Centimorgan to Relationship Calculator

A centimorgan (cM) is a unit of genetic distance, not physical distance — it measures how often crossover happens during meiosis. For genealogy purposes, the practical use is straightforward: more shared cM means a closer relationship.

The total human autosomal genome contains about 6800 cM (some define it as 7400; values vary slightly between testing companies). Two people share cM in chunks called segments. The total shared cM correlates with relationship distance.

Reference table from the Shared cM Project (compiled from millions of crowdsourced DNA matches):

• 3400+ cM: identical twins, parent-child, full siblings (parents share 0 cM with each other; their offspring share half with each parent)
• 2200-3400 cM: full siblings (the only relationship with this much sharing besides identical twins)
• 1300-2300 cM: aunt/uncle/niece/nephew, grandparent/grandchild, half-sibling
• 575-1330 cM: first cousin, great-aunt/great-uncle
• 200-620 cM: first cousin once removed, half first cousin, second cousin
• 75-360 cM: second cousin, half second cousin, first cousin twice removed
• 0-150 cM: third cousin and more distant; or half second cousin once removed; or unrelated

Note the overlap. A 200 cM match could be a half first cousin, a first cousin twice removed, a second cousin, or several other relationships. Total cM alone is rarely enough to determine the exact relationship — you need the largest segment size, age and ethnicity context, family tree information, and ideally additional matched relatives to narrow it down.

The “Shared cM Project” (created by Blaine Bettinger, hosted at dnapainter.com/tools/sharedcmv4) shows the full probability distribution for each cM value across all possible relationships. For 850 cM, for example, the most likely relationships are first cousin (40-50% probability) and half-aunt/half-uncle (20-30%), with smaller probabilities for several other options.

Half relationships share roughly half the cM of the equivalent full relationship. Half first cousins share about 425 cM on average; full first cousins share about 850 cM.

Removed relationships (the “removed” indicates a generation difference): first cousin once removed shares roughly 425 cM; first cousin twice removed shares about 213 cM. The “removed” generation halves the average shared DNA.

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.