🔄 DNA Percentage to Centimorgan Converter

DNA testing companies report shared DNA as either a percentage or a centimorgan (cM) total. Convert between the two using a commonly cited total autosomal genome length of approximately 6,800 cM. This is an approximation — the exact total used varies slightly by company.

Percent DNA shared (%)

OR Centimorgans shared (cM)

Total autosomal genome basis (cM)

One ratio, run in either direction

A cM total is a length, and a percentage is that length against the whole autosomal genome, so the conversion is one proportion. Whichever box you last typed in is treated as the input:

cM      = (percent / 100) × basis
percent = (cM / basis) × 100

basis = 6,800 cM (default) or 7,400 cM

Leave the default 25% in place and the tool returns 1,700.0 cM. Switch the basis to 7,400 and the same 25% becomes 1,850.0 cM — the percentage did not change, only what it is a percentage of. Going the other way, 850 cM against 6,800 reads 12.500%. Percentages print to three decimals and cM to one.

Choosing the basis and reading the result

Match the basis to the source of your number. The two options differ by about nine per cent, so a percentage taken from one company and converted on the other company's basis is wrong by roughly that much before you start.
Companies do not all report the same way. Some show a percentage, some a cM total, and their thresholds for small segments differ, so two sites can disagree about the same pair of testers.
A converted total is still just a total. A shared cM chart will return several possible relationships for it, because the published ranges overlap heavily and are observed distributions rather than rules.

Full siblings are the awkward case. Where both parents' copies match, the segment is fully identical, and companies differ on whether to count that stretch once or twice — which is why sibling percentages and sibling cM totals do not convert cleanly into each other.

Frequently asked questions

How many centimorgans is 25% shared DNA?

About 1,700 cM on the 6,800 cM basis, or 1,850 cM on the 7,400 one. A grandparent, an aunt or uncle and a half sibling all sit at that level, so the number cannot separate them.

What percentage is 850 cM?

12.5% on the 6,800 cM basis. That is the average for first cousins, though real first cousins run from roughly 575 to 1,330 cM.

Why does my match show a different cM total on another site?

Different genome bases and different minimum segment lengths. A gap of a few per cent is normal and does not mean either site is wrong.