📆 Generation Birth Year Estimator

Enter a known birth year and how many generations away (backward or forward) to estimate the approximate birth year range for an ancestor or descendant, using a typical generation length.

Known Birth Year

Generations Away

Direction

Avg. Generation Length (years)

The estimate is one multiplication

Generations are multiplied by the average generation length and added to or taken off the year you know. The generation length defaults to 28 years and can be set anywhere from 15 to 40:

offset    = generations × generation length
backward  = known year - offset      (ancestor)
forward   = known year + offset      (descendant)
range     = estimate ± 5 years

Someone born in 1980 looking three generations back at the default 28 years gets 3 × 28 = 84, an estimate of 1896, and a range of 1891 to 1901. Switch the direction to forward and the same inputs point at a great-grandchild born around 2064.

Choosing a generation length

25 to 30 years covers most families, which is why 28 sits in the middle of the field. Genetic genealogists often use figures in the 25 to 32 band for the same reason.
Maternal lines run shorter, paternal lines longer. A line of women marrying at 20 can average 22 to 25 years; a line of men remarrying and fathering late can pass 35.
Check the estimate against a record, then reset. Once you have a documented year for a middle generation, use that as the known year rather than compounding the guess through it.

The ±5 window is fixed and does not widen with distance, so at six or eight generations back it is optimistic. Treat the estimate as the centre of a search: at eight generations, a decade either side of it is a fairer net to cast over a parish register.

Frequently asked questions

What is the average length of a generation in genealogy?

Roughly 25 to 30 years, and this tool assumes 28 unless you change it. The figure is the parent's age when the child was born, not the parent's lifespan.

When was my 3x great-grandfather born if I was born in 1980?

Around 1840. He is five generations back, so 5 × 28 = 140 years, giving a search range of about 1835 to 1845 before other evidence narrows it.

Why does my estimate miss the real birth year by twenty years?

Because the errors compound in one direction. If three generations in a row were each born to a 40-year-old father, the tool is already 36 years adrift. Raise the generation length and try again.