Cousin Calculator

Cartoon by Vic Lee (2015) showing Einstein struggling to work out genealogical relationships

The difference between ‘second cousin’ and ‘first cousin once removed’ is not difficult to grasp.  The former is someone who shares the same great grandparents as you, whereas the latter is EITHER the child of your cousin OR you are the child of their cousin.  But in non-genealogy circles it’s surprising how many people get this muddled.  In fact I remember, myself, referring to my cousin’s children as my second cousins.  So this week here’s a little something for less experienced genealogists – or indeed for anyone having trouble calculating cousin relationships.  This becomes all the more important if you start to work with DNA and need to place likely matches, but there’s a DNA-specific cousin calculator to help with that aspect.  Today’s post is all about understanding how and why our cousins are ‘removed’.

The following ‘Cousin Calculator’ chart is really quick and easy to use (instructions down the right side).  It’s available from FamilySearch.  Click the link to download a higher resolution copy for your own use.

Grid enabling quick calculations of cousin relationships

This is really helpful in pointing you to the answer, but it still doesn’t explain why and how these people are so many times ‘removed’; and understanding this seems to me to be the main difficulty for many people.  I hope the following explanation will help.

It’s all about different generational ‘levels’
We know that these cousins are on two distinct, direct lines of descent from the ancestors they both have in common.  As set out on the above chart, first cousins share the same grandparents, second cousins share the same great grandparents, third cousins share the same GG grandparents, and so on….  However, the above only holds good when there is no generational difference between the two cousins.  We talk about cousins being ‘removed’ when there is a generational difference between them.  First cousin once removed, second cousin three times removed, and so on.

In fact, as an old hand now, dealing with this, I don’t use a chart to identify cousin relationships.  I find it quicker to look at those two individual lines of descent and do a couple of quick calculations:

  • First, I identify the Most Recent Common Ancestor(s)
  • Then I count how many generations down from them to my ‘cousin’ in the other line.  This gives us the ‘2nd cousin’, ‘3rd cousin’, (or whatever) part of the relationship.
  • Next, if they are older than (more accurately, ‘on a generational level above’) me, I look to see who is their ‘opposite number’ in my line.  That is, which of my ancestors is on the same generational level?
  • And finally I count down how many additional generations from that ancestor to me.  The number of additional generations is how many times ‘removed’ we are.
  • If my ‘cousin’ on the other line is on a generational level below me, then I look for my own ‘opposite number’ in their line, and count down how many additional generations to them, to get the number of times ‘removed’.
Family tree showing two lines of descent

This little family tree shows two lines of descent from my 3xG grandparents, George and Mary.  I’m descended from their daughter Annie Elizabeth.  The other line is descended from their daughter Martha.  A couple of years ago I made contact with Martha’s great grandson, called [Son] on the tree, to ask if he had any photos of Martha and Annie Elizabeth that he might share with me.  He didn’t, but he did have a little ‘family history’ that his aunt [Amy] had written sometime during the 1950s.  What a find!  There were some inaccuracies in it, but it gave a real insight into my great grandfather George’s life – information I couldn’t have got from anywhere else and which really helped me to understand the family dynamics.

So – the key people in that little story are [Son], his aunt [Amy] and [Me]. 

Amy is the same generational level as my granddad John.   The two of them are three generations below their Most Recent Common Ancestors (MRCA), George & Mary.  In other words, George & Mary are their great grandparents, making John and Amy second cousins (2C)

To calculate my relationship to [Amy] I need to count down from John to myself – that’s two generations.  So [Amy] is my second cousin twice removed (2C2R)

However, if I want to calculate my relationship to [Son], I don’t use my granddad John as the benchmark, because [Son] is on the same generational level as my Dad.  The two of them are four generations below their MRCA couple, their 2xG grandparents (George & Mary), making my Dad and [Son] third cousins.  I am one generation below [Son’s] third cousin (my Dad), so [Son] and I are third cousins once removed (3C1R), and my children are [Son’s] third cousins twice removed (3C2R).

Half cousins
Sometimes we see the term ‘half cousin’ or even something like ‘half third cousin twice removed’.  Wow – Scary! 😀 

The important thing to remember here is that the ‘half’ relates to the MRCA couple.  One of the ancestral couple married twice.  One of these half cousins is descended from the first spouse and the other from the second.  The rest of the calculation is exactly as above.  If the ancestor had married more than twice the same would apply – all descendents from that ancestor but with different spouses would always be ‘half’ plus something: half 4C, half 3C3R, etc.

I don’t know if this helps, or if any of my experienced readers have another way, but that’s how I do it.  Either way, if you didn’t understand why some cousins are ‘half’ or ‘removed’, I hope you do now.