Shared DNA and Relationship Chart: cM Ranges for Every Relationship

A shared DNA number on its own does not name a relationship. It narrows the possibilities, and how much it narrows depends on where the number falls. The chart below uses the Shared cM Project data, the standard reference compiled by Blaine Bettinger from tens of thousands of confirmed relationships, to show what each shared cM range typically implies. The overlap between ranges is the main point. The same cM amount can mean several different relationships, and the math alone cannot pick between them.

The chart

The numbers below are total shared cM. They are averages and observed ranges, not strict cutoffs, drawn from version 4.0 of the Shared cM Project as of early 2026.

Relationship	Average cM	Observed range
Identical twin	6,800	6,766-6,887
Parent or child	3,485	2,367-3,720
Full sibling	2,613	2,209-3,384
Grandparent, aunt or uncle, niece or nephew, half-sibling	1,750	1,160-2,436
Great-grandparent, first cousin, half-aunt or uncle, great-niece or nephew	850	396-1,397
First cousin once removed, half-first cousin	433	102-979
Second cousin, half-first cousin once removed	229	41-592
Second cousin once removed	122	14-353
Third cousin	73	0-217
Third cousin once removed, fourth cousin	35	0-127

The averages are tidy. The observed ranges are not, and the overlap between neighboring rows is the source of nearly every confused match interpretation.

Why the ranges overlap so heavily

Recombination is a random process. A first cousin is, on average, going to share around 850 cM with you, but the actual number for any given first cousin pair can fall anywhere from around 396 to around 1,397 cM. A half-first cousin shares around 433 cM on average, but the upper end of that range overlaps the lower end of the first cousin range. From shared cM alone, you cannot tell a low-end first cousin from a high-end half-first cousin.

The overlap gets worse as you move outward. A second cousin and a first cousin once removed share roughly the same average amount. By third cousin and fourth cousin, the ranges overlap so much that DNA alone cannot discriminate between them.

The hardest cluster: 1,300-2,300 cM

The most diagnostically frustrating range is 1,300-2,300 cM. Half-siblings, grandparents, aunts and uncles, and double first cousins all sit there. So do nieces and nephews. Two people who share 1,750 cM could be any of those relationships, and the cM alone will not pick. This is the range that drives the most search work for adoptees and donor-conceived people, and our finding half-siblings with DNA guide covers how to disambiguate it.

When cM is more definitive

The very top and bottom of the chart give cleaner answers. A shared cM total above roughly 2,400 is almost certainly a parent-child relationship, full sibling relationship, or identical twin. A shared total below about 130 cM is almost certainly a second cousin or more distant. The ambiguity lives in the middle.

Longest segment helps narrow some cases. A 1,750 cM match with a longest segment of 80 cM is more likely a half-sibling or a closer relationship; a 1,750 cM match with a longest segment of 30 cM and many smaller pieces is more consistent with a more distant relationship that just happens to share a lot of DNA by chance.

How to use the chart in practice

Look up the cM amount your match shares with you. Read the row it falls in, and the row above and below. Treat the union of those three rows as your candidate set. Use tree data, age differences, and other shared matches to narrow further.

The relationship predictor tool on this site automates the chart lookup and returns probabilities for each candidate relationship. It is more useful than a single-relationship guess because it respects the overlap rather than hiding it.

For the unit itself, see understanding centimorgans. For how matches are produced in the first place, see how DNA matching works. For the pillar overview, see the ancestry and genealogy guide.

A note on the source. The Shared cM Project is the work of Blaine Bettinger and many volunteer contributors who submitted confirmed relationship and cM data. The current version is 4.0. We use it because it is the largest and most carefully assembled dataset of its kind in consumer genealogy, and because it is open and updated periodically.