project, summarized at DNA Painter and described in detail on genetic genealogy blogs, collects real match data from many thousands of testers. Across that pool, second cousins share an average of about 229 cM, with most matches falling between about 100 and 350 cM and a long tail of lower and higher results.

That means a second cousin who shares 220 cM with you fits right on target. A match at 120 cM still fits second cousin territory, but it may also match patterns for other relationships, such as a first cousin twice removed or a half second cousin. A match at 400 cM sits near the upper edge for second cousins and might point to a closer tie, such as a first cousin once removed, a half first cousin, or even a more complex mix through cousin marriages.

At the same time, outliers exist. The Shared cM charts show that a small slice of second cousins can share under 100 cM. When that happens, the family tree still matches the second cousin position, but DNA shuffling has trimmed the shared segments.

How Testing Companies Label Second Cousin Matches

Each major testing company takes your total shared cM with a match and assigns a relationship label or category based on ranges. The exact ranges differ slightly by company, yet the broad approach is similar. Second cousin sized matches sit in the middle ground between close family and distant cousins.

Ancestry, for instance, places matches in a second cousin category when the shared DNA usually falls between about 200 and 400 cM, with some overlap into nearby categories. Their AncestryDNA match categories page explains how match categories are tied to shared cM bands and how more than one relationship type can fit the same band.

Other companies may not use the same wording, but they also lean on total centimorgans and the number of segments. Some tools let you enter a cM value into a calculator to see likely relationships. The Shared cM Project tool at DNA Painter is a popular example, since it shows both the average and the full range for second cousins and related relationships on one screen.

For you as a tester, that means the label on the match list is a starting point. It points you in the right direction, yet you still need to test the fit against your tree. A person shown as a second cousin may in fact be a half second cousin, a first cousin once removed, or share more than one line with you.

Why Second Cousin Dna Amounts Vary So Widely

Even when the paper trail is clear, second cousin matches can carry a wide spread of shared DNA values. Four main forces drive that spread: the randomness of recombination, half relationships, double relationships, and background relatedness inside a region or group.

Random Shuffling At Each Generation

When eggs and sperm form, each parent passes down a mix of their own parents’ chromosomes. That mix is not an even slice of each grandparent. Segments break and swap in a largely random pattern, so one child might carry slightly more DNA from one grandparent than another.

Over three generations between great-grandparents and second cousins, that random shuffle adds up. Some second cousins inherit long segments from the same great-grandparent and share more DNA. Others inherit shorter or fewer overlapping segments and sit near the lower edge of the range, even though the relationship is the same on the tree.

Half Second Cousins And Other Unequal Links

Not every second cousin shares both great-grandparents with you. In a half second cousin relationship, you share only one great-grandparent. That happens when one of the pair of great-grandparents had children with another partner in a later or earlier relationship.

Because half second cousins share fewer ancestors, they often share less DNA. A half second cousin might line up in the range for a third cousin or a second cousin once removed. Only by placing the person in your tree and checking the paths down from the shared ancestor can you tell that the match really is a half relationship.

Double Second Cousins And Endogamy

Sometimes two siblings from one branch marry two siblings from another branch. Their grandchildren then end up related through both lines. In that case, a person might be your second cousin in more than one way at once. Those double links add together, so the shared DNA can climb well above the usual average for second cousins.

In some regions, people marry within the same group for many generations. The same ancestors appear in several spots on the tree, so cousins connect in more than one way. That background relatedness can raise the shared DNA between people who sit on paper as second cousins or even more distant cousins.

Using Second Cousin Matches In Your Research

Genetic matches at the second cousin level can be valuable tools when you want to extend a family line, test a theory, or confirm a paper trail. Their shared DNA is high enough to be reliable, yet distant enough that they often hold branches of the tree you do not know yet.

Testing A Known Second Cousin

If you already know a second cousin from family records, testing that person brings several benefits. Once the results arrive, you can check whether the total cM lines up with the second cousin range. If it sits far outside the expected span, that may hint at a half relationship, an extra link, or a break in the tree that deserves more work.

Next, you can look at shared matches. Most testing sites show a list of people who match both you and that known second cousin. Those shared matches likely tie into the same great-grandparent branch, which helps you group matches and target the right part of your tree.

Working With Unknown Second Cousin Matches

When an unknown person appears on your list in the second cousin range, they often carry strong clues about a missing ancestor. Start by checking their public tree, if they have one, to see whether any surnames, places, or older ancestors look familiar.

If no tree is available, shared matches are again useful. Cluster the people who match you and that second cousin match, look for common surnames or shared hometowns among them, and compare those patterns with what you already know about your own tree.

In some cases, the shared DNA with a second cousin level match may uncover a new branch of recent family that you did not know existed. When that happens, progress slowly and kindly, since not everyone welcomes contact about new family ties right away.

Second Cousin Shared Dna Ranges And Odds

To pull the numbers together in one place, the next table sums up the main shared DNA ranges for second cousins and some nearby relationships. These figures come from large shared cM data sets and are meant as guideposts, not fixed rules.

Shared Dna (cM)	Likely Relationships	Comment
40–90 cM	Low second cousin, third cousin, other distant cousin	Fits second cousin only at the edge of the range
90–150 cM	Second cousin, first cousin twice removed, half second cousin	Tree details needed to sort the options
150–250 cM	Typical second cousin range	Most second cousins fall somewhere in this span
250–350 cM	High second cousin, first cousin once removed, half first cousin	Check ages and tree position for each person
350–450 cM	Upper edge for second cousin, likely closer cousin	Look for shared grandparents or great-grandparents
450–600 cM	Very high second cousin, close to first cousin once removed	Often points to a closer or double relationship
0–40 cM	Usually third cousin or more distant	True second cousins rarely sit in this range

When you see a second cousin label on a test result, compare the centimorgans with this table to decide whether the number matches the label. If it does, treat second cousin as your leading theory. If the number lines up better with a nearby relationship, such as first cousin once removed, keep both options in mind while you build out the tree.

Privacy, Consent, And Wise Use Of Dna Tests

Matches with second cousins can answer long-standing questions, but they also touch sensitive parts of family life. Before you share screenshots, charts, or detailed trees that include living people, make sure you have permission from anyone whose data is involved.

Take time to read your test provider’s privacy policy and match-sharing settings. Some companies let you turn off matching or limit what others see about you. Those tools help you control how much of your genetic information is visible to cousins who test in the same database.

Genetic tests are not medical tools. Shared DNA with a second cousin tells you about relatedness, not about health risks, diagnoses, or treatment choices. For those questions, health care professionals and clinical tests are the proper route.

Final Thoughts On Second Cousin Dna Sharing

So, how much dna do 2nd cousins share in real life? On average, they share just over three percent of their autosomal DNA, usually a total near 200 to 230 cM, with a wide normal span from about 100 up to the low 300s for most matches and rare results that fall below or above that band.

When you pair that range with what you know from your family tree, second cousin matches turn into practical anchors. They help you group distant relatives under the right great-grandparent couple, point out half or double relationships, and reveal new branches to research next.

The numbers on the screen never tell the whole story by themselves, yet once you understand how much DNA second cousins share and why the ranges look the way they do, those numbers become steady guides instead of confusing codes.