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How Many Years Are In A Generation?

Do you want to learn how many years are in a generation? In this post, find out the answer, and whether the number of years in a generation ever changes.

How Many Years Are In A Generation

Everyone uses the word “generation”, but it seems to mean different things to everyone. Sometimes, generations even get their own name (e.g. Generation X, Baby Boomers, etc).

The concept of a generation is important to us because it can help us understand how we are different from, and similar to, our ancestors. It can also help us have a better understanding of the passing of time, especially when it comes to our family trees.

How long is a human generation?

There are about 25 years in a generation. This is an average number that indicates the number of years that pass between the time a child is born and the time that they become a parent, and thus, the next generation is born.

We can come to the number 25 logically, since we probably know from experience in our own lives that most women become mothers for the first time before they turn 30 years old. However, science backs up our “educated estimate”.

A study human generation times over the past 250,000 years by researchers at Indiana University and Baylor College of Medicine reports that the average age to first become a parent is 26.9 years old. This is very close to our “25 years” number, which is, conveniently, also easy to use for mathematical calculations.

Does the number of years in a generation ever change?

The number of years in a generation has changed slightly over time, but the average still holds true. While there are many cultural, economic, and environmental factors that can change the exact number of years between generations over time, humans tend to be both motivated and constrained by our biological ability to start the next generation.

Factors that affect the length of a generation in a specific family

In an individual family, we are likely to see a lot more variation in the number of years in an actual generation. We can make all of the estimations using average numbers that we want, but we are likely to see different numbers when we study an individual family.

Over time, a family that has much shorter or much longer generations can add up to a big different in the number of total generations in a given amount of time. For example, a family that sees women becoming mothers at the age of 20 instead of 30 will have 12.5 generations in 200 years instead of 8.5.

Some factors that can affect the number of years in a generation in an individual family are listed below.

Cultural and religious expectations

As members of a society, our families often do what other families of the same place do. This may be influenced by religious or cultural habits and routines.

Over the past hundred or so years, we have seen governments around the world make progress on reducing or eliminating the marriage of females under the age of 18, though we do see this happen still, even in the United States.

Historically, it was much more common to see women married at younger ages. They usually began families soon after marriage.

Availability of partners

Some circumstances in history have reduced the availability of marriage partners. For example, the Civil War affected the average age of marriage for southern women, and World War 1 influenced the average age of marriage for women in Europe.

Family customs

Some families have expectations of members to marry at younger or higher ages. This is the type of information that most people know about their family from the experience of being a member of that family, or by checking out birth years on your family tree.

How to calculate generations in your own family tree

If you have built your family tree, like many of us have, you will be able to do your own informal study of generations to figure out the number of years in a generation in your own family. You might even find interesting details like differences in years in generations between lines of your family tree, which can lead you to learn more about your ancestors’ lives.

In order to figure out how many years is in a generation in your own family tree, you will need to know when your ancestor was born and what year their first child was born. Typically, we look at our female ancestors for this information, though you could certainly do this same analysis for your male ancestors.

For example, if your great-great-great grandmother was born in 1852 and had her first child in 1877, then you know that there was 25 years in that generation. However, if her mother was born in 1832 and had her first child in 1851, then there were 19 years in that generation of your tree.

You could carry this analysis back for as many generations as you have data in order to get the average. Just divide the total number of years for each generation by the number of ancestors that you are calculating information about to get the average for that line of your tree.

I found this was a fun exercise to do with my tree, and I realized that I had not researched all of my ancestors’ children on all lines of my tree. So, this was a good opportunity to see if I could find out more about my collateral ancestors on those lines.


I hope that this article has helped you understand everything you wanted to know about the number of years in a generation, whether this number ever changes, and how to figure out the number of years in a generation in your own family.

If you have any questions about something that you read in this article, or would like to share stories about how long generations are in your family, I would love to hear from you in the discussion below.

Thanks for reading today!

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Graham Watts

Sunday 23rd of July 2023

There are more years in a generation using male only data than there are using female data.


Sunday 23rd of July 2023

That's very true!

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