Centimorgan to Relationship Calculator

A centimorgan (cM) is a unit of genetic distance rather than physical distance. It measures how often crossover happens during meiosis, which is why it predicts inheritance where counting base pairs would not. For genealogy the practical use is simple: more shared cM means a closer relationship.

The autosomal genome used by every match table contains about 6,800 cM. You will sometimes see 7,400, which includes the X chromosome; the tables below and every figure on this site use 6,800.

Two people share cM in chunks called segments, and the total across all segments is what maps onto relationship distance.

Reference bands, from the Shared cM Project (compiled from millions of crowdsourced matches):

• Above 5,000 cM: identical twins only. Twins share essentially the whole genome, close to 6,800 cM. Nothing else comes near.
• 3,400 to 3,720 cM: parent and child. This band is remarkably tight, because a child inherits exactly half from each parent with no room for the shuffle to vary it.
• 2,200 to 3,400 cM: full siblings. The only relationship that lands here.
• 1,300 to 2,300 cM: aunt, uncle, niece, nephew, grandparent, grandchild, half sibling. All four sit on top of each other and DNA cannot separate them.
• 575 to 1,330 cM: first cousin, great-aunt, great-uncle, great-grandparent, half aunt or uncle.
• 200 to 620 cM: first cousin once removed, half first cousin, second cousin.
• 75 to 360 cM: second cousin, half second cousin, first cousin twice removed.
• Under 75 cM: third cousin and beyond, or genuinely unrelated.

A parent and a full sibling both average about half your DNA, and the way to tell them apart is not the total at all: it is the number of fully identical segments. A parent shares one copy of every chromosome with you and no fully identical regions; a full sibling has stretches where both copies match. Every testing company reports this, usually as “fully identical regions” or FIR.

Note how badly the bands overlap. A 200 cM match could be a half first cousin, a first cousin twice removed, a second cousin, or several other things. Total cM alone rarely settles the exact relationship. You need the largest segment size, ages, ethnicity context, family tree information, and ideally other matched relatives to narrow it.

The Shared cM Project (created by Blaine Bettinger, hosted at dnapainter.com/tools/sharedcmv4) shows the full probability distribution for each cM value across every possible relationship. For 850 cM the most likely answers are first cousin at 40 to 50%, and half-aunt or half-uncle at 20 to 30%, with smaller probabilities spread across several others.

Half relationships share roughly half the cM of the full equivalent. Half first cousins average about 449 cM against 866 for full first cousins.

“Removed” means a generation difference, and each removal roughly halves the shared DNA. A first cousin once removed averages about 433 cM; twice removed, about 213.

Endogamy effect: in populations with extensive intermarriage (Ashkenazi Jewish, Acadian French, Polynesian island groups), individuals share more DNA than the table predicts because they descend from multiple common ancestors. Real cousins in endogamous populations may show 30-100% more cM than predicted.

For the most accurate breakdown, plug your cM value into the DNA Painter “What Are The Odds” (WATO) tool, or compare against the official Shared cM Project chart at the link above. This calculator returns the most likely categories quickly; for unique cases the full distribution is more informative.