You spit in a tube, mail it off, and a few weeks later a DNA testing site sends you a list of people you are apparently related to.
Next to each name is a number followed by two letters, cM.
There is usually no explanation of what that number means, and no context for whether 900 cM makes someone a close cousin or a near stranger.
That number is a centimorgan, and once you understand what it actually measures, those match lists stop looking like a foreign language and start looking like a map of your real family.
What a Centimorgan Actually Measures
A centimorgan is a unit that measures genetic distance rather than physical distance.
It describes how likely a stretch of DNA is to stay together or get separated by a process called recombination as it passes from one generation to the next.
Every child inherits DNA from both parents, but the process is not a clean fifty fifty split of whole chromosomes. Instead the DNA gets cut and recombined in each generation, shuffling the genetic deck a little differently every single time it happens.
The more centimorgans two people share, the more closely their DNA lines up, and the more recently they likely shared a common ancestor.
A parent and child share close to the full amount possible because there has been no time for recombination to pull their DNA apart. A fourth cousin shares very little, because several generations of shuffling have worn that connection down to a thin thread.
Why DNA Companies Use cM Instead of Just Naming the Relationship
It would be simpler if a testing site could just tell you someone is your second cousin and leave it at that.
The problem is that DNA inheritance is not perfectly predictable. Recombination happens somewhat randomly in every generation, so two people with the exact same type of relationship will not necessarily share the exact same amount of DNA.
Take two different pairs of first cousins. One pair might share 1,100 cM and another pair might share 650 cM, and both pairs are still first cousins in every legitimate sense.
Because of that natural variation, testing companies give you the raw cM number and let genetic genealogists translate it into a range of likely relationships rather than a single guaranteed answer.
The cM Range for Every Common Relationship
Genetic genealogists have spent years collecting real world data from confirmed relationships to figure out which cM ranges line up with which relationships.
The numbers below reflect that collected data and are widely used across the genealogy community as a starting reference point.
Relationship | Average Shared cM | Typical Range |
Parent and child 3720 | 3400 to 3720 |
Full siblings 2629 | 1613 to 3488 |
Half siblings, grandparent and grandchild, or aunt or uncle and niece or nephew 1750 | 1160 to 2436 |
First cousins 866 | 396 to 1397 |
First cousins once removed 425 | 102 to 1075 |
Second cousins 233 | 41 to 592 |
Second cousins once removed 122 | 14 to 353 |
Third cousins 73 | 0 to 234 |
Fourth cousins 35 | 0 to 139 |
Distant match or statistical noise under 20 | 0 to 20 |
A few things stand out once you look at the full range.
Parent and child relationships are almost mathematically fixed, which is why DNA tests are so reliable for confirming or ruling out a parent.
Everything past first cousins starts to overlap more and more, which is exactly why a single cM number gets harder to pin down as the relationship gets more distant.
Why Some Relationships Share Almost the Same Range
You may have noticed that half siblings, grandparents and grandchildren, and aunts or uncles with nieces or nephews all land in roughly the same cM range.
That overlap is not an error. Each of those relationships involves exactly one shared ancestor connecting the two people through a single generational link, so the underlying math lines up almost identically.
This is one of the more frustrating parts of DNA genealogy. The cM number alone genuinely cannot tell you which of those three relationships you are looking at.
You need other clues to sort it out, like the age of the match, known family members, or a shared tree, before you can say with confidence which one actually applies.
When a Match Is Just Statistical Noise
Not every match on your list represents a real, traceable family connection.
Once you get below about 20 cM, the shared DNA segment becomes small enough that it may have survived by chance rather than through a specific traceable ancestor. Genetic genealogists often refer to matches this small as noise, meaning the shared DNA is technically real but rarely useful for building an accurate family tree.
This gets more complicated if your ancestors came from a small or historically isolated population, since people from those communities tend to share more background DNA with each other regardless of how closely related they actually are.
A 20 cM match pulled from a large, genetically diverse population usually means far less searching than a 20 cM match pulled from a small, tightly connected one.
How to Use the Table When a New Match Shows Up
The next time a new match appears on your DNA results, start by finding their cM number in the range table above.
Narrow the list down to the handful of relationships that number could represent, then look for outside information that can rule some of them out. A birth year close to your own makes a grandparent relationship unlikely. A shared surname or a public tree with an obvious connection can confirm a specific cousin relationship almost instantly.
Tools like the Shared cM Project and DNA Painter's relationship probability calculator take this a step further by showing the statistical likelihood of each possible relationship based on your exact cM number.
Those tools are worth using once you have a specific number in hand and want more precision than a general range can give you.
A Number That Points You Toward a Person
A centimorgan count will never tell you what someone's childhood was like, what they were proud of, or what they were afraid of.
What it can do is point you toward a real person and give you a starting number for figuring out how they fit into your family.
That part matters more than it might seem at first. Every newly confirmed relative is a doorway to another set of stories that might otherwise have stayed lost, another branch of the family who might be holding a photo, a letter, or a memory that fills in a gap nobody else could.
Once the DNA confirms the connection, the real work is getting the story out of the person behind the number, while there is still time to ask.
Memoracy was built for exactly that part, giving people a simple daily prompt to put their own life into words, so the next relative who comes looking does not find just a name on a chart, but an actual voice.
Sign up and start your first story on Memoracy today.
