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What a shared DNA number really means
A DNA test gives you a list of matches, and each match comes with a number: the amount of DNA you share, measured in centimorgans, or cM. The number looks precise, but the relationship behind it is not. The same amount of shared DNA can come from several different relationships, and the amount varies even between people with the same relationship. This estimator turns a cM number into a range of likely relationships, which is the honest way to read it.
Think of shared DNA as a clue, not a label. A parent and child share about 3,400 cM. Full siblings share a similar amount, but with more variation. First cousins usually share around 800 to 900 cM, but some share far more or far less. By the time you reach third cousins, the shared amount is small, and some real cousins share so little that they do not appear as matches at all. The estimator below maps your number to the group of relationships it fits best.
How to use the estimator
- Find the shared cM figure on your match’s page. Use the total shared cM, not the longest single segment.
- Enter the number in the tool.
- Read the likely range, not just the first relationship named.
- Compare the range with your family tree: which people in that range are missing from your tree?
- Check ages, places and shared matches before you conclude anything.
A worked example
Say a match shares 900 cM with you. The estimator puts that in the first cousin range. That does not mean the person is your first cousin. It means first cousins are the most common relationship at that level, but the range also covers great-aunts and uncles, half-aunts and uncles, and first cousins once removed, among others. Your next step is not to label the match, but to look at shared matches: if you share matches with your mother’s side only, the match probably sits on that side.
Now take a smaller number. A match shares 120 cM. That sits in the third cousin range, but the real relationship could be anywhere from second cousin once removed to fourth cousin. At this level, the tree matters more than the number. Look for a shared ancestor three or four generations back, and look at the match’s tree for surnames and places that overlap with yours. Small matches are often the ones that break down a brick wall, precisely because they point to a branch you had not explored.
Why the same number fits several relationships
DNA is passed down in chunks, and the chunks are shuffled each generation. Two siblings get different mixes from the same parents, so one sibling may share 2,600 cM with a cousin while the other shares 2,100 cM. The same shuffling means a grandparent and grandchild share about 1,700 cM on average, but the real figure varies. Add the fact that different testing companies measure shared DNA slightly differently, and you see why a single number can never name one relationship with certainty.
There is another twist. In families where cousins married cousins, or where a small community married within itself for generations, people can share more DNA than their closest relationship suggests. This is called endogamy, and it makes distant cousins look closer than they are. If your family comes from a small island, a remote valley or a close-knit community, treat small and medium matches with extra care, and lean on paper records more than on the number.
What this tool never does
This tool estimates relationship ranges from shared DNA. It never gives a definitive relationship, and it is not medical advice and not proof of ancestry. A cM number cannot prove who your father was, cannot diagnose anything, and cannot settle a legal question on its own. If a match raises a sensitive family question, pause before acting: talk to family, check paper records, and consider that the other person may not know what you now suspect. DNA results affect real people, not just trees.
Also be careful with very small matches. Below about 50 cM, a match may be distant, or the shared segments may be coincidence rather than a real shared ancestor. Many researchers ignore matches under 20 or 30 cM unless other evidence supports them. The estimator flags this range as distant or uncertain for that reason. A small number is a lead to test, not a cousin to claim.
Putting the range to work
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