Use Centimorgan Numbers to Identify DNA Matches July 10, 2023

Now we'll get into understanding DNA levels.  This will allow you to figure out the relationship between you and your DNA matches.  I always tell people, it's really all about the numbers.  Ignore those relationship descriptions you see on your match list that say things like "1st or 2nd cousin."  It's the centimorgan (cM) number that really matters.  This number tells you how much DNA you have in common with another person.  Another factor that's important is what generation you are in compared to your DNA match.  I'll explain more below.


As I mentioned before, 23 and Me doesn't always show you the centimorgan numbers for your DNA matches.  They use percents.  I prefer centimorgans when explaining this, so there is a way to convert percent to centimorgans.  Just multiply the percentage by 70.  That will give you an approximate cM number.  For example, if someone has 3.5 percent DNA in common with you, 3.5 x 70 = 245.  So, that person has about 245 centimorgans in common with you.  On 23 and me, if you click on the match and scroll down to "View DNA data," It will show you the cM number if that person has made the data available in their privacy settings.


Why are centimorgans so important?  A larger number usually means a closer relationship.  There are many factors involved in figuring out relationships though.  Again, I'm going to keep things as simple as possible and use nice round numbers to start with.  This can get very involved and strange numbers can result in the real world.  We've got to start simple.


In addition to the centimorgan number, the generation you and your DNA match are in is a huge factor.  Sometimes it's simple to figure out what generation someone is in.  Other times it can be much more difficult to figure out than you think.  My next post will be specifically about figuring out generations.  For this lesson we'll assume we know what generation everyone is in.  Also, make sure you have a good understanding of family relationships.  See my previous posts about that.  Knowing what to call these relationships is important.  For example, be aware that for someone to be called your first cousin, second cousin, third cousin, etc., they have to be in the same generation as you.  Otherwise, they are some kind of "removed" cousin, or other relationship.

Chart showing min, max, and average centimorgans


There are many DNA centimorgan charts available online showing you numbers for different relationships.  A popular one is shown above.  The chart shows you average numbers and min and max numbers for different relationships.  I find those min and max numbers to be extremely rare.  In fact, I've never seen a relationship reach those min and max numbers.  The average numbers shown are pretty accurate.  I'll give you some nice round numbers for relationships I often see.  Keep in mind, the numbers shown here are based on averages that I personally have obsevered.  Numbers shown are in centimorgans.

Parent/Child - 3500
Full Sibling - 2500-2900
Half Sibling - 1400-2000
Aunt/Uncle - 1400-2000
Niece/Nephew - 1400-2000
Grandparent - 1400-2000
1st Cousin - 700-1000
2nd Cousin - 200-300
3rd Cousin - 50-100


There's a whole lot more relationships than what is shown here and I can't stress enough that these are just average numbers to help get you started.  What about half relationships?  Well, a simple way to think about it is, half relationships will usually have about half as many centimorgans in common.  This is true, except for full siblings.  I find that half siblings have more than half, similar to Aunt/Uncle numbers (1400-2000cM).  Since I work with a lot of adopted people, I rarely see full sibling matches.  Half siblings show up quite often, and I think every one I've seen has been in the 1400-2000 range.  Everything else will be about half of what you see on the chart above.  A half first cousin will be about 350-500.  A Half aunt/uncle will be about 700-1000.


What about other people who are in a different generation than you (removed cousins).  For example, a first cousin once removed.  A good way to think about it is, each generation removed cuts the DNA level in half.  So, if a first cousin has 700-1000cM, a first cousin once removed will likely have 350 to 500cM.  How about twice removed?  Cut it in half again (125-250cM).  What about the child of your niece?  If you look at the chart above, a niece would likely have 1400-2000cM.  So, your grand niece would likely have half that, 700 to 1000cM.


Once you get into twice removed cousins, or third cousins, things really start to fall apart.  More and more relationships become possible and the centimorgan numbers really start to vary.  Here's an example...  It's possible for a third cousin to not show up on your match list.  The DNA level can be very low.  However, you might find another third cousin match with an unusually high amount, maybe even 200+cM.  Around 100cM is pretty common, but I've seen a variety of numbers when dealing with third cousins.  You often need to do a lot more research if you're looking into distant relatives.


As I mentioned, the numbers can vary quite a bit, but something to keep in mid is, they can only vary so much.  For example, let's say you have a DNA match to someone who you've always known as your first cousin.  In my chart above I mentioned a first cousin will likely have 700-1000cM in common with you.  In extreme cases, maybe it could drop down to 500cM.  Anything beyond that is really pushing the limits of DNA.  If that first cousin is matching you at something like 350cM, then you know there's a problem.  He's not really a full first cousin.  He's likely a half first cousin.  He may not know who his real grandfather is.  You'll want to investigate that!


That's enough to at least give you an idea of how to understand relationships to your DNA matches.  Again, the important things to remember are, bigger numbers mean closer relationships, and the generation you are in compared to your DNA match also makes a big difference when figuring out the relationship.  You may discover some things that don't quite add up.  Also remember, the numbers can vary quite a bit and it can get confusing.  You're not going to see nice round numbers like in my examples.  Don't get discouraged if you have problems, and don't be afraid to ask for help.  Finding someone with experience with these numbers is often the best way to go.  With some research and understanding of the numbers, you can usually solve your family mysteries.

My next post explains why determining generations can be confusing.

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