🔬

Shared DNA (centimorgans) to Relationship Calculator

Enter the centimorgans shared with a DNA match to see the percentage of DNA shared, the most likely relationships and which are ruled out, using published shared-cM ranges, plus the segment count and largest-segment sanity check and what to compare next.

Shared DNA

Number of shared segments (optional)

Largest segment (optional)

Testing company total (for %)

Loading calculator…

How it works

% shared = cM ÷ total autosomal cM; expected share halves with each meiosis: parent 50%, grandparent/aunt 25%, first cousin 12.5%, second cousin 3.1%, third cousin 0.8%; ranges from crowd-sourced data (Shared cM Project)

Autosomal DNA tests report matches in centimorgans, a unit of recombination distance; each generation halves the expected amount inherited from an ancestor, so a parent shares about 3,500 cM, a first cousin about 850 and a second cousin about 230. Because recombination is random, the actual amount for any relationship scatters widely — second cousins range from roughly 40 to 600 cM — and several relationships overlap at every level, which is why a match can only be narrowed to a group and must be resolved with trees, ages and shared matches. Segment structure helps: a few long segments mean a recent ancestor, while many short ones in a large total point to endogamy, where a population's intermarriage inflates shared DNA. The ranges here follow the widely used Shared cM Project compilation.

Worked example

230 cM is 3.4% of the genome — squarely the second-cousin average (229 cM), but also inside the ranges for first cousin once removed (102–980), second cousin once removed (14–353) and third cousin (up to 234). With 12 segments averaging 19 cM and a 45 cM largest segment the match looks like a genuine second cousin; ages and shared matches decide between the options.

Frequently asked questions

Why does my half-aunt show the same cM as a grandparent?

Grandparent, aunt/uncle and half-sibling all share about 25% — one meiosis further than parent/child on different paths. DNA cannot distinguish them; age and X-DNA or other relatives can.

What about X-DNA and matches under 10 cM?

X-DNA has its own inheritance pattern useful for eliminating lines. Segments under about 7–10 cM are often false (identical by state, not descent) and should not be used to claim a relationship.

Related calculators

Cousin Relationship Calculator (Degree & Removed)

Work out how two people are related from the number of generations each is below their most recent common ancestor: first, second or third cousin, once or twice removed, the genealogical degree of kinship, the expected shared DNA in centimorgans and percent, and the canon-law and civil-law degrees.

Ancestors per Generation & Generation Year Calculator

How many ancestors you have at each generation (2ⁿ) and in total, roughly when they were born, how many generations back a given year is, who any ahnentafel number is (relationship, side, sex, path), when pedigree collapse sets in, and how many of those ancestors actually passed you DNA.

Probability Calculator

Work out a probability with the steps shown: favorable ÷ total outcomes with odds, two events (and, or, neither, conditional), repeated trials with the binomial distribution, and the area under a normal curve.

Age Difference Calculator

Find the exact age gap between two people from birth dates, birth years or ages — in years, months and days, decimal years, months, weeks and days — plus who is older, both ages on any date, the age ratio, when one was twice the other’s age and a milestones table.

Relationship ranges are population statistics from crowd-sourced data; they estimate, never prove, a relationship. Endogamy, pedigree collapse and testing-company differences shift the numbers.