How Much DNA Do You Share With a First Cousin: A Clear Explanation

On average, you share about 12.5% (one eighth) of your DNA with a first cousin, because you each inherit roughly half of your DNA from the same set of grandparents. This makes first cousins the most closely related relatives who are not part of your parent–child or sibling circle, and the shared DNA is a direct result of how chromosomes are split and recombined across generations. The actual amount can vary, but the 12.5% figure captures the expected value.

What defines a first cousin

A first cousin is someone whose closest common ancestors with you are your grandparents. In other words, your parent and their parent are siblings. The children of two siblings are first cousins to each other. This connection is distinct from closer relatives, such as siblings or parents and children, who share noticeably more DNA, and from other cousin degrees, such as second cousins, who share a more distant set of common ancestors.

Each person inherits about half of their autosomal DNA from each biological parent. When your grandparents have children, their chromosomes recombine so each parent receives a different mixture of the grandparents’ DNA. When those parents have children, first cousins inherit some of the same chromosomal segments from the shared grandparents, but not all of them, and not in identical pieces. As a result, the total amount of shared DNA is large enough to identify close family, but small enough that two first cousins can look notably different in their genetic profile.

Key facts about inheritance and shared DNA

Attribute	Verified Detail	Source Type
Relationship	First cousins share two common grandparents	Standard kinship definition
Expected DNA shared	Approximately 12.5% of autosomal DNA (roughly 1,000 centimorgans)	Genetic consensus ranges
Typical range	About 9% to 14.5% depending on recombination	Published genetic studies
Shared segments	Many hundreds of centimorgans across multiple chromosomes	Chromosome mapping data
Close relative status	First cousins are the most recent relatives outside the nuclear family	Relationship classification

First cousins typically share approximately 12.5% of their autosomal DNA, which corresponds to an expected length of about 1,000 centimorgans. In practice, the observed amount often falls within a range of roughly 9% to 14.5%. Factors such as how chromosomes crossover and recombine during the formation of your parents affect the precise amount that ends up inherited. This overlap is large enough for close family algorithms to identify the cousin connection, but smaller than what you see with parents, siblings, or grandparents, who share around 50%.

Parent or child: roughly 50%
Full siblings: roughly 50%
Grandparent or aunt/uncle: roughly 25%
First cousins: roughly 12.5%
Second cousins: roughly 3%

How cousin DNA can differ between pairs

Two first cousins can share different amounts of DNA. One pair might inherit more of the same chromosome segments from their shared grandparents, while another pair might inherit fewer of the same segments. This explains why the measured shared DNA percentage can vary, even though the family relationship is the same. Some pairs may be at the higher end of the typical range, and others at the lower end, but both are still considered first cousins.

Why you should not use cousin DNA to estimate health risk

While identifying cousin matches is valuable for genealogy, you should not use the amount of shared DNA alone to evaluate health risk. Cousins share many genomic regions by descent, but each person also carries private variants. Health-related decisions should instead rely on clinical genetic testing, professional interpretation, and consultation with healthcare providers. Understanding the limits of DNA sharing helps prevent overstating what a relative match can tell you about disease risk or inherited conditions.

Knowing that first cousins share about 12.5% of DNA is useful for interpreting DNA test results, building family trees, and estimating how closely related two people are. It can also help set expectations for shared traits, such as physical features or carrier status for recessive conditions, while emphasizing that individuals within a family can differ. For genealogy research, this relationship level guides how you link branches of a family and prioritize connections in a growing tree.

Distinguishing first cousins from other relatives

First cousins occupy a clear spot in family structure. They are closer than second or third cousins, who share less DNA, and they are more distant than parents, children, or siblings, who share about 50%. Recognizing this midpoint helps you place new DNA matches, understand how many centimorgans to expect, and make informed decisions about which matches are likely to be close family versus more distant relatives.

Interpreting shared DNA in practice

When a DNA test labels a match as a first cousin, it is based on an estimated amount of shared DNA that typically aligns with about 12.5%. This estimate uses hundreds of thousands of markers across the genome and statistical models that account for variation in recombination. If your shared DNA falls near the edges of the typical range, the relationship may still be consistent with first cousins, especially when combined with additional genealogical evidence and family context.

Bottom line

You generally share roughly 12.5% of your DNA with a first cousin, reflecting the inheritance of DNA from common grandparents. This level of sharing is substantially more than more distant cousins but notably less than what you have with parents, children, or siblings. Actual percentages can vary across cousin pairs due to the randomness of chromosome recombination, but the relationship remains a stable and well-defined point in family structure.