On average, 3rd cousins share about 0.78% of their dna, around 53 centimorgans, though actual dna sharing can range from roughly 0% to 2%.
How Much Dna Do 3Rd Cousins Share?
Many testers ask how much dna do 3rd cousins share? after a new match appears on a site such as AncestryDNA or 23andMe. The textbook answer uses the simple rule that each generation halves shared dna. Third cousins sit at the level where you share great-great-grandparents, so the expected share is one part in 128, or about 0.78% of your autosomal dna.
In practice, dna testing companies talk in centimorgans, the unit that measures genetic linkage across the chromosomes. A widely used chart, based on data compiled by genetic genealogist Blaine Bettinger, puts the average third cousin match at about 53 centimorgans, usually spread over several small segments. Some known third cousins fall close to that average, while others drift above or below it because dna passes down in a random pattern.
| Relationship | Typical Shared Dna % | Typical Shared cM |
|---|---|---|
| Parent / Child | 50% | 3400 cM |
| Full Sibling | 50% | 2600–3400 cM |
| First Cousin | 12.5% | 575 cM |
| Second Cousin | 3.125% | 212 cM |
| Third Cousin | 0.781% | 53 cM |
| Third Cousin Once Removed | 0.39% | 35 cM |
| Fourth Cousin | 0.195% | 35 cM |
| More Distant Or Unrelated | <0.1% | 0–20 cM |
These figures line up with autosomal dna statistics collected by the International Society of Genetic Genealogy, which lists an expected 0.781% shared dna and about 53 centimorgans for third cousins under assumptions such as no endogamy and no extra relationships between the lines.
Third Cousin Dna Share By Percent And Centimorgans
Charts give a neat point value, yet real matches rarely land on that exact number. The question how much dna do 3rd cousins share? always needs a range as well as an average. Data from the Shared cM Project shows that confirmed third cousins can share anywhere from 0 cM to more than 200 cM, though most matches cluster near the middle of that span.
Centimorgan totals help narrow the field of possible relationships. A match well inside the expected third cousin band is more likely to sit near that level in your tree than a match at the edges. When the shared dna moves near 0 cM or near 200 cM, the label on the match list becomes more of a guess and you need extra context from trees and shared matches.
Why The Average For Third Cousins Sits Around 0.78%
Autosomal dna follows a simple rule on paper: you receive half from each parent, one quarter from each grandparent, and so on. Third cousins share great-great-grandparents, which puts them six steps apart in the family tree. If you multiply one half six times, you get one sixty-fourth. Because the relationship runs through both sides of the tree, the combined share for third cousins comes out to one part in 128, or 0.78125%.
Centimorgan Ranges Third Cousin Matches Often Show
The Shared cM Project, hosted at the DNA Painter site, collects real match data from tens of thousands of testers. In that dataset, third cousins average around 73 cM, with a reported range from 0 cM up to roughly 234 cM. That spread explains why two different third cousin matches can look nothing alike inside your match list even when the paper relationship lines up.
A match at the low end, with only one short segment around 20 cM, might still be a true third cousin. On the high end, a shared total above 150 cM might point to a slightly closer connection such as half second cousin once removed or two different paths to the same ancestors. The centimorgan number narrows the options, yet the family tree and shared matches still finish the puzzle.
Why Some 3Rd Cousins Share More Dna Than Others
Two third cousins descended from the same couple can share a wide range of dna. One pair might share near the textbook 53 cM, another pair might share over 150 cM, and a third pair might not share any detectable segments at all. The rules of inheritance set the overall odds, but chance governs which exact pieces of dna travel down each branch.
Random Recombination Across Generations
Every child receives a mix of dna segments that come from the grandparents through each parent. During meiosis, chromosomes swap small chunks of sequence, so each egg or sperm carries a different mosaic of segments. By the time five or six generations pass between the shared ancestors and the present testers, many small segments drop away or split into pieces too tiny for testing companies to count.
Pedigree Collapse And Endogamy
In families where cousins married cousins across several generations, the same ancestor can appear in the tree more than once. Genetic genealogists call this pedigree collapse or endogamy. In those settings, people who show as third cousins on paper might share extra segments because they descend from the shared couple through more than one branch.
Testing companies often respond by giving these matches a closer predicted relationship than the documented third cousin label. You might see a guess such as second cousin once removed while the actual relationship spans more generations. In endogamous groups, that question has a wider answer, and centimorgan totals need to be interpreted with extra care.
Half Third Cousins And Double Third Cousins
Some third cousins share only one great-great-grandparent instead of a couple. That pattern produces half third cousins, who usually share about half as much dna as a standard third cousin. Where a typical third cousin averages around 53 cM, a half third cousin might sit closer to 25 cM even when the paper research is firm.
The opposite situation appears with double third cousins. In that case, two siblings from one line married two siblings from another line, so descendants inherit dna from the same couple twice over. Double third cousins can share two to three times as much dna as ordinary third cousins, so their centimorgan totals sometimes overlap with second cousins in the match list.
Using Third Cousin Dna Matches In Your Family Research
Distant matches can feel small beside the long bars of close relatives, yet third cousins matter a lot for genealogical work. They often hold pieces of family history and records that your own branch never kept. When you read that percentage question as how much information they can add, the answer grows much larger.
Check The Reported Relationship Label
Each testing service tags matches with a predicted relationship band based on centimorgan totals. For a match in the third cousin range, the label might show third cousin, third cousin once removed, or second to fourth cousin. Before you build elaborate theories, check whether the shared amount fits within the ranges for third cousins in tools like the Shared cM Project tool.
If the number fits inside that band, treat third cousin as a strong contender instead of a fixed answer. Build out the match’s tree as far as you can, look for surnames or locations that overlap with your own tree, and then test the idea by adding records and more dna matches. When several lines of evidence point to the same pair of great-great-grandparents, you can be more confident that the relationship label fits.
Combine Dna With A Document Trail
Dna alone rarely names the exact ancestor you share with a match. To turn a centimorgan number into a story, you still need records such as census schedules, parish registers, civil registrations, and family letters. Third cousin matches give you a clue about which branch to work on, then the document trail narrows the search until a shared couple stands out.
The autosomal dna statistics page maintained by the International Society of Genetic Genealogy includes probability tables that show how often different testing companies report matches at each cousin level. In that chart, third cousins match around ninety percent of the time, which means you still have a fair chance to confirm a paper connection even when some cousins in the same generation do not match at all.
Use Third Cousins To Map Segments
When a third cousin match transfers their raw data to a site that offers chromosome browsing, you can see precisely which segments you share. If you also have dna from known relatives on the same branch, those shared segments help map which parts of each chromosome trace back to the mutual great-great-grandparents.
Over time, third cousin matches and slightly closer or more distant cousins can help you trace each chunk of your genome back to specific lines, even when the underlying dna share remains small in percentage terms.
| Shared Dna (cM) | Possible Situation | Practical Next Step |
|---|---|---|
| 0 cM | Documented third cousins with no detected dna | Test older relatives or siblings from both branches |
| 1–30 cM | Half third cousin or distant branch with small segments | Treat as tentative third cousin; confirm with records |
| 30–80 cM | Typical third cousin range | Prioritize tree building and shared matches |
| 80–150 cM | High sharing third cousin or slightly closer cousin | Check for multiple paths or second cousin scenarios |
| 150–234 cM | Upper edge of reported third cousin matches | Search for double relationships or pedigree collapse |
| >234 cM | Too high for normal third cousin match | Reassess; likely closer relationship than third cousin |
| Any value | Endogamy or dense cousin matches | Use segment location and multiple testers to sort lines |
Bringing Third Cousin Dna Sharing Into Perspective
Third cousin matches sit at a helpful spot for genetic genealogy. They share enough dna to show up in most databases, yet they reach branches of the tree that close relatives never touch. Once you understand the usual 0.78% share and its range, it becomes easier to place each match in your research.
Instead of chasing a single magic number, treat the question of how much dna third cousins share as a guide to likely relationships. Check that your match falls within published ranges, then use records, trees, and shared matches to confirm the shared ancestors. Small shifts from the average are common in third cousin dna match results.