Shared cM DNA Calculator
Use this tool to estimate the percentage of shared DNA and see likely relationship ranges based on shared centimorgans (cM).
What is a centimorgan (cM)?
A centimorgan is a unit genetic genealogy platforms use to describe how much DNA two people share. Bigger shared cM values usually indicate a closer biological relationship. Smaller values generally suggest a more distant relationship.
In practical terms, your match list at testing services is heavily organized by shared cM. That single number helps you quickly identify if someone might be immediate family, a close cousin, or a distant branch.
How this cm dna calculator works
This calculator uses two simple steps:
- Converts shared cM into an estimated DNA percentage using ~6,800 cM as a typical autosomal total.
- Compares your value to common relationship ranges to show likely possibilities (for example, first cousin, half sibling, grandparent/grandchild, and more).
Common shared cM ranges (quick reference)
| Relationship | Typical Shared cM Range |
|---|---|
| Parent / Child | 3300–3720 |
| Full Sibling | 1613–3488 |
| Grandparent / Grandchild | 984–2462 |
| Half Sibling | 1160–2436 |
| First Cousin | 396–1397 |
| Second Cousin | 46–515 |
| Third Cousin | 0–217 |
Why segment count can help
If you provide the number of shared segments, you get extra context. Two matches can share similar total cM, but one might have fewer larger segments (often closer relationship) while another has many smaller segments (often more distant and recombined over generations).
Simple interpretation guideline
- Larger average segment size often points to a closer, more recent relationship.
- Smaller average segments are more common in distant cousin-level matches.
Best practices for DNA match interpretation
- Start with your highest-cM unknown matches first.
- Build small “working trees” for each likely relationship path.
- Use shared matches to identify clusters from the same ancestral line.
- Confirm with records: census, vital records, obituaries, and local archives.
Limitations you should know
This tool is for educational and genealogy planning purposes. It does not provide legal, medical, or forensic certainty. Endogamy, pedigree collapse, and random inheritance can shift expectations.
If a result is surprising, double-check kit management settings, test type, and whether both people tested on comparable platforms with robust match reporting.
FAQ
Can two different relationships have the same cM?
Yes. That is common. Relationship categories overlap significantly, especially in cousin ranges.
What if I only have percent shared DNA?
Enter the percent value and leave cM blank. The calculator will estimate cM and then return relationship ranges.
Is this exact for every testing company?
No. Testing platforms can differ in matching thresholds and reporting methods, but this gives a useful cross-platform estimate.