DNA Relationship Calculator
Calculate the expected percentage of DNA shared with a relative.
From first cousins to half-siblings — understand genetic genealogy.
Expected Shared DNA
DNA relationship prediction uses the percentage of shared DNA (measured in centimorgans, cM) to estimate the likely relationship between two people. The more DNA shared, the closer the relationship.
The Formula:
Relationship probability = f(shared cM, number of shared segments)
DNA sharing percentage = Shared cM / Total genome size (≈ 7,400 cM)
Expected Shared DNA by Relationship:
| Relationship | Expected cM | % DNA Shared |
|---|---|---|
| Identical twins | 7,400 cM | 100% |
| Parent / child | 3,400–3,900 cM | 50% |
| Full sibling | 2,300–3,900 cM | 50% (avg) |
| Half sibling | 1,160–2,650 cM | 25% |
| Grandparent / grandchild | 1,160–2,650 cM | 25% |
| Full first cousin | 550–1,270 cM | ~12.5% |
| Half first cousin | 125–650 cM | ~6.25% |
| First cousin once removed | 175–680 cM | ~6.25% |
| Second cousin | 46–515 cM | ~3.125% |
| Third cousin | 0–173 cM | ~0.78% |
Worked Example:
You share 1,800 cM with a DNA match.
Possible relationships (overlapping ranges):
- Full sibling (2,300–3,900 cM): unlikely: too low
- Half sibling (1,160–2,650 cM): possible
- Grandparent/grandchild (1,160–2,650 cM): possible
- Uncle/aunt or niece/nephew (1,349–2,175 cM): most likely
Use family tree and age data to narrow down the specific relationship.
Practical Tips:
- Use the Shared cM Project (Blaine Bettinger) for the most comprehensive probability tables
- X-DNA inheritance follows a specific pattern: it only passes through certain lines, useful for narrowing down which family branch a match belongs to
- Multiple DNA matches in the same family group confirm predicted relationships
- DNA can’t distinguish between relationships of the same genetic distance (e.g., half-sibling vs. grandparent)