Generations and years.

Worked example

You were born in 1990 and want the 6th great-grandparents, 8 generations back. At 26.9 years per generation that is 8 × 26.9 = 215.2 years, so 1990 − 215.2 = 1774.8, or about 1775. Going the other way, 300 years at 25 years per generation is 12 generations, and 200 years at 30 years per generation is 6.7, so six full generations.

How it works

There is no single length for a generation. A common genealogy rule of thumb is 25 to 30 years, which is not a published standard. The other presets come from a study of whole-genome data that estimated generation times across the past 250,000 years: an average of 26.9 years, with fathers at 30.7 and mothers at 23.2. Those are long-run averages from changes in the mutation spectrum, not measurements of recent centuries, and the authors report a substantial increase in female generation times in the recent past. Where you have real birth dates, use them instead.

The tool multiplies generations by years per generation and subtracts from your birth year, or divides a span of years by years per generation. Errors add up: eight generations at 27 years can easily be off by a decade or more in either direction.

Sources and basis (opened 2026-10-08): Wang, R. J., Al-Saffar, S. I., Rogers, J., Hahn, M. W., “Human generation times across the past 250,000 years”, Science Advances (2023), doi:10.1126/sciadv.abm7047, abstract read via Europe PMC (26.9 years overall, fathers 30.7, mothers 23.2). The 25 and 30 year presets are a common rule of thumb, not a published standard.

FAQ

How many years is a generation?

There is no single answer. A common genealogy rule of thumb is 25 to 30 years, which is not a published standard. A study of whole-genome data found an average of 26.9 years across the past 250,000 years, with fathers at 30.7 years and mothers at 23.2 years.

How many generations are in 100 years?

About 3.3 to 4. At 30 years per generation 100 years is 3.3 generations, and at 25 years per generation it is 4.

How many generations are in 500 years?

About 17 to 20. 500 years divided by 30 is 16.7 generations and divided by 25 is 20.

How do I estimate when my great-grandparent was born?

Multiply 3 generations by the years per generation and subtract that from your birth year. For example, born in 1990 with 27 years per generation, 3 × 27 = 81, so about 1909. Use actual dates wherever you have records.

Why use different lengths for fathers and mothers?

In the study’s averages fathers were older than mothers at the birth of their children, 30.7 years against 23.2. Use the paternal figure when you follow only fathers’ fathers and the maternal figure when you follow only mothers’ mothers.