Shared centimorgans (cM) and DNA relationship prediction

Guide · Updated July 2026

A centimorgan (cM) is a unit of genetic distance, and the total number of centimorgans two people share is what predicts how closely they are related. More shared cM means a closer relationship: full siblings share roughly 2,600 cM, a grandparent or half-sibling around 1,700 cM, an aunt or uncle around 1,750 cM, and a first cousin around 866 cM. The ranges overlap, so a single number narrows the possibilities rather than settling them.

What a centimorgan measures

A centimorgan measures genetic linkage, not physical length. It is the distance over which there is about a one percent chance of recombination in a single generation. When a DNA test reports that you share, say, 1,730 cM with a match, it is summing the lengths of every DNA segment the two of you share.

Reading a shared cM chart

Because closer relatives share more DNA, the total maps onto a set of likely relationships. Approximate averages:

Approx. shared cM	Typical relationship
~3,485 cM	Parent / child
~2,600 cM	Full sibling
~1,700–1,750 cM	Grandparent, half-sibling, aunt/uncle, niece/nephew
~866 cM	First cousin
~433 cM	First cousin once removed / half first cousin
~229 cM	Second cousin
~74 cM	Third cousin

Notice the overlap: a grandparent, a half-sibling and an aunt can all fall around 1,700–1,750 cM. The number gives you a shortlist, and you use the tree and known ages to decide which fits.

The Shared cM Project

The standard reference for turning a cM total into a relationship shortlist is the Shared cM Project, a crowdsourced dataset of tens of thousands of submitted totals with the known relationship for each. It gives observed ranges, not just theoretical averages, which is what makes the overlap between relationships visible. AncestorOS predicts relationships by mapping a match’s shared cM onto bands derived from the Shared cM Project.

Where the prediction breaks down

Overlapping ranges — as above, one number can fit several relationships.
Endogamy — in populations that intermarried for generations (for example Ashkenazi Jewish or Acadian), people share more DNA than their genealogical relationship implies, so standard charts overestimate closeness.
Distant matches — below roughly 50–90 cM the relationship becomes very uncertain and some shared segments are coincidental.

Frequently asked

A centimorgan (cM) is a unit of genetic distance — the distance over which there is about a one percent chance of recombination per generation. In DNA testing, the total shared cM predicts how closely two people are related.

First cousins share roughly 866 cM on average, though the observed range runs from about 550 to 1,300 cM. Second cousins average about 229 cM.

Because the ranges overlap. A grandparent, half-sibling and aunt/uncle can all share around 1,700–1,750 cM. You combine the cM total with the tree and known ages to decide which relationship fits.

A crowdsourced dataset that collects tens of thousands of real shared-cM totals alongside the known relationship, producing observed ranges. It is the standard reference for interpreting a match's shared cM.

No. AncestorOS logs DNA matches by manual entry — there is no CSV import from AncestryDNA, 23andMe, MyHeritage or FamilyTreeDNA. Once a match's shared cM is entered, it predicts the likely relationship from that total.

Put it into practice

AncestorOS gives you Evidence Explained citation templates, Genealogical Proof Standard–aligned research logs, GEDCOM import and export, and DNA match tracking in one web-based workspace.

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