Centimorgans Explained: How to Work Out How You're Related to a DNA Match

Open your list of DNA matches and you'll see a number next to every name, such as "212 cM across 9 segments". That number is the key to understanding how you might be related to each person. Once you know how to read it, your match list stops being a wall of strangers and becomes a set of clues.

This guide explains what centimorgans are, how much DNA you typically share with different relatives, and a simple, reliable method for working out where a match fits into your family.

What is a centimorgan?

A centimorgan (cM) is a unit used to measure lengths of DNA in terms of how likely they are to be inherited together. You don't need to understand the biology in depth. For family history, it's enough to know that:

• The more centimorgans you share with someone, the more closely related you are likely to be.
• DNA is shared in segments. The longer the segments and the more of them there are, the closer the relationship tends to be.
• Your total autosomal DNA is roughly 7,000 cM, counting both copies of each chromosome. You share roughly half of that, about 3,400 to 3,700 cM, with each parent.

Why shared DNA varies

You inherit exactly half your DNA from each parent, but which half is random. That randomness compounds each generation. On average you share about 25% with a grandparent, but the real figure for any particular grandparent might be 20% or 30%.

So each relationship has an average amount of shared DNA, and a range around it. Ranges for different relationships overlap, which is why a single number can't tell you the exact relationship, only a set of possibilities.

Shared DNA for common relationships

The figures below are approximate, based on data compiled by the Shared cM Project, a large collaborative study of tens of thousands of known relationships. Different testing companies measure slightly differently, so treat these as guides.

• Parent / child: average about 3,485 cM (range roughly 2,375 to 3,720)
• Full sibling: average about 2,613 cM (range roughly 1,613 to 3,488)
• Half sibling, grandparent, aunt or uncle: average about 1,750 cM (range roughly 1,160 to 2,440)
• First cousin: average about 866 cM (range roughly 396 to 1,397)
• First cousin once removed: average about 433 cM (range roughly 102 to 980)
• Second cousin: average about 229 cM (range roughly 41 to 592)
• Second cousin once removed: average about 122 cM (range roughly 14 to 353)
• Third cousin: average about 73 cM (range roughly 0 to 234)
• Fourth cousin: average about 35 cM (range roughly 0 to 139)

Two things stand out. First, the ranges overlap a great deal. A match sharing 300 cM could be a second cousin, a first cousin once removed, a half first cousin, or several other relationships. Second, from third cousins onwards, it's possible to share no detectable DNA at all with a genuine relative.

The free Shared cM Project tool at DNA Painter lets you enter an amount of shared DNA and see which relationships fit, with probabilities. It's one of the most useful free tools in genetic genealogy.

A five-step method to work out a relationship

Step 1: Note the shared cM and the possible relationships

Enter the total shared cM into the Shared cM Project tool. Write down the most likely relationships. For example, 210 cM points most strongly to second cousin, half first cousin once removed, first cousin twice removed, or similar.

Step 2: Use ages and generations

Age helps you choose between relationships. If a match sharing 900 cM is 80 and you're 35, they're unlikely to be your first cousin and could more plausibly be your great-aunt or great-uncle, or a first cousin once removed. Check the match's profile, tree, or any clues they give about their generation.

Step 3: Identify which side of the family

Look at your shared matches, people who match both you and this person. If they include known relatives from your mother's side, this match is probably on your mother's side too. If a parent has tested, simply check whether the match also matches them.

Step 4: Look for the common ancestor

Compare trees. For a likely second cousin, you're looking for a shared set of great-grandparents. For a likely third cousin, shared great-great-grandparents. Look for matching surnames, places and dates.

Tools such as AncestryDNA's ThruLines and MyHeritage's Theory of Family Relativity can suggest a common ancestor automatically. Verify their suggestions against records.

Step 5: Test your theory

A good theory explains not just one match but several. If you believe you descend from a particular couple, you should find matches who descend from several of that couple's children, and their shared DNA should be consistent with the relationships you'd expect.

A worked example

Suppose you match a woman named Susan, sharing 245 cM across 11 segments. You're 45; Susan's profile suggests she's in her late 40s.

1. Possibilities: 245 cM is most consistent with second cousin, half first cousin once removed, or first cousin twice removed.
2. Ages: you're of similar age, which fits second cousin well.
3. Side: your shared matches with Susan include your known cousin on your mother's father's side.
4. Common ancestor: Susan's tree includes a great-grandmother named Annie Burke, born in County Clare, Ireland. Your maternal grandfather's mother was also Annie Burke, born in County Clare.
5. Conclusion: Susan is very likely your second cousin, sharing your great-grandparents, Annie Burke and her husband.

Segments: why they matter

As well as total cM, look at the number and size of segments.

• Close relatives share many long segments.
• Distant relatives tend to share a few short segments.
• Very small single segments, under about 7 to 10 cM, can sometimes be "false positives" or reflect very distant shared ancestry that's not traceable in records.

The size of the largest segment can help distinguish relationships, too. A match sharing a large total but in many small segments may be related through several distant lines, which is common in communities where families intermarried over generations, a situation called endogamy.

Endogamy and other complications

If your ancestors came from a small, close-knit population, such as a small island, an isolated rural area, or a community like Ashkenazi Jews, you may share DNA with matches through multiple lines. This inflates shared cM, making matches look more closely related than they are.

Other complications include:

• Half relationships: a half first cousin shares roughly the same DNA as a first cousin once removed.
• Double relationships: when two siblings from one family married two siblings from another, their children are double first cousins and share more DNA than ordinary first cousins.
• Unknown parentage: sometimes a match doesn't fit because a relationship in someone's tree isn't what it seems.

MyHeritage's chromosome browser can help with advanced analysis by showing exactly where you share DNA. See our AncestryDNA vs MyHeritage comparison for more on the tools each offers.

Understanding "removed" cousins

Relationship terms can be confusing, so it's worth a quick refresher. Cousins share a common ancestor; the number tells you how far back. First cousins share grandparents, second cousins share great-grandparents, and third cousins share great-great-grandparents.

"Removed" means you're in different generations. Your first cousin's child is your first cousin once removed, and so is your parent's first cousin. Your first cousin's grandchild is your first cousin twice removed.

A simple way to work it out: find the common ancestor, count how many generations down each of you is, and compare. If you're both three generations down (great-grandchildren), you're second cousins. If one is three generations down and the other is four, you're second cousins once removed.

Using DNA to confirm, not just discover

Centimorgans aren't only for identifying unknown matches. They're also a way to test the family tree you've already built from records. If your tree says a particular match is your second cousin, their shared DNA should fall within the second cousin range. If it falls well outside, something may be wrong, in your tree, in theirs, or in an assumed relationship somewhere along the line.

The strongest evidence comes from several matches who descend from the same ancestral couple through different children. If all of them share amounts of DNA consistent with their expected relationship to you, you can be far more confident in that part of your tree. This approach, combining records and DNA, is the gold standard in modern genealogy.

Keep track of which matches you've used as evidence in a research log, and note the shared cM alongside the expected relationship.

Which matches should you focus on?

For most family history goals, your most useful matches are those sharing roughly 50 to 400 cM. They're close enough that you can usually identify the common ancestor, and distant enough that they can reveal branches of your tree you don't already know.

Matches under 20 cM are numerous, but much harder to work with. Leave them until you've worked through the closer ones.

Getting more matches to work with

The more matches you have, the more clues you have. A larger database means more relatives, which is why AncestryDNA, with by far the largest database, is our top pick for genealogy. Uploading your raw data to other services such as MyHeritage adds still more.

New to DNA? Start with our guide to what to do when your results arrive.

Quick reference: what your cM number suggests

• Over 2,300 cM: parent, child or full sibling.
• About 1,300 to 2,300 cM: grandparent, grandchild, aunt, uncle, niece, nephew or half sibling.
• About 600 to 1,300 cM: most often a first cousin, or a great-grandparent, great-aunt or great-uncle.
• About 200 to 600 cM: commonly first cousins once removed or second cousins.
• About 75 to 200 cM: often second cousins once removed or third cousins.
• Under 75 cM: usually third cousins or more distant.

Use this as a starting point only. The full ranges overlap considerably, so always check the possibilities with a tool such as the Shared cM Project, and use ages, shared matches and trees to narrow them down.

Frequently asked questions

How many cM do second cousins share?

On average around 229 cM, but the range is wide, roughly 41 to 592 cM.

Can I share 0 cM with a real cousin?

Yes. Beyond about third cousins, it's increasingly common to share no detectable DNA with a genuine relative.

Why does my match show different cM on different sites?

Each company uses its own methods to identify and measure shared segments, and some filter out certain regions. Small differences are normal.

Is cM the same as percentage?

They're related. Dividing shared cM by roughly 7,000 gives an approximate percentage of shared DNA, but cM is the more useful measure for genealogy.