Third cousins usually share under 1% of their dna, with a wide range that sometimes reaches zero and sometimes looks closer to a second cousin.
When a test shows a match as a third cousin, it is natural to wonder how much dna that person actually shares with you and what that means for your family tree. The phrase how much dna do third cousins share? appears on charts, testing dashboards, and genealogy blogs, yet the numbers can look confusing. Some matches share only a small segment, while others sit much closer to you than you expected.
This guide breaks the topic down into everyday language. You will see the textbook fractions, the real data from testing companies, and the reasons third cousin matches sometimes vanish or sit outside the “normal” range. By the end, you will know what a third cousin usually shares, what the centimorgan numbers mean, and how much trust you can place in that label.
Family Tree Basics Behind Third Cousin Dna Sharing
Third cousins share a pair of great great grandparents. If you sketch a tree, you and a true third cousin each move back four generations to reach that shared couple, then come forward again along different branches. On paper, that means you both descend from the same ancestral pair, but the path back to them goes through separate great grandparents and grandparents.
Each step back in the tree usually cuts the expected shared dna by half. Parents and children share about fifty percent, grandparents and grandchildren share around twenty five percent, and first cousins share roughly twelve and a half percent. By the time you reach third cousins, the theoretical average drops below one percent of the genome shared between two people.
| Relationship | Expected Dna Shared % | Typical Range In cM |
|---|---|---|
| Parent / Child | 50% | 3300–3700 cM |
| Grandparent / Grandchild | 25% | 1600–2300 cM |
| First Cousin | 12.5% | 575–1330 cM |
| Second Cousin | 3.125% | 75–360 cM |
| Third Cousin | About 0.8% | Roughly 30–200 cM |
| Fourth Cousin | About 0.3% | 0–140 cM |
| Fifth Cousin Or Beyond | Below 0.2% | 0–70 cM |
On smaller screens, swipe or scroll sideways to see the full table.
Theory gives you that neat fraction of roughly zero point eight percent shared between third cousins. In practice, real test results scatter around that value. Data gathered by testing companies, such as the average percent dna shared between relatives from 23andMe, show that third cousins often fall near that expected value but can sit much lower or higher.
How Testing Companies Describe Third Cousin Matches
Consumer dna services translate complicated statistics into labels such as “third to fourth cousin” or “second to third cousin.” Behind that simple label sits a centimorgan count, written as a number like 76 cM or 142 cM. A centimorgan is a unit used by geneticists to measure the length of shared segments along the chromosomes.
One large direct to consumer service gives an average of about zero point eight percent shared dna for third cousins, with a relationship range from roughly zero point zero six percent to just over two percent. Expressed in centimorgans, third cousins commonly share somewhere between around thirty and two hundred centimorgans, while a much smaller number fall outside that span.
When another service describes match categories, it places typical fourth cousins between twenty and eighty five centimorgans and more distant cousins down to six centimorgans. That means a weak third cousin match can overlap the upper end of a fourth cousin or even fall below the detection threshold, while a strong third cousin match can nudge close to a second cousin once removed.
How Much Dna Do Third Cousins Share? Math Versus Reality
Most basic charts answer the question how much dna do third cousins share? with a neat fraction and a tidy percentage. That answer is useful as a starting point, yet it hides the randomness built into inheritance. At each generation, you pass on a shuffled mix of dna rather than a fixed slice from every ancestor. Some great great grandparents leave long segments in your genome; others leave almost nothing.
Projects that crowdsource real match data show this clearly. For third cousins, researchers have reported observed averages around seventy to eighty centimorgans, with some documented matches climbing near two hundred centimorgans and some true third cousins sharing no detectable segments at all. That real world range explains why your testing dashboard sometimes calls a match “second to third cousin” even though the two of you share a known set of great great grandparents.
This randomness also shows up when several siblings test. One brother or sister may share a healthy third cousin match, another may share only a short segment, and a third may not match at all, while the documented relationship stays identical.
Detection Odds For Third Cousins On Dna Sites
Because third cousins usually share a modest fraction of dna, they sit on the edge of what tests can pick up. One genetics education article notes that the expected average percentage for third cousins rests under one percent, while a support page from a major testing company estimates that around nine out of ten true third cousins will share enough dna to show up as a match.
That leaves roughly one in ten third cousins who are related on paper but invisible in your results. Genetic genealogy references explain that this absence does not cancel the relationship. Instead, it reflects the fact that each person inherited different pieces of the shared ancestors’ chromosomes, so no stretch of dna is long enough to cross the detection threshold on either side.
Seen this way, a third cousin match is reassuring but not required. When your test shows several third cousin matches around the same family line, they can support your tree. When a particular third cousin does not appear in the list, it sits within normal expectations rather than hinting at a mistake.
How Much Dna Do Third Cousins Share? Practical Ranges
Every testing company sets its own detection cutoffs, yet the practical ranges for third cousins tend to overlap. Many genealogists treat something in the middle band, perhaps fifty to one hundred centimorgans, as a comfortable third cousin level, then use tree data, shared matches, and segment charts to refine the relationship.
At the low end, a third cousin may only share one or two short segments. Matches around ten to twenty centimorgans may still be real but are harder to confirm on their own. At the high end, when you see a match above one hundred and fifty centimorgans labeled as “third cousin,” it may reflect more than one connection in your tree, such as cousins who descend from the same couple on two different lines.
| Shared cM With Match | Approximate Dna Shared % | Third Cousin Interpretation |
|---|---|---|
| 0 cM | 0% | Paper third cousin with no detectable segment |
| 5–20 cM | Below 0.3% | Could be third cousin or more distant; needs tree evidence |
| 20–60 cM | About 0.3%–0.8% | Firm third cousin region on many charts |
| 60–120 cM | About 0.8%–1.5% | Strong third cousin match, sometimes labeled “second to third” |
| 120–200 cM | About 1.5%–2.5% | May reflect third cousin with extra shared lines or one generation off |
| Over 200 cM | Above 2.5% | More likely a closer cousin or double relationship |
On smaller screens, swipe or scroll sideways to see the full table.
When Third Cousin Dna Numbers Look Strange
Sometimes third cousin dna sharing seems far off the charts. You might find a known third cousin who shares only a small segment with you, or another who shares far more than any table suggests. In both cases, the odd number usually has a straightforward explanation tied to how recombination shuffles dna each generation.
Very low or zero shared centimorgans between confirmed third cousins fit the normal expectation that around ten percent of third cousins will not share enough dna to register as a match. High shared centimorgans can reflect cousins who connect in more than one way, such as families where two siblings from one branch married two siblings from another branch, or where distant cousins married each other in a small community.
Shared dna numbers also depend on the algorithm used by each testing company. Updates to the way segments are called and stitched together can shift centimorgan counts slightly while leaving the underlying relationship unchanged. This is one reason education pages from test providers stress that predicted relationships are estimates rather than fixed labels, especially for third cousins and beyond.
Main Takeaways About Third Cousin Dna Sharing
Third cousins usually share far less than one percent of their dna, yet that small amount can still reveal a shared set of great great grandparents. The textbook answer gives an expected share of around zero point eight percent, while real world projects and test results show an average centimorgan range from a few dozen to around two hundred centimorgans, with occasional matches outside that band.
Because inheritance is random, some true third cousins do not share any detectable dna, and some share enough to edge into the range of second cousins once removed. Labels on testing sites reflect probabilities, not guarantees. Treat the centimorgan count and the third cousin tag as a helpful starting clue, then let traditional records and shared match patterns confirm exactly where each relative fits in your tree for you.