When you look at your match list, Ancestry assigns labels like “2nd Cousin” or “Close Family.” However, these labels are just estimates. The only hard data you have is the number of centimorgans (cM) shared between you and that person.
If you want to figure out exactly how a mysterious match fits into your family tree, you must ignore the label and look at the cM count. This guide explains what centimorgans are, where to find them, and how to use them to calculate the true biological relationship.
What Is a Centimorgan (cM)?
A centimorgan is a unit of measurement used to describe the length of DNA segments you share with someone.
- Simple Rule: The higher the number, the closer the relationship.
- Technical Definition: It measures the probability of genetic recombination. One centimorgan roughly equals a 1% chance that a section of DNA will split in the next generation.
Because you inherit DNA randomly, you might share 250 cM with one second cousin and 310 cM with another. However, you will never share 3,000 cM with a cousin—that amount is reserved for parents and children.
How to Find the Centimorgan Count on AncestryDNA
Ancestry displays this number on every profile, but you have to know where to look.
- Go to Your Match List: Open your main list of DNA matches.
- Look at the Middle Column: Next to the estimated relationship (e.g., “1st Cousin”), you will see small text that says: “Shared DNA: 850 cM across 32 segments.”
- Click the “i” Icon: Clicking the small information icon or the cM text itself opens a popup. This reveals the “Unweighted” vs. “Weighted” count (explained below) and a probability chart.
AncestryDNA Centimorgan Cheat Sheet (Relationship Ranges)
Use this table to translate the number into a real relationship. Note that ranges overlap heavily.
| Relationship | Average cM | Typical Range |
| Parent / Child | 3,400+ | 3,300 – 3,700 |
| Full Sibling | 2,600 | 2,200 – 3,300 |
| Grandparent / Grandchild | 1,750 | 1,300 – 2,300 |
| Aunt / Uncle | 1,750 | 1,300 – 2,300 |
| 1st Cousin | 870 | 550 – 1,200 |
| 2nd Cousin | 230 | 45 – 500 |
| 3rd Cousin | 70 | 0 – 200 |
| 4th Cousin | 30 | 0 – 120 |
- Note: If you are seeing a match with 0 cM but they appear in your tree, you may need to check your DNA percentage ethnicity to see if the connection is too distant to show up genetically.
Weighted vs. Unweighted DNA on AncestryDNA
When you click the cM number, you will often see two different numbers.
- Weighted (Timber): Ancestry’s algorithm removes “noise” or DNA that is common in a specific population (Endogamy). This is usually the more accurate number for determining relationships.
- Unweighted: This is the raw total.
When to check Unweighted: If you and your match are from a community that married within itself frequently (like Ashkenazi Jewish, Amish, or Island populations), the “Weighted” score might be too low. Always compare both numbers if the relationship seems off.
What Does “Once Removed” Mean in DNA Relationships?
You will often see categories like “1st Cousin 1x Removed” in your Ancestry DNA match categories.
- Removed means “generations apart.”
- 1x Removed: You are not in the same generation. For example, your mother’s first cousin is your First Cousin Once Removed.
- 2x Removed: Your grandmother’s first cousin.
Because “Removed” cousins share less DNA, a 1st Cousin 2x Removed can look exactly like a 3rd Cousin in terms of cM count (both ~200 cM). You must look at their age to figure it out. If the match is 40 years older than you, they are likely a “Removed” cousin, not a direct 3rd cousin.
Conclusion
Centimorgans are the ruler by which you measure your family tree. While names and spelling can be wrong in paper records, the cM count rarely lies. If you find a match that shares 1,800 cM with you but you don’t recognize them, do not ignore it—that is the range of a half-sibling or grandparent, indicating a significant discovery waiting to be made.
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