What Is a 2nd Cousin? The Complete Family Guide

A 2nd cousin is a person who shares a set of great-grandparents with you, but not a set of grandparents. That single sentence is the whole 2nd cousin definition. If you and another person can both trace your family line back to the same great-grandmother and great-grandfather, and you are each exactly three generations down from them, you are second cousins. Everything else people argue about at reunions grows out of that one sentence.

The confusion usually starts because families don't talk in generations. They talk in nicknames, in "your mom's cousin Dee," in "the people from the Ohio side." So when somebody asks what does second cousin mean, they are rarely after a dictionary line; they want to know how the person across the table actually connects to them. Let's untangle it properly, with the counting rule, the DNA numbers, the marriage question, and the part where "once removed" finally makes sense.

The One Rule That Solves Every Cousin Question

Count how many generations each person is from the most recent common ancestor. The cousin number is the smaller count minus one, and the "removed" number is the difference between the two counts.

That's it. Genealogists call the shared ancestor the MRCA, which stands for most recent common ancestor. Once you find the MRCA, you count upward from each living person, and the math falls out. It is also why you never need a separate rule to define 2nd cousin as opposed to third or fourth: one piece of arithmetic covers every degree.

Here's how the counting works in practice:

1. Find the closest ancestor the two of you both descend from. For second cousins, that's a great-grandparent couple.
2. Count generations from person A up to that ancestor. Your parent is 1, your grandparent is 2, your great-grandparent is 3.
3. Count generations from person B up to that same ancestor.
4. Take the smaller number and subtract 1. That's the cousin degree.
5. Take the difference between the two numbers. That's how many times removed.

For two true second cousins, both counts are 3. So 3 minus 1 gives you second cousins, and 3 minus 3 gives you zero removals. Clean and symmetrical.

If counting generations in your head makes your eyes glaze over, the free cousin calculator on cousinchart.com will do it in a click. But it genuinely helps to understand the rule underneath, because that's what lets you explain it to your aunt.

The Great-Grandparent Test

The fastest way to confirm someone is your second cousin is to ask whether your grandparents were siblings.

Think about the chain. Your great-grandparents had children. Two of those children are your grandparent and your second cousin's grandparent, so those two are siblings. Their children (your parent and your second cousin's parent) are first cousins to each other. And their children, which is you and the other person, are second cousins.

So the ladder looks like this:

• Your parents are siblings → you are siblings
• Your grandparents are siblings → you are first cousins
• Your great-grandparents are siblings → wait, that's not right, and this is where people trip

Careful with that last line. It's not that your great-grandparents are siblings. It's that your grandparents are first cousins to each other, which happens because their parents were siblings. Say it the clean way:

• Your parents were siblings → you're siblings
• Your parents were first cousins → you're second cousins
• Your parents were second cousins → you're third cousins

That's the cleanest shortcut in the whole subject. Your parents' first cousins' children are your second cousins. The cousin degree goes up by exactly one with each generation, as long as nobody is skipping a rung.

Full Cousin Degree Table With Shared DNA

Second cousins share about 3.125% of their DNA on average, which works out to roughly 229 centimorgans on a consumer DNA test. Here's how that sits alongside the neighboring relationships.

The observed ranges come from the Shared cM Project, Blaine Bettinger's crowdsourced dataset built from tens of thousands of real, documented relationships. It's the honest source for this, because it reports what people actually measure rather than what the textbook predicts.

Notice how wide those ranges are. A second cousin pair can land anywhere from 41 cM to nearly 600 cM. That's not sloppy testing. That's how inheritance works, and it's worth a section of its own.

Why the Percentages Are Averages, Not Promises

DNA percentages between cousins are averages across many pairs, and any individual pair can land far from the average because inheritance past the parent-child level is random.

You get exactly 50% of your DNA from each parent. That part is fixed, and it's the only fixed part. But which 50% your parent passed to you was decided by a shuffling process called recombination, and it happened independently for your cousin.

So two siblings do not pass identical pieces of their parents down. One might hand down a big chunk of great-grandma's chromosome 4. The other might hand down almost none of it. Follow that down two more generations and the variation compounds.

A few things follow from this that are genuinely useful:

• A cM number alone cannot prove a specific relationship. A 229 cM match is a great second cousin candidate, but it's also a plausible first cousin twice removed or half first cousin once removed.
• A low number doesn't disprove a documented relationship. If your paper trail says second cousin and the test says 60 cM, the paper trail is probably fine.
• Essentially 100% of true second cousins share detectable DNA. This is the good news. At the second-cousin level, the odds of sharing nothing are effectively nil, which is why second cousins are so reliable for confirming a family line. That reliability drops off fast at third and fourth cousins, and by the time you reach a 5th cousin a large share of true pairs share no detectable DNA at all. Where that line falls is covered in the third cousin guide.

For context, the total measurable genome on these tests is roughly 6,800 cM, with about 3,400 to 3,500 cM coming from each parent. When you see 229 cM, you're looking at a little over 3% of that total.

What "Second Cousin Once Removed" Actually Means

A second cousin once removed is either your second cousin's child or your parent's second cousin, because "removed" always means a generational gap, never a distance sideways.

This is the single biggest misunderstanding in cousin terminology. People hear "removed" and picture someone further away in the family, a more distant relative. It doesn't mean that. It means the two of you sit on different rungs of the family ladder.

Run the counting rule. One person is 3 generations from the great-grandparent MRCA. The other is 4 generations from that same couple. The smaller number is 3, so 3 minus 1 gives second cousin. The difference is 1, so once removed. Together: second cousin once removed.

Two ways to hold it in your head:

• Looking down: your second cousin's kids are your second cousins once removed. Their grandkids are your second cousins twice removed.
• Looking up: your parent's second cousins are your second cousins once removed. Your grandparent's second cousins are your second cousins twice removed.

Both directions get the same label, which trips people up. Your second cousin once removed might be a toddler or might be eighty. The term describes the shape of the relationship, not the ages involved.

Second Cousin vs. First Cousin Once Removed

A second cousin and a first cousin once removed are not the same relationship, and they are not even close genetically. A first cousin once removed shares roughly double the DNA.

This mix-up happens constantly because both people can be around the same age and both can feel like "sort of a cousin." Here's the difference laid out plainly.

First cousin once removed: - One person is 2 generations from the MRCA, the other is 3 - The MRCA is a grandparent for one of you, a great-grandparent for the other - It's your first cousin's child, or your parent's first cousin - Average shared DNA: ~6.25%, about 433 cM

Second cousin: - Both people are 3 generations from the MRCA - The MRCA is a great-grandparent couple for both of you - It's your parent's first cousin's child - Average shared DNA: ~3.125%, about 229 cM

The test question that settles it: is the shared ancestor the same distance from both of you? If yes, nobody is removed and it's a plain second cousin. If one of you is a generation closer, someone is removed and the cousin number comes from whoever is closer.

Here's a concrete family scene. Your mom has a first cousin named Rita. Rita is your first cousin once removed. Rita's daughter Priya is your second cousin. Same family, one generation apart, half the DNA. Rita and Priya are not interchangeable, even though everyone at the picnic calls them both "cousin."

How Many 2nd Cousins Do You Have?

Most people have somewhere around 150 to 200 second cousins, though this is a rough estimate that depends completely on how many children were born in each branch of the family.

Common ballpark figures, and every one of these should be treated as a soft estimate rather than a fact:

• Around 20 to 30 first cousins
• Around 150 to 200 second cousins
• 1,500 or more third cousins for people from large families

The numbers balloon at each step because you're multiplying across eight great-grandparents' worth of descendants instead of four grandparents'. Every sibling those great-grandparents had, every child, every grandchild feeds into the count.

But family size drives everything. If your great-grandparents each had two kids, you might have a dozen second cousins total. If they were part of families of ten, and their children had large families too, you could pass 400 without leaving one branch. Immigration patterns, wars, infant mortality, and religious traditions around family size all move this number enormously. Any single number you see quoted online is an average of averages.

The practical takeaway: you almost certainly have second cousins you've never met, and probably some you've met without knowing the connection.

Can 2nd Cousins Legally Marry in the US?

Yes. Second cousins can legally marry in all 50 US states, with no restrictions anywhere in the country.

Every state restriction on cousin marriage in the United States applies to first cousins or closer. Some states permit first cousin marriage outright, some prohibit it, and some allow it under conditions such as age or infertility. None of that reaches second cousins.

The genetic reasoning is straightforward. Second cousins share about 3.125% of their DNA on average, which is a small fraction, and the elevated risk of recessive genetic conditions in their children is very close to the risk for any unrelated couple. Geneticists generally treat second cousin pairings as carrying no meaningful additional risk.

Marriage law changes, and it varies by country. If this is a live question for you rather than a curiosity, confirm the current rules with your state or country's marriage license office rather than relying on any article, this one included.

Finding Your Second Cousins on DNA Testing Sites

Second cousins are the easiest useful match to spot on a DNA site, because they land in a distinctive cM band and almost always come with a recognizable surname.

A few things that make the search go faster:

Look at the cM number, not the label. Testing companies bundle predictions into categories like "2nd–3rd cousin," which is a wide net. The centimorgan figure is the real data. A match in the 150–350 cM range is a strong second cousin candidate, though it overlaps with first cousin twice removed, half first cousin once removed, and great-aunt or great-uncle relationships.

Work the shared matches tool. If a mystery match shares a cluster of matches with your known maternal cousins, you've just narrowed the search to one grandparent's line. This single feature does more work than anything else on these sites.

Build back four generations, not two. To identify a second cousin you need all eight of your great-grandparents named. Most people's trees peter out at grandparents, which is exactly one generation short of useful.

Compare surnames sideways. Your second cousin may not carry any surname you recognize, because the connection can run through two mothers. Look at the surnames in their tree's grandparent generation, not their own last name.

Expect the tree to be private. Many matches never build one. A short, friendly message that names your shared great-grandparents and asks a single specific question gets a much better response rate than a generic hello.

What to Call Them at the Reunion

There is no special title for a second cousin in English, and "cousin" is the correct and universally accepted thing to call one.

English is unusually thin here. Dictionaries define cousin so loosely that the one word stretches from the person you grew up beside to somebody you have never met, and the language does nothing to narrow it. Many languages have distinct words separating your father's brother's child from your mother's sister's child, and some mark cousin degree directly. English gives you "cousin" and a numbering system almost nobody uses out loud.

So in practice: call them cousin. If you need to be specific, say "we're second cousins, our grandmothers were sisters," which lands better than the numbering alone because it points at a person everybody remembers.

A note on the older generation. In many families, your parent's first cousin gets called "Aunt" or "Uncle" as a courtesy, even though the technical term is first cousin once removed. That's a warm custom, not an error, and correcting it at a funeral will not win you friends.

Frequently Asked Questions

Is my second cousin's child my third cousin?

No. Your second cousin's child is your second cousin once removed. They become a third cousin to your children, not to you. The cousin number only advances when both people move down a generation together, which is exactly what the counting rule captures.

How much DNA do 2nd cousins share?

About 3.125% on average, roughly 229 centimorgans on a consumer DNA test. The observed range in the Shared cM Project runs from about 41 cM to 592 cM, so individual pairs vary widely. Essentially all true second cousins share some detectable DNA, which makes this a reliable relationship to confirm by testing.

What is the difference between a second cousin and a first cousin twice removed?

A second cousin is 3 generations from your shared great-grandparents, same as you. A first cousin twice removed is 2 generations from the shared ancestor while you are 4, or the reverse. They share more DNA on average than second cousins and sit on a different generational level, so despite similar-sounding names they're distinct relationships.

Can second cousins have children safely?

Yes. The genetic risk to children of second cousins is very close to that of unrelated parents, because the two share only about 3% of their DNA. Any couple with concerns about inherited conditions can speak with a genetic counselor, but second cousin pairings are not considered a meaningful risk factor.

Why do I share more DNA with one second cousin than another?

Because DNA inheritance is random once you get past the parent-child step. Each ancestor's contribution gets shuffled and cut differently in every line of descent. Two people who are both genuinely your second cousins can easily differ by a factor of five in shared centimorgans, and neither result says anything is wrong.

Do second cousins show up on every DNA test?

Almost always, yes. At the second cousin level, detection is effectively certain on the major autosomal tests, so if a documented second cousin doesn't appear in your match list, the likeliest explanations are that they tested with a different company or that their results aren't set to be visible to matches.