Blood Percentage and Genetic Relatedness Calculator

Why shared DNA is not a fixed number

The simple rule is that you inherit exactly 50% of your DNA from each parent. Beyond that, every generation gets shuffled by recombination: chromosomes randomly cross over during egg and sperm formation, so siblings do not share an identical 50% with each other. They share about 50% on average, but the actual number can range from 38% to 61% across full siblings of the same parents. This is part of why two full siblings can look quite different and why DNA tests sometimes give surprising “you are related less than expected” results.

Centimorgans, not just percentages

DNA testing companies (Ancestry, 23andMe, MyHeritage) report shared DNA in centimorgans (cM) rather than percentages. A centimorgan measures genetic distance, not physical distance: it is how much of the genome tends to travel together through a generation.

The autosomal total used throughout genetic genealogy, and by the Shared cM Project, is about 6,800 cM. So one percent is roughly 68 cM, and:

• 50% is about 3,400 cM (a parent and child, or full siblings on average)
• 25% is about 1,700 cM (half siblings, grandparents, aunts and uncles)
• 12.5% is about 850 cM (first cousins, great-grandparents)
• 6.25% is about 425 cM (half first cousins, first cousins once removed)
• 3.125% is about 213 cM (second cousins)

You will also see 7,400 cM quoted. That figure includes the X chromosome, which most match tables exclude, and mixing the two is where a lot of confused arithmetic starts. This site uses 6,800 throughout.

Expected against actual

The theory column is the arithmetic. The average column is what millions of real test results actually show, collected by the Shared cM Project. They are close but not identical, and the range column is the reason the averages matter more.

Notice that a great-grandparent and a first cousin sit at the same 12.5% and share almost exactly the same amount of DNA. They get there by different routes. A great-grandparent is three steps straight up one line: ½ × ½ × ½ = 12.5%. A first cousin is four steps around a corner, ½ to the fourth, which is 6.25%, but doubled because you descend from two shared grandparents rather than from one person. Both land in the same place, and DNA has no idea which of the two it is looking at.

Notice too how far the observed ranges overlap. A 1,200 cM match could be a half sibling, a grandparent, an aunt or a great-grandparent, and no amount of staring at the number will separate them. Ages and known family structure do that.

Where DNA tests can mislead

At third cousin and beyond, random inheritance can leave two genuine relatives sharing 0 cM (no detectable DNA in common) even though they descend from a common great-great-grandparent. This is why genealogy DNA matches fade out around 4 to 5 generations back. The further back you go, the more shared DNA falls below detection thresholds.

The tests are very accurate inside 3 generations (parent through second cousin) and progressively less reliable beyond. Treat any zero match outside that range as “we cannot tell” rather than proof of no relationship.