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How many greats is that?
Everyone hits this question in family research. You find an ancestor born in 1900, you were born in 1990, and someone asks: is that your great-grandparent, or your great-great-grandparent? The answer depends on how long the generations in between lasted, and generations vary. This finder works it out: enter your birth year and the ancestor’s birth year, and it gives the likely number of generations between you, plus the possible range.
The maths is simple. Divide the years between you by a typical generation interval. At 20 to 35 years per generation, a 90-year gap means between three and four generations, with three the most likely. The range matters as much as the likely number, because it tells you when to stop guessing: if family stories say the ancestor is two generations back but the maths says three or four, the stories or the dates need a second look.
How to use the finder
- Enter your birth year.
- Enter the ancestor’s birth year. It must be earlier than yours.
- Read the years between you, the likely generation count, and the range.
- Translate the count: 1 back is a parent, 2 back a grandparent, 3 back a great-grandparent, and so on.
- Check the answer against your tree: does the named relationship fit the people in between?
A worked example
You were born in 1990, and you found an ancestor born in 1900. The gap is 90 years. Ninety divided by 35 is about 2.6, and ninety divided by 20 is 4.5, so the range is three to four generations back, with three the most likely. That means the ancestor is probably your great-grandparent, but could be a great-great-grandparent if your family married young. The next step is to fill in the middle: find the people between you and the ancestor, and see whether there are two or three of them.
Now take a closer case. You were born in 1990, and the ancestor was born in 1960. The gap is 30 years, which fits one generation of 20 to 35 years, so the ancestor is most likely a parent. That sounds obvious, but the tool earns its keep on the less obvious cases: a gap of 55 years could be two generations of late marriages or three of early ones, and the range tells you not to assume. When the range spans two answers, the tree, not the maths, decides.
Why generations vary so much
Generation length follows marriage age, and marriage age follows work, war, money and custom. A farm labourer who waited to inherit a tenancy might marry at 35. His daughter might marry at 20. Their grandchildren are then born nearly a generation apart in calendar years, even though they are the same generation in the tree. This is why cousins can be decades apart in age, and why a “generation” is a slippery unit: it measures family steps, not years.
Second marriages stretch generations further. A man widowed at 40 who remarries and has more children creates two sets of children a generation apart in age but the same generation in the tree. Migration does the same: a family that emigrated often delayed marriage until settled, pushing the next generation later. When your tree shows cousins with a forty-year age gap, this is usually why, and it is a sign to trust the family steps, not the calendar.
Using the answer in your tree
The generation count is a check, not a conclusion. If the finder says three generations back but your tree shows only two people between you and the ancestor, one person is missing, or one date is wrong. That mismatch is a research lead: look for a missing parent, a first marriage, or a birth year that belongs to a different person with the same name. If the finder says the range is three to four and your tree shows three, the maths and the tree agree, and you can move on with confidence.
The count also helps you talk about your tree. “My great-great-grandfather” is clearer than “an ancestor from the 1800s”, and it helps relatives place the person. It also helps you search: a great-grandparent’s records sit in a different set of archives than a great-great-great-grandparent’s, and the generation count tells you which set to open. Keep the range in mind when you search, and do not let a likely number harden into a certainty before the middle generations are found.
Limits to remember
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