In This Article
In This Article
A centimorgan relationship calculator predicts how closely related you are with someone based on the amount of DNA you share with them.
Shared DNA is measured in centimorgans or cMs. The more you have in common, the closer the connection likely is.
Centimorgans are interpreted with a centimorgan chart, which allows you to estimate the type of relationship you have with a person.
It’s how you can tell if someone is your sibling, aunt, grandmother, and so on.
DNA testing companies like 23andMe and Ancestry often use these tools to predict your relationships with DNA matches or people you may be related to.1
Summary
Centimorgans measure the total amount of DNA you share with a potential relative and may help predict the type of relationship you have.
A centimorgan (cM) is a unit of measurement that describes the size of shared DNA segments between people. Let me explain:
Every cell in the body has 46 chromosomes, and each chromosome contains chunks of genetic material called DNA segments.
Geneticists use centimorgans to identify the number of DNA segments that a person shares with another person.
Since we inherit half of our genes from each parent, we tend to share several segments or cM counts with a potential relative.2,3
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Centimorgans can be expressed as a percentage that shows how much out of 6,800 cMs is shared or as a count that shows the number of shared cMs.
You can calculate the percentage of shared DNA with this formula:
If you share 1,800 cMs with a family member, here’s what it would look like:
The result won’t tell you what relationship type you have with a potential relative. It only estimates the closeness of your relationship.
The higher the percentage, the closer the relationship likely is.
If you have the percentage of shared cMs, you can also calculate the total shared DNA with this formula:
Let’s say you share 40.5% cMs with a match. Here’s what it would look like:
Knowing how much DNA you share with a person is important because it will allow you to predict genealogical relationships with a centimorgan chart.
A centimorgan chart is a reference that you can use to estimate your relationship with a DNA match or a potential relative.
Also known as the Shared cM Project (SCP), the chart shows the many probabilities of how you may be related to someone.
Blaine Bettinger, a genealogy expert, first developed the system in 2016. The Shared Centimorgan Project is currently on its third version.
Research on the cM chart is still ongoing. It helps genetic genealogists and DNA companies predict possible relationships between people with matching DNA.
To use the centimorgan relationship chart, you’ll need the number of centimorgans you share with someone (e.g., 2754 cMs).
Once you have that number, you can look it up on the Bettinger DNA cM chart to predict your relationship. There are two ways to do this:
Step 1: Look at the Cluster Chart below and compare your total cM shared with the ranges indicated in the 95th percentile. Take note of which clusters your cM falls into.
Source: The Shared cM Project, The Genetic Genealogist
Step 2: Compare your total shared cM with the average and see which clusters it is closest to. This will help you identify all possible relationships with a genetic match.
For accurate relationship predictions, use your results from Step 2 and compare them with the histogram below.
Relationship types closest to the peak are your most probable relationships with someone.
A person can have up to 6,800 centimorgans. The number of shared cMs reveals the closeness of your genetic relationship with a relative.
You will have more DNA segments in common with close relatives such as a parent, sibling, grandparent, child, aunt, or uncle.
However, you will have less shared DNA with distant relatives, like a cousin or a cousin once removed.
Most at-home DNA tests will show how much cMs you share with DNA matches and predict how you’re likely related to each other.
At-home genetic tests like 23andMe, Ancestry, Family Tree DNA, and MyHeritage DNA help you find possible relatives through DNA matches.
DNA matches often include information on your shared centimorgans. Depending on the test, you will get one or more of these reports:
Here is a sample report from Ancestry DNA which shows the number and percentage of centimorgans that Mickey shares with a possible relative who may be his first or second cousin:
The 23andMe kit offers a similar report for your DNA matches. But it also shows your shared cMs with friends and relatives who have taken the same test:
DNA tests only show your most probable relationship with a genetic match. It’s possible that you’re related to them in another way than what your results tell you.
However, you can use your DNA results to explore other relationship probabilities with the help of a centimorgan calculator and relationship chart.
DNA testing companies may help you find relatives on their database and estimate your probable relationships.
Although accurate, keep in mind these are just predictions. The only way to know you’re related is to investigate further and take additional DNA tests.
Here’s what you can do to establish genealogical relationships:
If your DNA relative is open to the idea, consider taking a DNA test designed to confirm genetic relationships, like maternity and paternity testing.
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