A genealogy relationship calculator helps you figure out how two people are related based on their common ancestors, and look up how much DNA (in centimorgans) different relationships typically share. Use the cousin calculator tab to find the relationship name, or the DNA cM tab to identify possible relationships from a DNA test result.
Enter how many generations each person is from your closest shared ancestor (grandparent = 2, great-grandparent = 3, etc.).
Quick Reference — Common Relationships
| Your Gens | Their Gens | Relationship | Avg cM |
|---|---|---|---|
| 1 | 1 | Parent / Child | 3,700 |
| 2 | 2 | First Cousins | ~850 |
| 2 | 3 | First Cousin Once Removed | ~425 |
| 3 | 3 | Second Cousins | ~230 |
| 3 | 4 | Second Cousin Once Removed | ~115 |
| 4 | 4 | Third Cousins | ~90 |
How to Use the Genealogy Relationship Calculator
Understanding family relationships is essential for genealogy research — both for building family trees and for interpreting DNA test results. This genealogy relationship calculator has two functions: a cousin calculator to find relationship names, and a DNA cM reference to identify likely relationships from test results.
Using the Cousin Calculator
Count the number of generations from you to the common ancestor (grandparent = 2 generations, great-grandparent = 3 generations, 2x great-grandparent = 4 generations). Do the same for your relative. Enter both numbers. The calculator outputs the relationship name and explains the "removed" concept if your counts differ.
Understanding "Removed" Relationships
"Removed" describes a generational difference between two people in a cousin relationship. Your first cousin's child is your first cousin once removed — you are both descended from the same great-grandparent, but one of you is one generation further away. "Second cousin once removed" means you share 3x great-grandparents but with a one-generation gap. Your own children will be one generation further removed from all of your cousins.
Using the DNA cM Reference
After taking a DNA test (AncestryDNA, 23andMe, MyHeritage, FamilyTreeDNA), you receive a list of matches showing how many centimorgans (cM) you share with each person. Enter that number in the DNA cM lookup tab to see which relationships fall within that range. Many cM ranges overlap between relationship types — the reference table shows the average, min, and max for each relationship based on thousands of verified tested pairs from Blaine Bettinger's Shared cM Project.
Why Shared DNA Ranges Overlap
DNA inheritance is not perfectly predictable. Each child randomly inherits half of each parent's chromosomes, and recombination events further vary which segments are passed down. Over multiple generations, this variation compounds — a second cousin might share 500 cM (well above average) while another second cousin shares only 40 cM (below average). Never rely on shared cM alone to determine a relationship without corroborating family tree evidence.
Frequently Asked Questions
Is this genealogy relationship calculator free?
Yes, completely free with no signup required. All calculations run in your browser and no data is transmitted anywhere.
Is my data private when using this tool?
Absolutely. Everything runs locally in your browser using JavaScript. No family or DNA data is stored or transmitted.
What is a first cousin once removed?
A first cousin once removed is either the child of your first cousin, or your parent's first cousin. The 'removed' means one generation difference between the two people. Your first cousin's child is your first cousin once removed because you both share a great-grandparent — but one of you is one generation further from that ancestor.
How do I use the cousin calculator?
Count how many generations back each of you must go to reach your first common ancestor. For example, if you share a great-grandparent: you are 3 generations from them, and so is your relative. The cousin degree is one less than the smaller number (3-1=2, so second cousins). The 'removed' count is the difference between the two generation counts.
What is a centimorgan (cM) in DNA testing?
A centimorgan measures the length of a DNA segment shared between two people. The more centimorgans two people share, the more closely related they are. A parent and child share about 3,400–3,900 cM, while second cousins share about 40–500 cM. DNA testing services report your total shared cM with each DNA match.
Why is the DNA cM range so wide for some relationships?
DNA inheritance is random — not every child inherits exactly half of each parent's DNA, and chromosomes recombine differently in each generation. This random variation means two half-siblings could share anywhere from 1,160 to 2,650 cM. The ranges given are real observed ranges from thousands of tested pairs, not theoretical estimates.
Can shared DNA cM tell me the exact relationship?
Sometimes, but many relationships have overlapping cM ranges. For example, a half-sibling and a grandparent share similar cM ranges (~1,700 cM average). You need to combine cM evidence with other tools — family tree, age of match, known relatives who also match — to determine the most likely relationship. A genetic genealogist can help with complex cases.