Quick Tip – Calculating Cousin Relationships Easily

Lots of people struggle with figuring out exactly how two people are related.

Most genealogy programs include a relationship feature, but what if you are working with a new genetic cousin whose line isn’t yet in your genealogy software? Hopefully, that happens often!

There are also nice reference charts available, like this one provided by Legacy Tree Genealogists.

However, rather than trying to figure out who fits where, it’s easier and quicker for me to quickly sketch this out by hand on a scrap piece of paper. I can do this while looking at someone’s tree or an e-mail much more easily than I can deal with charts or software programs.

Rather than make you look at my chicken scratches, I’ve typed this into a spreadsheet with some instructions to make your life easier.

Common Couple Ancestor

This first example shows a common couple ancestor – as opposed to calculating a relationship to someone where your common ancestor’s children were half siblings because the ancestor had children by two spouses. 

Down one side, list your direct line from that ancestor couple to you.

On the other side, list your matches direct line from that ancestral couple to them.

The first generation, shown under relationship, will be siblings.

The next generation will be first cousins

The next generation will be second cousins, and so forth.

You can see that Ronald and Louise are one generation offset from each other. That’s called “once removed,” so Ronald and Louise are third cousins once removed, or 3C1R.

If Ronald’s child had tested, instead of Ronald, Ronald’s child and Louise would be third cousins twice removed, because they would be two generations offset, or 3C2R.

See how easy this is!

Half Sibling Relationships

In the circumstance where Ronald and Louise didn’t share an entire ancestral couple, meaning their common ancestor had a different spouse, the relationship looks like this:

The only difference in the relationship chart is that Jane and Joe are half siblings, not full siblings, and each generation thereafter is also “half.”

The relationship between Louise and Ronald is half third cousins once removed.

It’s easy to figure relationships using this quick methodology!

Update:  I can tell from the comments that the next question is how much DNA to these various relationships share, on average.  The chart below is from the article Concepts – Relationship Predictions, where you can read more about this topic and the chart.



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18 thoughts on “Quick Tip – Calculating Cousin Relationships Easily

  1. Thanks, Roberta. I just recently came across the term “half-cousin” and was having a problem figuring out what they meant. Your explanation sure cleared it up. Thanks, Eileen

  2. Does it ever happen, that when dna is compared, and the test comes back saying the two people are half siblings, are ever truly full siblings?? Can dna be off that much?

    • The minimum full and the maximum half are close, but I’ve never seen it be confused. One way to tell is that full siblings will have several areas where their DNA is entirely identical (FIRs) called Fully Identical regions and half siblings won’t, because they don’t share both parents. I’ve updated the article with a chart at the end showing expected relationship amounts.

  3. Thank you so much for sharing this information!
    Does the ‘half-cousin’ relationship come with a decrease in the amount of shared DNA?
    My great-grandfather was adopted; by a combination of Y-DNA testing and then autosomal DNA testing, we feel virtually certain that we have identified his birth father.
    Have some good clues concerning the birth mother…
    I’m just wondering if the percentage of DNA shared would change with a half-cousin situation…

  4. Hi Roberta, Slightly off topic, but I am currently using Big Y results and the interpretation service at YFull to establish relationships between individuals and branches on one of my male lines. Those relationships fall outside of the 5 to 6 generation limit of autosomal testing, and beyond our documented family tree. Your June 5th post addressed this. While I am focused on the SNP analysis of those male lines, it occurred to me that there is no equivalent process for females. Don’t females have SNPs ?? And if so, do they not develop novel variants in successive generations ?? And if they do, why can’t we use them to establish the genetic distance of females on a maternal line. That in conjunction with a FMS test could indicate succession on a female line. Could you explain why we don’t have this for women ?? Thanks, Paul.

    • We do have that. In the full sequence at Family Tree DNA, you’re shown the common mutations and the extra or missing mutations, which are the same thing as novel variant. It’s just up to people to figure out the path of descent of the people they match. I did this with my own mitochondrial DNA which proved that the common ancestor was indeed in Scandinavia, and the two of us with ancestors found in Poland and Germany did not descend from each other’s direct line, but from different lines both found in the home group in Scandinavia. I think that males have more invested in “their surname” that people do in mitochondrial DNA where the surname changes every generation, at least historically. No one has put the effort into mtDNA that goes into Y DNA. Having said that, the recent V17 update showed new subhaplogroups which were formed from those extra and missing SNPs, or Novel Variants in Y DNA terms.

  5. Thanks Roberta, very clear –
    I’ve used this method for years and find it much faster also. It reflects the “Shared Ancestor Hints” parallel lines at Ancestry as well. A word of warning, though – I’ve found several instances at Ancestry where the siblings are actually the same person – this person is the genuine common ancestor and not their parents. It skews the relationship calculation if you don’t notice it.
    By the way, I have some Estes ancestors from the Knoxville area and have enjoyed your personal stories of relatives from that area immensely. Helps me to visualize their lives and struggles so much better.

  6. Thanks so much for this! This fits the way my mind works. The usual matrix format is something I can puzzle out, but it is always a little confusing. Just is not something intuitive for me….
    Love this method!

  7. Roberta’s cousin explanation is so clear and helpful! I especially love her little chart: I rather like the “chicken scratches,” Roberta! When attending family gatherings and reunions in the past, I’ve always had the usual conversation, “how are we related?” ….Y’all know what I’m talking about…and there’s always the usually much older family member who insists that we substitute another level of cousin for a generational “removal.” Not wishing to offend, many of us will say to the older relative, well, we’re cousins, no matter how we’re cousins…..had to throw that in, y’all! But for the future, Roberta’s little chart is SO GOOD, for genetic genealogists! …I just hope we can keep these family reunions from dying out!!

    • I do too. There are none left in my family, on any line, except one which may be holding their last reunion this year. The interest has disappeared – much to the fact that people have moved away and are scattered across the country now.

  8. Hmmm… I just count the number of Gs in the shorter path and then the removes are how many more Gs in the other path. So my cousin who shares my GGGrandparents (3 Gs) but they are her GGGGrandparents (4Gs) is a 3rd cousin once removed … fast and easy

  9. Greetings, Roberta:

    A question for you to way in on please: if two males in 2017 have yDNA testing and the same surname, and find 108 out of 111 alleles exact, and the other 3 markers off by one, is it possible that the 3 markers changed over the 250 years allowing them to be 10th or 12th cousins or such? My thinking is that between the two lines leading back to the mid 1700s, there is room for two lines to have three spontaneous changes. Kind of a 250 years x 2… What do you think, assuming that my question is intelligible!

    Thank you, Robin Gaynor in Arlington, WA

  10. i’m seriously questioning the accuracy of relation estimation chart. i’ve come across two people who share 25 cm of dna but so far i can’t find a common ancestor between then in past 200+ years. so there’s no way they could be third cousins once removed like that chart claims.

  11. Pingback: Shared cM Project 2017 Update Combined Chart | DNAeXplained – Genetic Genealogy

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