Expanded Explanation

If you’ve taken a genealogy DNA test, you’ve probably seen numbers like:

• 25 cM,
• 210 cM,
• or even 3,400 cM.

Those numbers represent shared centimorgans.

In genetic genealogy, centimorgans are used to estimate how closely two people may be biologically related based on the amount of shared DNA between them.

Generally speaking:

• higher cM numbers usually suggest a closer relationship,
• while lower cM numbers often indicate more distant relationships.

For example:

• parents and children typically share around 3,400 cM,
• first cousins often share several hundred cM,
• while distant cousins may only share small amounts such as 20–50 cM.

But DNA inheritance is not perfectly even, which means centimorgan totals are estimates rather than exact relationship guarantees.

That’s one of the reasons DNA genealogy works best when combined with traditional genealogy research and family trees.

Genealogy Context

Centimorgans are one of the core measurement systems used in modern genetic genealogy.

When genealogy websites display DNA matches, the amount of shared DNA is usually shown in centimorgans.

Genealogists use cM values to help:

• estimate biological relationships,
• separate maternal and paternal matches,
• identify unknown relatives,
• confirm direct ancestor lines,
• and organize DNA match groups.

One important thing to understand is that shared centimorgans alone cannot usually identify an exact relationship.

For example, two people sharing the same cM amount could potentially be:

• second cousins,
• half first cousins once removed,
• great-great-aunt relationships,
• or several other possibilities.

That’s why genealogists usually combine cM data with:

• family trees,
• shared matches,
• historical records,
• geographic locations,
• and surname research.

And honestly, learning how centimorgans work is often the moment DNA genealogy starts feeling less mysterious and more understandable for beginners.

Approximate Shared cM Examples

These ranges overlap heavily, which is why centimorgan values should always be interpreted carefully rather than treated as exact answers.

For a more in-depth approach to understanding shared cM, I encourage you to visit The Shared cM Project at DNAPainter.com.

Examples

A few common examples of centimorgan use in genealogy include:

• grouping DNA matches connected to a Scottish paternal line,
• identifying maternal cousin clusters from rural Ireland,
• or estimating how closely an unknown DNA match may connect to a family tree.

Centimorgan analysis can also help genealogists identify:

• adoptions,
• misattributed parentage events,
• previously unknown relatives,
• or connections between families separated across countries and generations.

Sometimes even relatively small shared cM amounts become extremely important once combined with traditional genealogy evidence.

Why It Matters in Family History

Centimorgans matter because they help genealogists interpret biological relationships through DNA evidence.

They provide a measurable way to estimate how closely people may be related while helping organize and interpret DNA matches more effectively.

Understanding centimorgans can help researchers:

• separate maternal and paternal matches,
• estimate family relationships,
• confirm family tree connections,
• solve unknown ancestry questions,
• and better understand how DNA inheritance works across generations.

And honestly, one of the most interesting things about centimorgans is that they turn genealogy into something measurable while still leaving room for interpretation, probability, and detective work.