Degree and removal explained
Cousin relationships have two independent components, and confusing them is why the terminology feels harder than it is. The degree — first, second, third — counts how many generations back the shared ancestor is. First cousins share grandparents, second cousins share great-grandparents, third cousins share great-great-grandparents.
The removal counts the generation difference between the two people. Two people of the same generation are cousins with no removal. If one is a generation further from the shared ancestor, they are "once removed".
So your first cousin's child is your first cousin once removed, not your second cousin — a distinction almost universally got wrong in casual use. Your parent's first cousin is also your first cousin once removed, since the relationship is symmetrical. Your second cousin is the child of your parent's first cousin.
How much DNA relatives actually share
Expected shared DNA halves with each degree of separation, and the numbers are useful for interpreting genetic testing results. Full siblings share about 50 percent, half siblings, grandparents, aunts and uncles about 25 percent, first cousins about 12.5 percent, second cousins about 3.1 percent, and third cousins about 0.78 percent.
How many cousins do I have?
Nobody can give you an exact number without a full family tree, but the arithmetic of descent gives a usable range. Each degree further out multiplies the count enormously, because it counts every descendant of an ever-larger set of ancestors. The ranges below assume typical family sizes over the last few centuries — a family with many children per generation lands at the high end, a small one at the low end.
| Relationship | Shared ancestor | Typical number | DNA shared | Detected range |
|---|
| First cousins | Grandparents | ~5–8 | ~12.5% | 850–1,300 cM |
| Second cousins | Great-grandparents | ~20–30 | ~3.13% | 200–620 cM |
| Third cousins | 2× great-grandparents | ~150–190 | ~0.78% | 0–234 cM |
| Fourth cousins | 3× great-grandparents | ~900–1,300 | ~0.20% | 0–139 cM |
| Fifth cousins | 4× great-grandparents | ~4,700–17,000 | ~0.05% | 0–117 cM |
| Sixth cousins | 5× great-grandparents | ~20,000–170,000 | ~0.01% | Usually none |
| Seventh cousins | 6× great-grandparents | ~100,000–1.7M | ~0.003% | Usually none |
| Eighth cousins | 7× great-grandparents | ~400,000–17M | ~0.0008% | Usually none |
| Ninth cousins and beyond | 8× great-grandparents or earlier | Millions | Negligible | Effectively none |
Two things worth noting. Past about the sixth degree the counts stop being meaningful, because family trees overlap — the same ancestor appears in your tree by several routes, so the theoretical count badly overstates the number of distinct people. And beyond third cousins a DNA test frequently detects nothing at all, so a genuine relative can share no measurable DNA with you.
Those are averages, and actual sharing varies considerably because inheritance is random beyond the parent-child relationship. Two first cousins might share anywhere from roughly 7 to 19 percent. By the fourth cousin level, the expected share is small enough that a meaningful number of genuine fourth cousins share no detectable DNA at all — so an absence of shared DNA does not disprove a distant documented relationship.
This is also why shared DNA alone cannot distinguish some relationships. A half sibling, a grandparent, and an aunt or uncle all share about 25 percent, so testing services present a range of possibilities rather than a single answer, and resolving it requires documentary evidence or additional relatives.
Terminology varies more than you would think
The English cousin system is unusual in collapsing many distinct relationships into one word. Many kinship systems distinguish maternal from paternal relatives, and cross cousins — the children of a parent's opposite-sex sibling — from parallel cousins. Chinese kinship terminology has separate specific terms for a large number of relationships English handles with "cousin" plus qualifiers.
Some English usage differs regionally too. "Second cousin" is used colloquially in places to mean any more distant relative, and "cousin-brother" appears in Indian English for a close male cousin. Removal is frequently dropped entirely in speech.
Legal definitions are their own matter and vary by jurisdiction. Rules on marriage between cousins differ substantially between countries and between US states, and inheritance law defines degrees of kinship by its own formal method rather than by the cousin terminology. This calculator describes the genealogical relationship; anything with legal consequences should be checked against the relevant law.