What Is a Centimorgan and Why Does It Matter?

You've taken a DNA test, your results are in, and now you're staring at a list of matches — each one showing a number followed by "cM." What does that mean? And why does everyone in the genealogy world seem to treat it like such a big deal? This guide breaks it all down, from the basics to the nuances that even experienced researchers sometimes miss.

The Simple Answer: A Unit of Genetic Distance

A centimorgan (cM) is a unit used to measure the length of a shared DNA segment — or the total amount of DNA two people share. The name honors Thomas Hunt Morgan, the American geneticist who pioneered our understanding of how genes are inherited. The "centi" prefix means one-hundredth, though in genetic terms, centimorgans measure probability of recombination rather than physical distance on a chromosome.

For genealogists, the practical definition is simpler: the more centimorgans you share with a DNA match, the more closely related you are. A parent and child share roughly 3,500 cM. A fourth cousin might share anywhere from 0 to about 60 cM — or nothing at all.

Why Centimorgans — Not Percentages?

Testing companies often show your shared DNA as a percentage, and that's fine for a quick overview. But percentages can be misleading for closer analysis. Two people who share "about 12.5%" of their DNA could be half-siblings, grandparent/grandchild, aunt/niece, or half-aunt/niece — very different relationships that a percentage alone can't distinguish.

Centimorgans give you a more precise number to work with, and that precision becomes essential when you're trying to figure out exactly how — and through which ancestral line — you're connected to a match.

The Relationship Reference Table

This table shows typical cM ranges for common relationships. Keep in mind these are ranges, not fixed numbers — the actual amount you share with any given relative will vary due to the random nature of DNA inheritance.

Relationship	Typical cM Range	Average cM
Parent / Child	3,330 – 3,720 cM	~3,500 cM
Full Sibling	2,200 – 3,900 cM	~2,620 cM
Grandparent / Grandchild	1,156 – 2,311 cM	~1,754 cM
Half Sibling	1,160 – 2,650 cM	~1,759 cM
Aunt / Uncle / Niece / Nephew	1,156 – 2,311 cM	~1,754 cM
First Cousin	553 – 1,225 cM	~850 cM
Half First Cousin	125 – 650 cM	~449 cM
First Cousin Once Removed	141 – 851 cM	~433 cM
Second Cousin	41 – 592 cM	~229 cM
Third Cousin	0 – 173 cM	~74 cM
Fourth Cousin	0 – 139 cM	~35 cM

Notice the significant overlap between many of these ranges. A match sharing 1,700 cM could be a grandparent, half-sibling, aunt, or uncle — the cM value alone can't tell you which. That's why documentary research always has to work hand-in-hand with DNA evidence.

Why the Ranges Are So Wide

DNA inheritance isn't perfectly equal. Each time DNA is passed from parent to child, chromosomes undergo a process called recombination (or crossover), where segments are shuffled and exchanged before being passed on. This means two full siblings can inherit different amounts of DNA from each grandparent — and by extension, share very different amounts with their cousins.

This randomness is why the ranges in the table above are so broad, especially for more distant relationships. A third cousin might share 74 cM on average, but plenty of real third cousins share far more — or nothing at all. In fact, roughly 10% of fourth cousins share no detectable DNA, meaning they simply won't appear as a match even though the relationship is real.

Total Shared cM vs. Longest Segment

When you look at a DNA match, you'll typically see two numbers: the total shared cM and the longest segment. Both matter, and understanding the difference helps you evaluate your matches more accurately.

• Total shared cM is the sum of all DNA segments you share with that match. This is the primary number used to estimate relationship distance.
• Longest segment (in cM) tells you the length of the single largest unbroken piece of shared DNA. Longer segments tend to indicate a more recent common ancestor, since DNA gets broken into smaller pieces with each passing generation.

A match with a large total cM but many small segments may indicate a distant relationship — or could be a sign of endogamy, where ancestors repeatedly married within a close community, accumulating many small shared segments across multiple lines. A match with fewer but longer segments more reliably points to a single, recent shared ancestor.

A Note on Endogamy

If your ancestry includes populations with significant endogamy — Ashkenazi Jewish, Acadian/Cajun, certain island communities, or populations that remained geographically isolated for many generations — cM values will often overstate how closely related you are to a given match. You may see hundreds of matches in the 200–400 cM range that aren't first cousins at all, but rather distant cousins related through multiple ancestral lines simultaneously.

Recognizing endogamy in your results is crucial to avoiding misidentified relationships, and it's one of the areas where working with an experienced genetic genealogist can save you significant time and frustration.

Putting It All Together: A Practical Example

Say you have a match showing 412 cM across 18 segments, with a longest segment of 58 cM. What do you know?

• 412 cM falls in the range for a half first cousin, first cousin once removed, or second cousin — among other possibilities.
• The longest segment of 58 cM suggests a relatively recent connection, likely within the last three to four generations.
• 18 segments is a moderate number — not so many tiny fragments that endogamy is the obvious explanation.
• Your next step is to look at their family tree (if they have one), check your shared matches, and see which ancestral surnames or locations overlap with your own research.

The cM value opened the door. Documentary research is what walks you through it.

Tools That Help

You don't have to memorize relationship ranges. Several excellent free tools can help you interpret cM values quickly:

• DNA Painter's Shared cM Tool — Enter your cM value and it shows you all possible relationships with probability percentages. One of the most useful free tools in genetic genealogy.
• The Shared cM Project — Blaine Bettinger's crowd-sourced dataset of real cM values across known relationships. The gold standard reference for relationship ranges.
• WATO (What Are the Odds?) — A tool for working through complex match scenarios when you're trying to identify an unknown relationship using multiple matches simultaneously.