Do you know how many ancestors you have? Of course not. Let’s simplify the question: How many ancestors do you have in the past one thousand years? Many people do not know the answer to that question. Care to guess? (The answer is given below but please don’t peek just yet.)

The number of ancestors is simple to calculate as it is a simple mathematical progression: every person has two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents and so on. The number doubles with each generation. As you go back in years, the numbers soon become very large.

For this example, I have assumed that a new generation appears on an average of every twenty-five years:

**Number of Ancestors**

Generation Number |
# of Years Before Your Birth |
Number of ancestors in that generation |
Total ancestors (this generation plus all later generations) |

1 | -25 | 2 | 2 |

2 | -50 | 4 | 6 |

3 | -75 | 8 | 14 |

4 | -100 | 16 | 30 |

5 | -125 | 32 | 62 |

6 | -150 | 64 | 126 |

7 | -175 | 128 | 254 |

8 | -200 | 256 | 510 |

9 | -225 | 512 | 1,022 |

10 | -250 | 1,024 | 2,046 |

11 | -275 | 2,048 | 4,094 |

12 | -300 | 4,096 | 8,190 |

13 | -325 | 8,192 | 16,382 |

14 | -350 | 16,384 | 32,766 |

15 | -375 | 32,768 | 65,534 |

16 | -400 | 65,536 | 131,070 |

17 | -425 | 131,072 | 262,142 |

18 | -450 | 262,144 | 524,286 |

19 | -475 | 524,288 | 1,048,574 |

20 | -500 | 1,048,576 | 2,097,150 |

21 | -525 | 2,097,152 | 4,194,302 |

22 | -550 | 4,194,304 | 8,388,606 |

23 | -575 | 8,388,608 | 16,777,214 |

24 | -600 | 16,777,216 | 33,554,430 |

25 | -625 | 33,554,432 | 67,108,862 |

26 | -650 | 67,108,864 | 134,217,726 |

27 | -675 | 134,217,728 | 268,435,454 |

28 | -700 | 268,435,456 | 536,870,910 |

29 | -725 | 536,870,912 | 1,073,741,822 |

30 | -750 | 1,073,741,824 | 2,147,483,646 |

31 | -775 | 2,147,483,648 | 4,294,967,294 |

32 | -800 | 4,294,967,296 | 8,589,934,590 |

33 | -825 | 8,589,934,592 | 17,179,869,182 |

34 | -850 | 17,179,869,184 | 34,359,738,366 |

35 | -875 | 34,359,738,368 | 68,719,476,734 |

36 | -900 | 68,719,476,736 | 137,438,953,470 |

37 | -925 | 137,438,953,472 | 274,877,906,942 |

38 | -950 | 274,877,906,944 | 549,755,813,886 |

39 | -975 | 549,755,813,888 | 1,099,511,627,774 |

40 | -1000 | 1,099,511,627,776 | 2,199,023,255,550 |

Answer to the earlier question: If we assume that there is a new generation every twenty-five years, an ancestor born 1,000 years before you would be 40 generations removed from you. You would have 2,199,023,255,550 (that’s 2 trillion, 199 billion, 23 million, 255 thousand, 550) unique ancestors born in the previous 40 generations, assuming no overlap (that is, none of your ancestors were cousins to other ancestors).

1,000 years doesn’t even take you back to the years in which Charlemagne lived! (April 2, 742 AD to January 28, 814 AD)

Now, how many ancestors have you had in the past 10,000 years? 100,000 years? I’ll leave it to you to figure out the mathematics involved. However, the answers obviously are huge numbers!

There is but one problem: all of these numbers are far more than the total number of people who ever lived on the face of the earth.

The reality is that all families can find lots of cousins somewhere in the limbs of the family tree, resulting in the same ancestor(s) showing up in multiple places in the pedigree charts. Ask anyone who has done French-Canadian genealogy or has researched any families that lived for generations in one small village almost anyplace on earth.

Obviously, you and everyone else have cousin marriages in your ancestry, resulting in individual ancestors showing up in multiple places in your family tree.

## 22 Comments

I have always figured that each generation was at 20 years not the 25. I can go back 10 generations to Switzerland and his birth date was 1626 and mine is 1946, only 320 years difference. Therefore the 20 years idea seems to be in error. Nice number anyway.

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—> I have always figured that each generation was at 20 years not the 25.

I suspect that varies a lot by ethnic groups. In my own ancestry, I find that French-Canadian girls often got married at 15 or 16 years of age although their husbands typically were about 10 years older. However, my New England ancestors, with families that came from England, typically got married in their early to mid 20s.

Then you realize in the days of large families, these families often continued to have children for 20 years, sometimes longer. For instance, my own grandmother was 15 years old when she got married and then she and my grandfather continued to have 16 children over the next 28 years. (They had grandchildren older than the youngest of their own children. My brother was older than 2 or 3 of his aunts and uncles.) So I think the average age has to be based on the parents’ ages when children were born, not on the marriage dates of the parents. Even with my French-Canadians, I suspect the AVERAGE age of the parents when their children were born was probably 25, maybe older.

In your example of 10 generations over 320 years, that equals an average of children being born to 32-year-old parents. Obviously, that’s an average. Many times the age of the parents was undoubtedly higher or lower.

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This is truly a mystery. The Bible gives 70 years to a generation and it is interesting to count back 35 doublings to come to a number that is nearly the number of people alive on Earth today. If we start subtracting back 35 steps to the first two people on Earth we come to the biblical timelines of Noah 70 steps back from each individual today. At some point a subtraction must start and for some reason the number 70 has great significance in Scriptures in relation to generations. Only our Creator knows the answer to this.

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Fantastic job in explaining this. I have tried to explain it several times but you have done it much better. Your chart help tremendously. This also explains why just about anyone who does the research will find many famous people they are related to in their ancestry. With that large a number of ancestors there has to be many that are famous. Thank you.

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Oh! Thank you…seeing this is so very helpful and makes sense. While there will obviously be differences, this is a terrific bell-weather for calculating generational ancestors.

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The best average age per generation to use might even be longer than 25 years. See: https://isogg.org/wiki/How_long_is_a_generation%3F_Science_provides_an_answer

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Very good article.

However, to further complicate the calculations, I suspect that the number of one’s children is a function of one’s place in the birth order. Perhaps first born children, traditionally being more likely to inherit the bulk of their parents’ wealth, were seen as more attractive suitors and better able to begin procreating sooner and were better able to support larger families. Children born later in the birth order, on the other hand, stereotypically going into the military or clergy, would have had fewer children on average.

The result would seem to be that while the average age of one’s parents at one’s birth would be as stated in the article, the average duration of a generation would be slightly less. I wonder if the Icelandic or other data sets offer enough resolution to confirm such a trend.

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While some cultural groups may be different, in my experience with Southern families, women had children, on average, for 22 years, from about age 20 until they turned age 42. Yes, some women had a baby after age 42, but the average is 42 when you look at many women. If you are estimating generations, you should use a middle number in that range of 22 years — 31 years for women. On average, men had their children from age 25 through 47, placing the middle for them at 36. A much better estimate for generations is 33 years. Twenty-five is too low a number. Your ancestors are typically going to move back and forth between being an early child in the family and being a late or last child. Of course, if your line is the first child of the first child of the first child, your generational number will be low. But on the other hand, if your line is the last child of the last child, etc, your generation number will be high. Go for the average. When you are looking at 200 or 300 years, it won’t stay as low as 25 years per generation.

If you have six or more generations in your known lineage, go calculate the average age between generations. If you have enough data, it won’t be 25 years and certainly not 20 years. 33 to 34 years per generation is more accurate.

Ann B.

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I tried to address this topic as it applies to my own family blog/research – see https://nelsonloeb.wordpress.com/the-big-picture/, using this insight about the exponential growth of ancestors to help define the boundaries of my overall research project. I would appreciate any critical feedback about the way I present I the concept. Thanks for your post!

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I’ve noticed in my own family, that my maternal ancestor was the second wife, as perhaps the first wife died in childbirth ( presumptive) but that affects the age question due to the years of remarriage and next-generation birthing. It is somewhat common on my Italian side.

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I use to substitute elementary through high. Sometimes when I was subbing a fifth grade and during the math section of the day, I would drive my students absolutely bonkers telling them they were much more closely related to each other than they could ever imagine – especially if there were various races within the class. I wrote on the chalk board something similar to the chart Richard posted and told them if we could trace a student’s ancestors (I would pick out a white boy, for example) and another student’s ancestors (I would usually pick out a black girl or another boy), we might see they are related to one another – a VERY distant cousin, but still related. This opened up a LOT of discussion and questions of how this could be. It was a lot of fun and a great opportunity to teach kids that race and gender isn’t very important in the grand scheme of things. Just our hometown, for example. When I told them that EACH of us have 2,199,023,255,550 ancestors, I’d ask how could this be? Then, multiply each of us in the classroom – times 30 kids – then the whole school – then all of Rockford, IL (population about 150,000) – then the whole nation and world! Then, I’d ask them how could this be? There would be more ancestors than have ever lived on the planet since the beginning of the earth. They just didn’t get it. So this is when I’d talk and they would ask trying to cope with and understand the sheer magnitude of this simple yet complex fact of existence. A REAL eyeopener for a 10 year old, let alone all of the rest of us!

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There have been a number of recent studies that support the average generation as about 33 to 34 years. You don’t look at it as “how early can a couple conceive a child” and use that number to determine how long a generation is, because “generations” spring from children who were born during the full range of the fertile years of a couple. The average length of a generation should be calculated as the average age of a father when his “middle child” was born. I tested this concept against my own Scottish tree going back to 1685 and it correlated with that concept – it came out as roughly 34 years.

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Its then amazing that anyone could feel lonely.

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I am only an amateur family genealogist, but here is my take on this:

Re the number of years in a generation, it doesn’t matter, because your family is not the same as every other family and even within your own line the generations are not the same.

Example: My husband’s mom had him when she was 15. My mom had me when she was 31. It has no effect on our son doing his tree if he wants to. No one cares. The average number is only listed to give you an idea in general. It helps when doing “guesstimates”.

In reality, you do your own tree and if you find an ancestor that you identify as your 17th great-grandfather, you can then look at the chart to see how many other 17th great-grandparents you have. It doesn’t matter when each one was born, so long as each one gives birth to your 16th great grandparents, and so on down the line.

There are obviously thousands if not millions of duplicates the further back you go in generations, but in a perfect tree you would want to account for them from each of their children that leads down to you. Most of us are lucky if we can account for all of our 4th great grandparents.

Thanks for the chart Dick. There are a number of them online and yours is the easiest to quickly view and print out too.

Char

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I run statistical anlysis on my genealogy data (dozenz of thousands of persons) and average generation is 29,7 years.

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Okay, so now let’s figure back down the other way. If I have some ancestors born about 1500, how many descendants might they have by now?

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How long a generation is does not matter. Each generation back would still have the same number of ancestors. So on one side of my family, 10 generations back may be 1600 and on the other side, 1500 or before. There would still be 1024 ancestors at that grandparent level.

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I have a genealogy chart that lists my ancestors back to Charlemagne. It’s 40 generations and I was born in 1945. That would make the average generation about 30 years. Maybe my ancestors were late bloomers!

I felt pretty special until I realized Charlemagne is just one in a couple of trillion of my ancestors in his generation.

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Did not my grandparents have 4 grandparents each, just as I did? That would mean I had 16 great grandparents, not 8.

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Your grandparents grandparents are not your great-grandparents. Their grandparents are your 2nd great-grandparents and you do have 16 of those.

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If we go back to my 32 great great great grandparents and assuming that each generation has six children, 3 boys and 3 girls, and that all children marry and have 4 to 6 children, how many descendants are there likely to be of my 32 great great great grandparents?

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You left out the most obvious reasons for these giant unreal numbers, father daughter/father grand daughter/brother sister incest is the only real answer whether people like it or not. Read the story of Lot in the holy bible, first cousin marriages would be some of the total only but not all, after the norman conquest of England who became our landlords fathered many of our ancestors. The number crunchers need to factor in a fairly large percentage of incesteral relationships to come up with a number that is realistic.The only true way to decipher this mess is to use your father and his father etc up to 20 generations, this will give you perhaps 20 paternal lines with a percentage of these 20 fathered some of the other 20 would make your direct paternal line perhaps 15 fathers. You may I may not see my point, but I have to be right. I also believe in 5 gens per century for the eldest child, regardless of the age of the parents.

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