Understanding cM & Relationships
Understanding cM & Relationships
Your match list is full of different numbers followed by “cM”. Learn exactly what they mean and how to decode your biological relationships with ease.
When you log into your DNA results at Ancestry, MyHeritage, or 23andMe, you are greeted by thousands of genetic cousins. But next to each name, there is a technical number like “Shares 250 cM” or “74 cM across 4 segments”.
To the untrained eye, this looks like complicated biology. But in reality, this number is the single most valuable key to mapping your family tree. Once you understand it, you can bypass paper brick walls and trace your cousins with mathematical certainty.
🧬 What is a Centimorgan?
A centimorgan (cM) is not a physical measurement of length or physical size. Instead, it is a biological scale representing the genetic probability of segments of DNA being passed down together intact through generations without shuffling.
🌳 The Simple cM Golden Rule
Because DNA dilutes by roughly half with each generation, the rule of thumb is straightforward: The higher the shared cM number, the closer your biological relationship is. Parents share massive amounts of cM, while distant cousins share tiny amounts.
🚗 The “Genetic Highway” Analogy
Think of your chromosome as a long stretch of highway, and genes as exit markers along the way. A centimorgan doesn’t measure the miles between exits. Instead, it measures the probability that you can drive from Exit A to Exit B without encountering a construction detour (a recombination event). If two exit markers are very close together (low cM), the odds of a detour splitting them up is almost zero, so they are passed down together.
Test Your Matches: The cM Predictor Widget
Got a match you don’t recognize? Type the exact number of shared centimorgans (cM) you share with them below to see the most likely relationship brackets instantly!
The Ultimate Shared cM Reference Scale
While ranges can overlap due to the random nature of genetic shuffling (recombination), here is the standard reference scale of shared cM used by professional genealogists to group DNA matches:
| Typical Range | Relationship Class | Generations to Ancestor | Average Sharing |
|---|---|---|---|
| 3,300 – 3,700 cM | Parent / Child or Identical Twin | 1 Generation | ~3,400 cM (50%) |
| 2,200 – 3,300 cM | Full Sibling | 1 Generation (shared parents) | ~2,600 cM (37.5% – 50%) |
| 1,300 – 2,300 cM | Grandparent, Aunt/Uncle, Half-Sibling | 2 Generations | ~1,700 cM (25%) |
| 550 – 1,220 cM | First Cousin, Great-Grandparent | 2 – 3 Generations | ~850 cM (12.5%) |
| 200 – 540 cM | Second Cousin, Great-Aunt/Uncle | 3 – 4 Generations | ~340 cM (6.25%) |
| 45 – 220 cM | Third Cousin, Second Cousin once removed | 4 – 5 Generations | ~75 cM (1.5%) |
| 10 – 45 cM | Fourth Cousin, Third Cousin once removed | 5 – 6 Generations | ~25 cM (0.35%) |
| 0 – 10 cM | Distant Cousin / Random Overlap | 6+ Generations | ~5 cM (< 0.10%) |
The Recombination Shuffle: Why Ranges Overlap
You might have noticed that some ranges in the cheat sheet overlap. For instance, a match of 200 cM could technically be a distant Second Cousin, a Great-Aunt, or a First Cousin twice removed.
Why is this? It all comes down to the random nature of recombination (the genetic shuffle). When your parent passes down DNA to you, their maternal and paternal chromosomes align and swap random blocks of code. Your sibling might inherit a slightly different set of blocks than you do from the exact same parents.
Because of this randomness, there is no single fixed number of centimorgans for any given cousin relationship. Instead, we always talk in terms of ranges of probability. The closer the relationship, the narrower and more predictable the range is. The more distant, the wider the overlap becomes!