Second cousins usually share about 3 percent of dna, though real test results can run a bit lower or higher.
People ask how much dna second cousins share when they open a test result and see a new cousin near the top of the list. The number looks small compared with a parent or sibling, yet large enough to matter.
This guide explains who counts as a second cousin, how much dna second cousins share on average, why the range is wide, and how to use those numbers in family research.
Where Second Cousins Fit In Your Family
You and a second cousin share a pair of great-grandparents. Each of you descends from a different grandchild of that couple, so your parents are first cousins to each other.
Each generation cuts the shared dna roughly in half. Parent and child share about half their dna, first cousins share around one eighth, and second cousins share only a small slice that survives through four generations.
How Much Dna Does Second Cousins Share? Average And Range
Autosomal dna tests scan hundreds of thousands of markers across your chromosomes and look for long stretches that match another person. For second cousins, classic genetic math predicts that you will share about 3.125 percent of your dna, because you both inherited pieces from the same pair of great-grandparents.
Real test data lines up with that theory. A large consumer testing company reports that second cousins share an average of about 3.4 percent of their dna, with a typical range from just over 1 percent up to a little more than 6 percent. Another major provider lists an average near 3.13 percent with a similar band. Together, these charts show that most second cousin matches cluster close to three percent shared dna.
| Relationship | Average % Dna Shared | Typical Shared cM Range |
|---|---|---|
| Parent / Child | 50% | 3300–3800 cM |
| Full Sibling | 50% | 2300–3300 cM |
| Grandparent / Grandchild | 25% | 1300–2300 cM |
| First Cousin | 12.5% | 550–1400 cM |
| First Cousin Once Removed | 6.25% | 220–650 cM |
| Second Cousin | ~3.1% | 90–450 cM |
| Third Cousin | ~0.8% | 20–230 cM |
The exact figures in this table come from a blend of company help pages and large relationship studies. The relationship chart published by 23andMe lists about 3.4 percent shared dna for second cousins with a clear range around that value. Summaries from projects such as the Shared cM Project show similar centimorgan ranges for the same relationship.
How Much Dna Second Cousins Share On Different Tests
Every autosomal test reports shared dna in two ways: percent and total centimorgans, usually written as cM. Percent gives a quick sense of closeness, and centimorgans show how much of the genome lined up between two people.
Different companies use different marker sets and rules for trimming short segments. As a result, the same pair of second cousins might share slightly different totals in cM on different sites, yet still sit in the same expected band. Tools based on the Shared cM Project gather thousands of match reports and show that second cousins tend to share a little over 200 cM on average, with a wider range from roughly 86 cM up to a bit over 400 cM.
How Much Dna Does Second Cousins Share? Common Ranges In Practice
So how much dna does second cousins share when you read the numbers on screen? For most people, the match will sit somewhere around three percent shared dna with a shared total near 220–250 cM. Many results land a bit lower or higher, and that is still normal as long as the value stays inside the broader band.
Studies that combine company data and user reports show that second cousins often fall in these rough zones:
- Near 220–260 cM: fits a textbook second cousin very well.
- Near 300–400 cM: can still be a second cousin, though closer options such as first cousin once removed may also fit.
- Near 90–150 cM: sits at the low end for second cousins and may overlap with third cousins or other distant relationships.
These bands highlight how random inheritance works. Each person gets half of their dna from each parent, but the lengths and positions of the segments vary. Over four generations, that shuffle makes some second cousin pairs share more than expected and others share less, even when the genealogical distance is the same.
Why Some Second Cousin Matches Share More Or Less Dna
Two second cousins on paper can show very different shared dna values. Several common factors shape how much dna second cousins share in practice.
Random Recombination
During the formation of egg and sperm cells, parental dna is shuffled into new combinations. One sibling might inherit long blocks from one grandparent, while another receives shorter fragments from the same person. By the time you reach second cousins, that shuffle can widen the gap between their shared cM totals.
Multiple Family Links
Sometimes two branches of a family intermarry more than once. When that happens, cousins can be related along more than one ancestral path. A match labelled as a second cousin might then share more dna than expected, because segments come from two pairs of shared ancestors instead of one.
Population Background And Matching Rules
In groups with long shared history, distant relatives can share extra short segments that come from very old lines, not just recent great-grandparents. Testing companies apply different rules to trim or keep those segments. Those choices can nudge the reported cM up or down without changing the true relationship.
Using Second Cousin Dna Matches In Genealogy
Second cousin matches sit in a helpful middle ground for family research. They are close enough that the predicted label is usually right, yet far enough out that they can point to great-grandparent lines you may not have studied yet.
Start by confirming the paper trail. A cousin chart from a genealogical group or library shows how many generations stand between you and a match. Resources such as the cousin chart from FamilySearch explain how to count back to shared grandparents or great-grandparents in a clear grid.
If a match has a shared total on the edge of the range, use a centimorgan calculator built around the Shared cM Project to test other relationship options such as second cousin once removed or half second cousin. These tools show probability curves for each possibility at a given cM value.
Table Of Shared Dna Ranges For Second Cousins
This table gathers the main numbers for second cousins from theoretical genetics, company charts, and large user projects. Values are rounded for clarity.
| Source Type | Average Shared Dna | Typical cM Range |
|---|---|---|
| Theoretical Expectation | 3.125% | About 220–240 cM |
| Testing Company Chart A | 3.4% | About 110–630 cM |
| Testing Company Chart B | 3.13% | Near 150–400 cM |
| Shared Cm Project Data | Near 222–238 cM | 86–426 cM |
| Company Help Page Example | About 3% | Roughly 90–450 cM |
| Library Dna Handout | About 3% | Overlap with other charts |
| Practical Genealogist Reports | About 3% | Mostly 150–300 cM |
Taken together, these sources paint a steady picture. Second cousins hover around three percent shared dna. Most matches land in the low to mid-200 cM range, with some stretching higher or lower.
Key Points About How Much Dna Second Cousins Share
When you see a match flagged as a second cousin, the shared dna number tells you that you and this person almost certainly share a pair of great-grandparents.
From a dna standpoint, second cousins share enough genetic material to stand out clearly from random matches, yet not so much that they crowd your close-family list. Charts from major testing companies and long-running projects place them near three percent shared dna with totals often close to 220–250 cM. That knowledge makes your match list easier to read and sort.