Ancestors per generation.

Worked example

Five generations back are your 3rd great-grandparents, also written great-great-great-grandparents. There are 2⁵ = 32 of them, 16 on each parent’s side. Counting every ancestor from your parents through that generation gives 2⁶ − 2 = 62 people. If DNA were split evenly, each of the 32 would account for 100 ÷ 32 = 3.125% of your DNA on average.

How it works

Each person has two parents, so the count doubles with each step back: 2 parents, 4 grandparents, 8 great-grandparents, 16 2nd great-grandparents. Generation n has 2ⁿ ancestors, half on each parent’s side, and all generations up to n total 2ⁿ⁺¹ − 2. Each ancestor contributes 1 ÷ 2ⁿ of your DNA on average; the real share varies from person to person.

Real trees have fewer distinct people than this. Without any shared ancestors a person’s tree would hold about 2³⁰, roughly 1 billion, ancestors 30 generations back, more than the world population at the time of the High Middle Ages. The paradox is explained by shared ancestors, called pedigree collapse. For example the child of two first cousins has at most six great-grandparents, not eight.

Sources and basis (opened 2026-10-08): Pedigree collapse - Wikipedia (binary-tree count, 2³⁰ example, first cousins have at most six great-grandparents); DNA Painter, Shared cM Project 4.0 tool v4 (last updated 26 March 2020) (great-grandparents average 887 cM of the 7,440 cM total).

FAQ

How many great-grandparents do I have?

Eight. You have 2 parents, 4 grandparents and 8 great-grandparents, and the count doubles with each generation.

How many 3rd great-grandparents do I have?

Thirty-two, which is 2 to the power of 5. Sixteen are on your mother’s side and sixteen on your father’s.

What is a 2nd great-grandparent?

A 2nd great-grandparent is the parent of your great-grandparent, also called a great-great-grandparent. They are four generations back and you have 16 of them.

Why do real family trees have fewer ancestors than 2 to the power n?

The same person can appear more than once in a tree when an ancestor’s parents were related. This is called pedigree collapse. The child of two first cousins has at most six great-grandparents instead of eight.

How much of my DNA comes from a great-grandparent?

On average one eighth, or 12.5%. Actual amounts vary. In the Shared cM Project 4.0 data, great-grandparents share an average of 887 cM out of 7,440 cM, about 11.9%, with a range of 485 to 1,486 cM.