What a centimorgan is
A centimorgan measures a stretch of shared DNA. The more you share with a match, the closer the relationship, but the ranges overlap a lot. A second cousin and a first cousin twice removed can share exactly the same amount.
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What does your shared cM mean?
Enter the centimorgans (cM) you share with a DNA match. You'll see every relationship that commonly shares that amount, closest fit first.
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Ranges are approximate, based on crowd-sourced data like the Shared cM Project. Endogamous communities share more DNA than these numbers suggest. For percentage probabilities, use the DNA Painter Shared cM tool.
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Good to know
Why one number isn't enough
DNA is passed down randomly, so two people with the same relationship can share very different amounts. Narrow it down with ages (a match 40 years older is more likely a generation up), shared matches you can identify, and the match's tree checked against records.
Proving it
DNA supports a paper-trail conclusion; on its own it rarely proves a specific ancestor beyond close relationships. Build out the shared matches, find the common ancestor in records, and cite every source.
A note on surprises
DNA can reveal unexpected parentage or unknown relatives. Take your time, think carefully about who to contact, and consider support communities for DNA surprises.