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Two people, one common ancestor
Every family relationship between two people can be described by counting upward. Start from the first person and count generations up to the ancestor you both share. Do the same from the second person. Those two numbers are all you need. If both counts are one, the two people are siblings, because they share a parent. If one count is zero, one person is the direct ancestor of the other. If both counts are two, they are first cousins, because they share grandparents. The rest of the family tree follows from the same two counts, and this calculator does that counting for you.
This tool is about deriving a relationship from ancestor distances. You tell it how many generations each person sits below the common ancestor, and it names the relationship. That is a different job from the cousin calculator on this site, which starts from a cousin degree and a number of times removed and explains what that label means. The two tools answer opposite questions: this one asks “what are we?”, the other asks “what does this label mean?” Keep that distinction in mind and you will always pick the right tool.
How to use the calculator
- Find the most recent ancestor both people share.
- Count generations from the first person up to that ancestor.
- Count generations from the second person up to the same ancestor.
- Enter both counts and read the relationship name.
The counting itself is where most people slip. Your parent is one generation up, your grandparent is two, your great-grandparent is three. A common error is counting yourself as a generation, which shifts every answer by one. Another is picking the wrong common ancestor: always use the most recent one you both share. If two people share both grandparents and great-grandparents, the grandparents are the most recent common ancestors, and the relationship is first cousins, not second. Using a more distant ancestor gives a more distant-sounding answer that is simply wrong.
A worked example
Suppose Ana is two generations below her grandmother, and Ben is three generations below the same grandmother. The smaller count is two, so the cousin degree is one less than that: first cousins. The difference between the counts is one, so they are first cousins once removed. “Removed” is the word genealogy uses for a generation gap between cousins, and it confuses almost everyone at first. It does not mean removed from the family. It means the two cousins sit in different generations, and the number tells you how many generations apart they are.
Now suppose one count is one and the other is two. The smaller count is one, which means one person is a child of the common ancestor and the other is a grandchild. That is an aunt or uncle with a niece or nephew, not a cousin at all. Cousin relationships only begin when both people are at least two generations below the common ancestor. The calculator handles that boundary for you, so you do not need to remember the rule, only the two counts.
Why the names matter
- Records use these terms, so reading an old will or letter correctly depends on them.
- DNA matches are often described as “second cousin” or “third cousin once removed”, and the label predicts how much DNA you should share.
- Family trees drawn from memory often skip a generation; the calculator exposes that gap.
- Legal and medical family histories also use generation counts, so a correct label avoids real confusion.
Research aid only. Family records vary in accuracy, spellings change, and relationships recorded in old documents are not always biological ones. Treat this tool’s answer as a labelled starting point, then confirm it against primary sources such as birth, marriage and census records before you write it into a family tree or share it as fact. Estimates and labels are approximate; primary sources decide.
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