Understanding Centimorgans

Here’s what centimorgans are and how to use them to understand your estimated genetic relationships to your DNA matches.

shared centimorgans measuring dna matches.jpg

I recently spent some time in New Zealand and I was reminded again of how simple the metric system is. In the metric system, a centimeter is 100 millimeters, a meter is 100 centimeters, and a kilometer is 1,000 meters. Everything is so simple and even, compared to the Imperial system in which one foot equals 12 inches, it takes 3 feet to make a yard, and 5,280 feet to make a mile! And don’t even get me started on temperature! It is such a good idea to make freezing at zero, don’t you think? 

Centimorgans measure genetic relationships

When we are looking at genetic relationships, there are also many ways we can measure them. Ultimately, we want the testing company to tell us how likely it is that a particular individual shares a single, recent common ancestor with us. One factor in this calculation is to take into account the total amount of DNA we share with that match. Currently, all the testing companies are reporting this sum in centimorgans (cMs).  Every company reports to you the total number of shared cMs, as outlined below. 

AncestryDNA: Click on the match to access the personal profile page for that match.  In the second section, under Predicted Relationship, you will see the confidence level. To the right of the confidence level you will see a grey circle with a little “i” in it. Clicking there will show you the total amount of shared cMs as well as how many pieces of DNA you share. 

Family Tree DNA: On the main match page for your Family Finder results, you will see the total amount of shared cMs in the third column. 

23andMe: You can see the percentage of shared DNA from the main DNA Relatives home page. To convert the percentage into centimorgans, just multiply your percentage by 68 (that will at least get you close). You can also see total shared cMs in the chromosome browser tool (go to Tools > DNA Relatives > DNA). 

MyHeritageDNA: The total amount of shared DNA is shown on the main match page under the title Match Quality

It is very tempting to think of a cM just like you would think of an inch or a centimeter, and for all practical purposes, that is ok. But it is actually much more complicated than that. A cM is a actually more like a crystal ball. It is there to help us predict how likely it is that the piece of DNA we are looking at looks exactly like it does right now a generation ago. This in turn helps us calculate how far back we should be looking for the common ancestor between two people. 

How to use centimorgans

But for our practical purposes, you can use the total amount of shared DNA, in combination with this chart compiled by Blaine Bettinger and The Shared cM Project, to better assess your genealogical relationship with your match based on your genetics. To use the chart, take the total amount of shared DNA you have with a match, and look up that number in the chart to get an idea of what kind of genealogical relationship might best fit the genetics that you see. For example, if I share 90 cM with my match, we might be 3rd cousins. But we might also be second cousins two times removed. How do you figure out which one? Simply put: DO GENEALOGY! 

I know this science stuff isn’t everyone’s favorite topic, but I do hope you can see by understanding it all just a little bit better you are more likely to have the tools you need to put your genetics to work in solving your family history mysteries. 

What to read next? I recommend this article on teasing apart complicated genetic relationships in tight-knight communities.


Originally published June 2016 on genealogygems.com.