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How Many Ancestors Do You Have?

Are you wondering how many ancestors you have?  There are many reasons you might want to learn this information, and in this post, I’ll teach you how to figure out how many ancestors you have – with a slight catch. 

If you really want to know how many ancestors you have, you have to learn a little bit about pedigree collapse.

How Many Ancestors Do You Have_(1)

For those of you looking for a simple answer, we’ll start off by looking at the easiest way to estimate the number of ancestors that we have:

The number of ancestors that we have doubles each generation

Each person, with exceptionally few exceptions, has two biological parents.  Each person’s parents has two parents, and their grandparents have two parents, and so on and so on. 

This means that each generation that you go back into your family tree, the number of ancestors that you have doubles.

Don’t want to do the match?  No worries, I have done it for you. 

Below, you can find the number of ancestors that you have going back 20 generations, or about 500 years (assuming an average of 25 years per generation):

  • Parents: 2 (cumulative: 2)
  • Grandparents: 4 (cumulative: 6)
  • Great-grandparents:  8 (cumulative: 14)
  • Great-great grandparents:  16 (cumulative: 30)
  • Great-great-great grandparents:  32 (cumulative: 62)
  • Great-great-great-great grandparents:  64 (cumulative: 126)
  • Great-great-great-great-great grandparents: 128 (cumulative: 254)
  • Great-great-great-great-great-great grandparents:  256 (cumulative: 510)
  • Great-great-great-great-great-great-great grandparents:  512 (cumulative: 1,022)
  • Great-great-great-great-great-great-great-great grandparents:  1,024 (cumulative: 2,046)
  • Great-great-great-great-great-great-great-great-great grandparents:  2,048 (cumulative: 4,094)
  • Great-great-great-great-great-great-great-great-great-great grandparents:  4,096 (cumulative: 8,190)
  • Great-great-great-great-great-great-great-great-great-great-great grandparents:  8,192 (cumulative: 16,382)
  • Great-great-great-great-great-great-great-great-great-great-great-great grandparents:  16,384 (cumulative: 32,766)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 32,768 (cumulative: 65,534)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 65,536 (cumulative: 131,070)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 131,072 (cumulative: 262,142)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 262,144 (cumulative: 524286)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 524,288 (cumulative: 1,048,574)
  • Great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great grandparents: 1,048,576 (cumulative:  2,097,150)

Holy ancestors!  That’s a lot of people! 

Like, a LOT of people.  Most of us (all of us?) will never be able to build a family tree going back on every line – on all branches, on both sides of the family – all the way to 20 generations. 

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There are just too many variables, and besides, not all of our ancestors were born where good records were kept.

How many ancestors do we really have?  Realistically?

So, do we really have more than one million 18th great-grandparents?  At risk of having one person e-mail me explaining that they are the exception, not one of us has more than one million 18th great-grandparents – and I’ll tell you why not. 

Basically, it’s impossible for the number of ancestors that we have to simply keep doubling every generation.

To demonstrate why it’s impossible, let’s go further back – just six generations – to 26 generation back, using the doubling method.  We’ll begin with our 20th generation, and the 1,048,576 18th great grandparents that we would theoretically have at that point:

  • 21 generations back:  2,097,152 ancestors
  • 22 generations back: 4,194,304 ancestors
  • 23 generations back: 8,388,608 ancestors
  • 24 generations back: 16,777,216 ancestors
  • 25 generations back:  33,554,432 ancestors
  • 26 generations back:  67,108,864 ancestors

As you can see, doubling the numbers every generation allows the number of ancestors to get really, really big – and really quickly!  This leads us to one reason why this can’t possible be true. 

I don’t have 33 million ancestors 25 generations ago – it’s impossible.  My ancestry, at a basic level, goes back to Europe (British Isles, Western Europe, and Eastern Europe), and 26 generations ago (approximately 650 years ago), population estimates for the entire European continent were right about 78 million people.

… you might be able to see where I am going with this by now.

If I go back one more generation – to 27 generations – and still within the 1300s (when about 78 million people lived in Europe), I supposedly have 134,217,728 ancestors.  More than 134 million ancestors! 

That’s almost twice as many people who were even living in the entire continent of Europe at the time.  It’s very obvious to me that I don’t have 134 million ancestors!

To demonstrate how ludicrous it is to keep doubling the number every generation, you should know that at 30 generations back we’d have more than 1 billion ancestors, and at 40 generations, we would have more than 500 billion ancestors.

This is many more ancestors than the number of humans who have ever lived in the history of the world, which is estimated to be at a little over 107 billion people in total.

What this means is that some of our ancestors are our ancestors on more than one line of our family tree. The technical genealogical term for this is “pedigree collapse”.

Pedigree collapse is real, and everyone has it

We are now led to the obvious question.  We exist, of course, and we also – most definitely – have ancestors.  So how many ancestors do we really have?

In most people’s family trees, the number of ancestors starts at two (your parents) and doubles going back several generations.  This means that the calculations going back a few generations are accurate – you probably do have 64 distinct great-great-great-great grandparents. 

(Note: Some of us might find that some of our great-great-great-great grandparents are the same people – more on this – and why – below)

Assuming you could accurately trace your family tree back on all lines, you would eventually start to see the same people pop up on multiple lines of your tree.  We know that this happens in most “modern” family trees starting at about ten generations back.

Depending on your unique ancestry, you might see this happen more recently than ten generations, and almost everyone will see it at least once by the time they’ve researched all their 1,024 8th great-grandparents. 

Most people will never be able to know who each and every one of their 1,024 8th great-grandparents are, so it’s really tough to prove or disprove this statement.

Why is there pedigree collapse in every family tree?

Back in the day, people lived in small communities.  It was beneficial to stay where you were born, generally speaking, since you had an established social network that made survival easier. 

When people grow up and choose mates, they generally choose people who they know.  This meant that most people that you know were related to you in some way, even it was distant and you didn’t even know for sure that you were related. 

It was also more socially acceptable to marry people who were related to you, which is demonstrated by the fact that was many as 80% of historical marriages were between people who were were second cousins or closer.

This happened in my own family tree, and not even ten generations ago.  Two of my 64 great-great-great-great grandparents (six generations ago) were first cousins. 

They shared grandparents, which means that the number of ancestors that I have going back eight generations is two less than the 256 6th great-grandparents that many people have, and I can’t simply double that number to get back one generation further, since many of the ancestors are the same.

Conceivably, I will find this more and more as I go further back in my tree, and eventually, the number of ancestors that I have will start to shrink as more and more of them become the same. 

And I can guarantee that this same situation occurs in your family tree, too, at some point.

So, is there a way to really know how many ancestors you have?

The only way to be sure about the number of ancestors that you have is to actually count them.  But since researching all of your ancestors, equally, on every line, can prove difficult, it’s fine to use the “doubling” method to estimate your number of ancestors going back about 10 generations to your 8th great-grandparents.

Check out this graphic I made to help you visualize the number ancestors you have going back 10 generations.  I didn’t put 20 generation on the chart because, well, you’ll have to read the rest of the post to find out. 

Don’t forget to share this graphic on your favorite social media site or group – I appreciate you!

How many ancestors do you have_


I hope that this post helped you learn about how many ancestors you have, pedigree collapse, and how this effects how many ancestors you have further back.  If you have any questions about something that you read in this post, I’d love to hear from you in the discussion below.

Thanks for stopping by today!

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Saturday 28th of November 2020

I have a question. How far back would you have to go to find a common ancestor of everyone living today?


Tuesday 1st of December 2020

This is a great question! We all do share common ancestors, and I think there would be some variation in the answer. However, I found these two links interesting. They suggest two different answers. One suggests that about 20 generations would be enough to find a common ancestor shared between a group of people. The other article says that about 1,000 years (that's about 40 generations). I think that these are great explanations, but that we might have to go back further in history to find common ancestors shared between populations that have been geographically isolated from each other for many thousands of years.

Terry Williams

Saturday 24th of October 2020

That has explained why my mathematics are so poor! I have traced my ancestry back via my 4th grand father Thomas Williams who married Elizabeth Tudor about 1800. Now the Tudor line is fairly visible to the early days of the Welsh lords, & then even back to the kings of France, & Dukes of Bavaria, & Milan, taking me back to around the years 8 to 900 A.D. the Williams’s seem to disappear after five generation (so far)? But of course with your Pedigree Collapse, the correct way to view Ancestry, is to descendants, if I’m correct?


Saturday 21st of November 2020

Your Elizabeth Tudor's grandfather is unknown however : so whether or not they are related in any way to the welsh Tudors is unknown as well.

Georzetta Lynn Ratcliffe

Tuesday 10th of March 2020

Thanks for the information on pedigree collapse. I see this in my own family tree as most of the last few generations came from small enclaves in the Appalachian Mountains around and about Eastern Kentucky. I never knew what it was called. I will be subscribing to your newsletter.


Thursday 12th of March 2020

Thank you, Georzetta! I am so glad that you found the information helpful :)

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