Use my centimorgan relationship calculator to help you figure out family relationships. November 21, 2023

In several of my older posts I explained centimorgan numbers and I gave some examples of possible relationships based on those numbers.  In another post I discussed generations and how they affect centimorgans.  Below is a calculator I made that allows you to enter a number and choose the generation of your DNA match.  You don't always know what generation your match is in compared to yourself.  Sometimes you have to start out by guessing and then see if things work out.  In my posts about family relationships, I explain what each relationship means.  For example, a half first cousin means, you have one grandparent in common.  If you've forgotten or are still confused, I'll include some relationship descriptions here as well.  Hopefully this online tool will help you learn about your DNA matches and their relationships to you.  Just enter a centimorgan number, choose a generation and press "Show Relationships."  If you need additional help or notice any problems with the calculator, feel free to contact me.

Keep in mind, the calculator will tell you "likely" relationships.  Crazy things can happen with centimorgan numbers though.  There are rare cases where numbers can really be thrown off for various reasons.  For example, in a previous post I mentioned the "double cousin" relationship.  I didn't mention it in my calculator because it's not common.  Another reason you could get odd results would be, if your parents are related to each other.  Even if they are third cousins, you could see some unusual numbers.  Incest is another big problem when doing these calculations.  Something else that can happen is, your DNA match could be related on multiple lines of your tree.  Even if your parents are not related to each other, a match could be getting DNA from somewhere in each of your parent's families.  I've seen it happen, but again, it's not common.  So, keep that in mind. This tool is just to provide you with some common possibilities for relationships.


Here are some odd results for you from my past experiences so you can understand how the numbers can really get tricky.  Again, these are not common, but I want you to be aware that things like this can happen.  For example, what's the lowest cM number for a confirmed full first cousin DNA match I've ever seen?  478cM.  Yeah, I had a hard time believing it myself.  Using data from other DNA matches, there was no doubt this person really was a full first cousin.  As you might know, a full first cousin normally has about 700 to 1000 cM in common with you.  Why would something like that happen?  Because, the DNA segments you inherit from an ancestor is random.  So, even though you have the same two grandparents, you and your cousin just happened to inherit different parts of their DNA.  The same can be true when expecting a low number, but you get something high.  What's the highest cM number for a third cousin match I've seen?  216cM.  As you may know, a third cousin is often in the range of 100cM or less.  This DNA match just happened to inherit a whole lot of the same DNA segments from their ancestors.  Here's another one... I once had someone with a half-sibling DNA match with 2294cM.  That's in range, but pretty uncommon.  I've seen many half sibling matches.  Normally, 1400-2000cM is what you'd expect.  Keep in mind though, the numbers only vary so much.  You're not going to see a third cousin with a 1000cM match or something like that.  So, again, use the calculator to see what is likely.  Then do more research to see if it's correct.


This is a tip that might come in handy for you.  Let's say you have two people on your match list with family trees that look identical.  The two people are siblings, but they have very different centimorgan numbers for their DNA match to you.  Joe has a 200cM match to you, and David has a 40cM match to you.  Seems unlikely for two siblings to have numbers that vary that much, right?  Well, it can happen.  Back when I first started learning about this stuff, I would have thought for sure that Joe was somehow related to me in a different way than David.  No, probably not.  This is a good example of how numbers can vary.  It often happens in distant relationships.  If these two guys are related to you in the same way, try averaging the two numbers.  200 plus 40 divided by 2 equals, 120cM.  If they are in the same generation as you, my guess is, both are your third cousins.  A third cousin match can vary that much.  It is even possible for a third cousin to be so weak they don't even show up on your list.  120cM is a fairly common value for a third cousin.  It's a good guess because both numbers are in range for that relationship.  What if they are in a different generation than you.  My guess would be second cousin once removed.  the 200 would be high for that relationship, and the 40 would be low.  Together, 120 is perfect for that relationship.  Remember, the numbers can be tricky at times, but they will give you a place to start looking.  Good luck in your search.

Next, Listen to an interview in which I discuss various genealogy topics.

Comment Box is loading comments...