You open your DNA match list and there it is, right next to a name you don’t recognize: 847 cM shared. Is that a lot? Is that a little? Should you be excited, or is this just another stranger on a very long list?
Most DNA testing platforms touch on centimorgans somewhere in their help documentation. This is for you if you want to understand what the number actually represents before you start using it.
A Unit Named After a Scientist
The centimorgan is named after Thomas Hunt Morgan, an American geneticist who spent the early 1900s working out how traits get passed from one generation to the next. He won the Nobel Prize for it in 1933. Morgan was building on the pattern work that Mendel had done with his peas decades earlier. Mendel found the patterns. Morgan found the mechanism. The unit named after him measures the probability that two points on a chromosome will get separated during inheritance. One centimorgan equals a 1% chance of that separation happening in a single generation.
What that means in practice is simpler: centimorgans measure how much genetic material two people share. The more centimorgans you share with someone, the more recently you shared a common ancestor.
The Math Behind the Centimorgan Ranges
If you remember anything from science class about basic genetics, the logic here should feel familiar. You receive roughly half your DNA from each biological parent. Not exactly half, but close — somewhere between 47% and 53% from each.
If you’ve ever fed a sourdough starter, you already have a picture of this. You pull out a portion of what’s there and discard the rest. What you kept carries forward into the next generation. What you didn’t simply doesn’t. DNA inheritance works something like that. At every generation, roughly half gets selected and passed on. The other half doesn’t travel with it.
Follow that forward. If you got roughly half from each biological parent, and each of them got roughly half from their biological parents, then your biological grandparents each contributed about a quarter of your total DNA. Your great-grandparents, about an eighth. The same math holds as you move sideways in the tree, to cousins and their children, though the numbers stay logical while the ranges start to overlap in ways that can get confusing fast.
Centimorgans are just a way of measuring that contribution. Here is roughly what to expect across common biological relationships:
- Biological parent and child: 3,400 to 3,900 cM
- Full biological siblings: a similar range, with more variation
- Biological grandparent and grandchild: around 1,700 cM
- Biological first cousins: roughly 550 to 1,200 cM, with 850 cM as a common average
As relationships get more distant, the numbers drop quickly. Second cousins share roughly 75 to 360 cM. By fifth cousins, you might share only 20 to 40 cM, and some fifth cousins share nothing detectable at all.
That last point matters. DNA inheritance is random. You do not receive a perfectly even slice from every ancestor. Close biological relationships show up reliably. Relationships further out are hit or miss, and the miss is not a mistake. It just means the shuffle of inheritance did not deal matching cards from that particular ancestor. The DNA Painter Shared cM tool maps this out visually and is worth bookmarking early.
Centimorgans Measure Shared DNA, Not Closeness
When we see a cM count, the instinct is to translate it immediately into relationship distance. How close is this person? That is a natural question, but it is not quite what centimorgans are measuring. They measure how much DNA two people share. That is related to closeness, but it is not the same thing. What it is really reflecting is how much genetic material two people happened to receive from the same ancestors.
And because the shuffle is random at every step, two full biological siblings can receive noticeably different amounts from the same parents. The range exists because that 47% to 53% variance applies at every step independently. The low end of the first cousin range reflects what happens when both halvings landed toward 47%. The high end reflects both landing toward 53%. Most people fall somewhere in between. Every relationship range on the shared cM chart is that same calculation run out to however many generations separate you from the shared ancestor. The further back the shared ancestor, the more steps, the more the variance compounds, and the wider the range gets.
Why the Same Number Can Mean Different Things
Here is where it gets complicated, and why centimorgans are a starting point rather than an answer.
The same centimorgan count can point to multiple different relationships. 400 cM could mean first cousin twice removed, or half first cousin, or great-grandparent. Those relationships are actually similar in distance from you on the tree. A great-grandparent is roughly three generations removed, and so is a first cousin twice removed. The tree positions are different, but the genetic math lands in the same neighborhood.
This is also where surprises show up. When the cM numbers do not match what the family tree says they should, it often means the shared ancestors were not who everyone assumed. Adoptions, informal child placements, or other circumstances that were never recorded or spoken of. The DNA reflects the biology accurately. It just does not know the story behind it.
One more variable worth naming: all of this assumes the lines in your family tree are distinct. If you have endogamy in your background, the cM counts will run higher than the ranges suggest. Endogamy is what happens when one or more ancestors appear on more than one branch of the family tree. Relatives marrying relatives. It shows up in closed communities defined by religion, culture, or social custom, but it does not have to be dramatic. Early settlers in isolated regions often simply married who was nearby, because nearby was all there was. When that pattern repeats across generations, two people can share significantly more DNA than the standard ranges would predict. The numbers are still accurate. What they are pointing to may need a second look.
How to Use Centimorgans Without Getting Paralyzed
A tool people reach for when doing this kind of research is a shared cM chart. The DNA Painter Shared cM tool is a good one. Enter the centimorgan count and it shows you every probable relationship along with the likelihood of each. At 847 cM, for example, it will show you that first cousin is the most likely relationship, but it will also show you the other possibilities and their relative probabilities.
That gives you a working theory. What confirms or eliminates the theory is everything else: other DNA matches you share with this person, which side of the family those shared matches come from, and whatever paper records connect the two of you.
This is where triangulation comes in. If you have a mystery match and one of your biological parents is in the same database, you can often tell within minutes which side of the family the match belongs to. That narrows the field considerably. From there, the shared matches start pointing toward specific surnames and branches.
The centimorgan count opens the question. The research answers it.
When the Match List Feels Overwhelming
When you first open your DNA match list, the number of people on it can feel staggering. Hundreds of matches, sometimes thousands, almost none of them familiar. Part of what is happening is that the platforms are surfacing very distant relationships, sometimes down to 6 or 8 cM, where the connection is so thin it may not be genealogically meaningful.
A 20 cM match is not a close family discovery. It is a distant possibility — someone who may share a common ancestor from five, six, or seven generations back. Start with your highest centimorgan matches and work down. The people who share 500 cM or more are your most informative matches. The rest can wait.
The Short Version
Centimorgans measure shared DNA. More centimorgans means a more recent common ancestor on average, but the same count can point to multiple relationships because the random shuffle of inheritance compounds across generations. Like every tool in this work, centimorgans are a starting point. They raise a question. The records, the shared matches, and the family history you can verify are what answer it.