A centimorgan (cM) is a unit that measures genetic distance between two points on a chromosome. One centimorgan equals a 1% chance that a recombination event, the shuffling of DNA that happens when egg and sperm cells form, will separate two genetic markers in a single generation. If you’ve taken a DNA test and found matches sharing a certain number of centimorgans, that number tells you roughly how much DNA you have in common and, by extension, how closely you’re related.
How Centimorgans Work
When your body creates egg or sperm cells, pairs of chromosomes line up and swap segments with each other. This process, called recombination, means the chromosomes you pass to a child aren’t identical copies of what you inherited. They’re reshuffled versions. Centimorgans measure how likely two spots on a chromosome are to get separated during that shuffle.
Two markers that are 1 cM apart have a 1% chance of being split in any given generation. Markers 50 cM apart have roughly a 50% chance. The closer together two pieces of DNA sit on a chromosome, the more likely they are to be inherited as a package. The farther apart, the more likely recombination will break them up over time. This is why close relatives share long, unbroken stretches of DNA measured in many centimorgans, while distant relatives share only short fragments.
Centimorgans vs. Physical Distance
A common rule of thumb is that 1 cM corresponds to about 1 million base pairs (1 megabase) of physical DNA. But this average is misleading in practice because recombination doesn’t happen evenly across chromosomes. Certain narrow regions called hotspots, typically only 1 to 2 kilobases long, concentrate recombination activity, while the stretches between them see almost none. Individual hotspots can have recombination rates ranging from below 0.001 cM to as high as 3 cM. So a segment that’s 5 cM long might span a very different physical length depending on where it sits on a chromosome.
There’s also a significant difference between biological sexes. Female recombination rates are roughly 1.6 times higher than male rates across most of the genome, meaning a mother’s chromosomes get reshuffled more than a father’s during cell division. This is why the total genetic map of the human genome measured in centimorgans is longer when calculated from female meiosis than from male meiosis.
Shared Centimorgans and Relationships
DNA testing companies report how many centimorgans you share with each match. The more you share, the closer the relationship. But because recombination is partly random, there’s a range for every relationship type, not a single fixed number. The Shared cM Project, a large collaborative dataset visualized by DNA Painter, provides these average values and ranges:
- Parent-child: 3,485 cM on average (range: 2,376 to 3,720 cM)
- Grandparent-grandchild: 1,754 cM on average (range: 984 to 2,462 cM)
- Aunt or uncle: 1,741 cM on average (range: 1,201 to 2,282 cM)
- Great-grandparent: 887 cM on average (range: 485 to 1,486 cM)
- First cousin: typically in the range of several hundred to around 1,000 cM
- Second cousin: around 122 cM on average (range: 14 to 353 cM)
- Third cousin: around 48 cM on average (range: 0 to 192 cM)
Notice how the ranges overlap. A grandparent and an aunt or uncle share similar average cM values, which is why DNA testing companies sometimes can’t distinguish between these relationships from shared DNA alone. Additional context, like the age of each person or the pattern of shared segments, helps narrow things down.
Why the Same Relationship Can Give Different Numbers
You inherit exactly 50% of each parent’s DNA, which is why the parent-child range is the tightest. But as you go further back, the randomness of recombination creates wider variation. You don’t inherit exactly 25% from each grandparent. One grandparent might contribute 27% while the other contributes 23%. By the time you reach third or fourth cousins, some relatives who are genuinely related by blood may share no detectable DNA at all simply because the shared segments were lost through generations of reshuffling.
This is also why siblings who aren’t identical twins don’t share exactly the same amount of DNA with a given relative. Each sibling received a different random mix from each parent, so one sibling might share 900 cM with a cousin while another shares 650 cM with that same cousin.
When Small Segments Are Unreliable
Not every shared DNA segment represents a real common ancestor. Very small segments can appear as matches simply because certain DNA patterns are common in a population, a phenomenon geneticists call identical by state (IBS) rather than identical by descent (IBD). The size of a shared segment determines how seriously you should take it:
- Under 6 cM: roughly a 1% chance of being a true ancestral match. These are almost always coincidental.
- 6 to 16 cM: about a 15 to 50% chance of being a genuine match. Treat these as possible but unconfirmed leads.
- Over 45 cM: 99%+ probability of a real shared ancestor. These are highly reliable.
This is why most genealogy platforms set a minimum threshold before reporting a match. If you’re investigating a specific connection, focus on your largest shared segments first. Small segments below about 7 cM are generally not worth building a family tree around unless you have additional evidence supporting the connection.
Using Centimorgans in Practice
If you’ve taken a test through a service like AncestryDNA or 23andMe, your match list will show total shared centimorgans for each person. To interpret a match, compare that number against known relationship ranges. Free tools like the DNA Painter Shared cM Tool let you enter a cM value and see all the possible relationships it could represent, along with the probability of each one.
For close family (parents, siblings, grandparents), the cM value alone usually points to one or two possible relationships. For more distant matches, you’ll often see a dozen or more possibilities at a given cM level. That’s where traditional genealogy research, building out family trees, checking records, and comparing multiple DNA matches, becomes essential. Centimorgans tell you how much DNA you share. Figuring out exactly how you’re connected still takes detective work.