Generation Calculator

How long is a generation?

A generation is the average age of parents when their children are born, and it is longer than many people assume. Studies of family records across Europe and North America put the long-run average at about 30 years for all lines, about 26 to 29 years along female lines and 30 to 33 along male lines; it has been as low as the mid-twenties in some periods and places and is above 30 in much of the world today. Using 25 years, as older genealogy books did, overestimates the number of generations by about a fifth. From a person born in 1990 to an ancestor born in 1750, 240 years at 30 years per generation is 8 generations; at 25 years it would be 9.6.

Ancestors double every generation

Everyone has 2 parents, 4 grandparents and 8 great-grandparents; generation g holds 2^g ancestors. Eight generations back there are 256 ancestor slots, the 6× great-grandparents, and 510 in total from parents to that generation. Thirteen generations, roughly back to 1600, gives 8,192 slots in a single generation. Thirty generations would be over a billion, more people than were alive at the time. The paradox is resolved by pedigree collapse: the same people fill many slots, because distant cousins married, and in practice everyone of the same region shares most of their distant ancestors.

How much DNA a distant ancestor passes on

On average, each ancestor in generation g contributes a fraction (1/2)^g of your autosomal DNA. A grandparent contributes about 25%, a great-great-grandparent about 6.25% and one of the 256 ancestors 8 generations back about 0.39%, around 27 cM of about 6,800. But DNA is inherited in large segments, and the number of segments drops each generation, so the average hides a lot of variation. By about 8 to 10 generations, a sizeable share of your genealogical ancestors have passed you no DNA at all, and you inherit DNA from only a fraction of them. A common misconception is that a DNA test can show every ancestor; beyond about six generations, the paper trail includes ancestors the DNA no longer reflects.

Using the estimate

Generation estimates are a planning tool. They suggest which century to search, how many direct ancestors a project involves and whether a claimed descent from a historical figure is plausible in the time available. A line that claims 10 generations in 150 years implies an average of 15 years per generation and should be checked carefully. Always replace estimates with documented birth years as research progresses.

How to use it

1. Enter the birth year of the living person and of the ancestor.
2. Choose an average generation length, about 30 years unless the line suggests otherwise.
3. Read the number of generations, the ancestor relationship and the number of ancestors in that generation.
4. Use the expected DNA share to judge whether DNA evidence can reach that far.