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AncestryDNA, 23andMe and FTDNA use centimorgans to compare DNA matches

What Are centiMorgans and How Do You Use Them to Find DNA Matches?

When you get an autosomal DNA test whether from AncestryDNA, 23andMe or Family Tree DNA, you can use the results to find possible DNA matches.

All genealogical DNA testing services use centiMorgans to estimate (emphasis on ‘estimate’) how close you are with someone else. Depending on the number of centiMorgans across your shared DNA segments they could be close family members such as a parent or grandparent or a relative further away such as a second cousin.

So what exactly is a centiMorgan (abbreviated as cM) and how do you use it to find and analyze DNA matches.


Recombinant DNA

Image showing Recombinant DNA that forms via crossover of DNA segments between DNA molecules.

A centiMorgan is the measure of the probability of a crossover event. That is, the likeliness of two different DNA segments crossing over between two different DNA molecules.

Genetic recombination is how we get physical traits passed down from our parents.

Interesting fact: The term ‘centiMorgan’ was coined by Alfred Henry Sturtevant in honor of his teacher, Thomas Hunt Morgan, a famous American geneticist. The original unit was called a Morgan but is rarely used today.

A centiMorgan describes the frequency of recombination or crossover. Specifically, a centiMorgan represents a one out of a hundred chance (1%) that a crossover within a DNA segment will occur within one generation.

On average one cM is equal to around 1 million base pairs (the two nucleotides that make up one rung of the DNA helix ladder). That means that a crossover is likely to happen for every DNA segment consisting of 1 million base pairs.

The definition of a centiMorgan can be a bit confusing and many people tend to see it as a measurement of distance. It’s not. Rather, it is a measurement of probability.

It’s important to understand that a centiMorgan is not a definitive unit of measurement. This is because not all DNA segments have the same recombination frequency. Like a lot of things in genealogy, it’s a close estimate.

But it is very handy in determining the closeness of two people who share DNA. In fact, it may provide a more accurate prediction than just looking at shared DNA segments.

Using centiMorgans to Analyze DNA Matches

Most DNA testing services will include the number of number of centiMorgans in matching DNA segments. You don’t have to carry out any complicated calculations.

You’ll need to order a DNA test first. See our buying guide for an in-depth comparison of the top three DNA ancestry testing services.

After the results are in go to your results page in your secure account and check whether there are any matches.

In AncestryDNA for instance, you click on a specific match and then click or hover on the small i next to the confidence label. This brings up a box showing the number of centiMorgans on shared segments.

Shared centiMorgans on AncestryDNA

The higher the number of centiMorgans the more significant the match is. It means you both have inherited a number of traits from a recent ancestor.

There is a minimum number of cMs required to establish any kind of familial relationship. It’s usually 7cMs.

The higher the number goes the closer you are to your match. It can go as high as 3,000cMs for a very close match such as a parent or a child.

Here is a table by AncestryDNA showing how the number of shared centiMorgans corresponds with different levels of relationships.

Shared centiMorgans table on AncestryDNA

As you can see, centiMorgans are not precise measurements. For instance most people share around 120cMs with a third cousins but in other third-cousin matches they can be as few as 90 or as high as 200.

Converting centiMorgans into Percentages

Another way to express how much DNA you share is in form of percentages.

You can convert centiMorgans into percentages simply by dividing the total number of cMs by 68.

Why 68?

There are roughly 6,800 possible centiMorgans in a person’s DNA. So if you share 3,400 centiMorgans with a match you can say you share 50% of DNA (that is, 3400/6800*100 or simply 3400/68).

A 50% match is likely a parent or a child while a 1% match is likely a third cousin.

Complement Your Research

You shouldn’t use centiMorgans on their own to determine familial relationships. They can be misleading at times especially for more distant matches.

Combine what you learn from your 23andMe, FTDNA or AncestryDNA test with your family history and historical records to draw a more accurate timeline and establish more accurate ancestral links.

About the Author Charles McKnight

I'm just another amateur genealogist investigating my American-Scots-Irish lineage. I built after buying all of the leading DNA tests to discover everything I could about my family history. Hopefully, this site will save you time and demystify the emerging science of DNA-based genealogy, for your family project.

Leave a Comment:

John Drach says November 10

Very helpful, thanks!

Gene L Shefland says January 12

Thank You, Ancestry DNA didn’t give me this info with my results.

Maura says March 23

Thanks, this was very helpful (in common man English) 🤔

Wes Paul Gerrard says April 12

This has been really helpful. It’s a pity more amateur genealogists don’t get involved. (many waste money on a DNA test and then couldn’t be bothered to get in touch or even set up a family tree). What I can’t figure out is why after its been established that we all came out of Africa originally that we all don’t have a common link with hunter gatherers from that region.
Thanks for setting up this page

Kind regards
Wes Paul

Bill cole says May 17

That great, You got me on the right track

dianne says June 27

Thanks for helping me understand.

Jen says July 27

Thank you so much for the info explaining cMs. As someone mentioned – in plain language

Cecelia says August 13

Why are the number of cM different (substantially different) between the same 2 people on different sites. I am tested at ftdna and ancestry – a match is also tested on both. Our cM match is 60 on ftdna but only 20.7 on ancestry. Why is this?

    Charles M. says October 31

    This is because the statistical DNA database that each test service uses is different/unique.

    All cM match figures are in essence just statistical correlations.

    To calculate a cM match, your DNA record is first compared with the service’s entire database of DNA results. Then, the other person’s record is compared the same way.

    Then, your two records are compared to calculate a match.

    Because the service databases are different, their calculations for each of you will differ and your comparison will differ.

    The amount of difference between your DNA ancestry and the average weighted ethicity/tribal ancestry of other DNA records in the database also matters… for example, if one service has very few records from your tribe (haplogroup), it will increase the error in that service’s results when calculating a match.

    Hope this makes sense. A lot of people don’t understand how the calculations are made. I didn’t either, for quite a while!

Sylvia Hanna says January 6

very helpful for me to understand the relationship with dna

Brian says April 8

Great site, a complex concept explained simply.

Lorene Honea' says August 23

I was wondering why the cMs for my parents are 1698-2524 and my 4th great grandfather is 2524cMs.

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