DNA Cousinship Calculator
Estimate the probable relationship between two individuals based on the amount of shared DNA (percentage or centiMorgans). Results show the most likely relationships and the degree of cousinship.
Formulas & Methods
Shared DNA Conversion:
Shared cM = (Shared % / 100) × 7,400 cM
The human autosomal genome is approximately 7,400 centiMorgans (cM) in total.
Expected Shared DNA by Relationship:
Expected % = (1/2)n × 100
where n is the number of meioses (generational steps) separating the two individuals through their most recent common ancestor(s).
Coefficient of Relationship (r):
r = Shared % / 100
This represents the probability that any given allele is identical by descent.
Degrees of Separation:
Degrees = −log₂(Shared % / 100)
Provides a continuous estimate of genetic distance.
Cousin Degree Formula:
For nth cousins: Expected Shared % = (1/2)2(n+1) × 100
1st cousin (n=1): (1/2)⁴ × 100 = 6.25% per side × 2 sides = 12.5%
Assumptions & References
- Relationship ranges are based on Blaine Bettinger's Shared cM Project (2020 update), which aggregated data from over 60,000 real DNA relationships.
- The total autosomal genome is treated as ~7,400 cM for percentage-to-cM conversion.
- DNA inheritance is random; actual shared amounts vary significantly even for the same relationship type (e.g., full siblings can share 38–61%).
- This calculator uses autosomal DNA only. X-DNA, Y-DNA, and mtDNA follow different inheritance patterns and are not considered here.
- Ranges overlap between relationships — a single shared DNA value can correspond to multiple possible relationships. Always consider family history alongside DNA evidence.
- Matches below 6–7 cM are often considered unreliable due to the possibility of identical-by-chance (IBC) segments rather than true identical-by-descent (IBD) segments.
- Reference: ISOGG Wiki — Autosomal DNA statistics; Bettinger, B.T. (2020). The Shared cM Project 4.0.
- Reference: Donnelly, K.P. (1983). The probability that related individuals share some section of genome identical by descent. Theoretical Population Biology.