Blood Percentage and Genetic Relatedness Calculator
Calculate shared DNA percentage between family members.
Understand genetic relatedness for parents, siblings, half-siblings, cousins, and other relationships.
Why shared DNA is not a fixed number
The simple rule is that you inherit exactly 50% of your DNA from each parent. Beyond that, every generation gets shuffled by recombination: chromosomes randomly cross over during egg and sperm formation, so siblings do not share an identical 50% with each other. They share about 50% on average, but the actual number can range from 38% to 61% across full siblings of the same parents. This is part of why two full siblings can look quite different and why DNA tests sometimes give surprising “you are related less than expected” results.
Centimorgans, not just percentages
DNA testing companies (Ancestry, 23andMe, MyHeritage) report shared DNA in centimorgans (cM), not percentages. A centimorgan is a unit of genetic distance, not physical distance. The total human genome is about 7,400 cM. So:
- 50% shared ≈ 3,500 cM (parent/child; identical twins share all 7,400)
- 25% shared ≈ 1,750 cM (half siblings, grandparents)
- 12.5% shared ≈ 875 cM (first cousins, half-uncles)
- 3.125% shared ≈ 218 cM (second cousins)
Companies report cM because it survives the random recombination noise better than raw percentages do.
Expected vs actual table
| Relationship | Theoretical % | Test range | Typical cM |
|---|---|---|---|
| Parent / child | 50% | 47 to 54% | 3,485 to 3,600 |
| Full sibling | 50% | 38 to 61% | 2,200 to 3,500 |
| Half sibling | 25% | 18 to 32% | 1,300 to 2,300 |
| Grandparent | 25% | 17 to 34% | 1,200 to 2,400 |
| First cousin | 12.5% | 5 to 20% | 575 to 1,300 |
| First cousin once removed | 6.25% | 2 to 13% | 250 to 700 |
| Second cousin | 3.125% | 1 to 6% | 75 to 360 |
| Third cousin | 0.78% | 0 to 2% | 0 to 220 |
Where DNA tests can mislead
At third cousin and beyond, random inheritance can leave two genuine relatives sharing 0 cM (no detectable DNA in common) even though they descend from a common great-great-grandparent. This is why genealogy DNA matches fade out around 4 to 5 generations back. The further back you go, the more shared DNA falls below detection thresholds.
The tests are very accurate inside 3 generations (parent through second cousin) and progressively less reliable beyond. Treat any zero match outside that range as “we cannot tell” rather than proof of no relationship.