What this centimorgan relationship calculator does
The Centimorgan Relationship Calculator compares an observed shared DNA amount with a reference amount and reports how far the observed value is above or below that reference as a percentage. Enter both values in centimorgans (cM), the unit commonly used by genetic genealogy services to describe shared DNA. The result is a straightforward arithmetic comparison. A positive percentage means the observed amount is higher than the reference, zero means they are equal, and a negative percentage means it is lower.
This comparison can help organize notes or make two reported amounts easier to compare proportionally. For example, a difference of 20 cM has a different scale relative to a 50 cM reference than it does relative to a 500 cM reference. Expressing the difference as a percentage makes that relationship explicit. It does not convert centimorgans into a family relationship, estimate a precise degree of kinship, or establish that two people share a particular ancestor.
The calculator uses two inputs. Observed shared DNA is the amount reported for the pair being considered. Reference amount is the comparison value you choose, such as a value from your own research notes or a documented comparison relevant to your question. Since the reference is user-selected, the output depends on that choice. It should be read as a percentage difference from that specific number, not as a general measure of how closely related two people are.
How to use the Centimorgan Relationship Calculator
Enter the observed amount and the reference amount in cM. Both fields accept values from 0 to 1,000 cM, while the reference must be at least 1 cM so the formula can divide safely. The default values are 100 cM in both fields, which produce a difference of 0%. Change them to the values you want to compare, then read the result as a percentage.
- Use the same unit for both entries. Do not enter one value in cM and the other in a different unit.
- Check that the observed value belongs to the pair or segment total you intend to analyze.
- Choose a reference amount that has a clear meaning in your notes.
- Keep the displayed sign. A negative result indicates a lower observed amount relative to the reference.
Worked example: suppose a match shows 120 cM and your chosen reference is 100 cM. The formula is ((observed cM − reference cM) ÷ reference cM) × 100. Substituting the values gives ((120 − 100) ÷ 100) × 100 = 20%. The observed amount is 20% above that reference. If the observed amount were 80 cM with the same 100 cM reference, the result would be −20%, meaning it is 20% below the reference.
How the Centimorgan Relationship Calculator formula works
The calculation first subtracts the reference amount from the observed amount. That gives the signed difference in cM. It then divides this difference by the reference amount to scale the gap relative to the chosen baseline. Multiplying by 100 expresses the ratio as a percentage. In symbols, the calculation is:
((observed cM − reference cM) ÷ reference cM) × 100
The denominator matters. A difference of 10 cM is 10% of a 100 cM reference, but 2% of a 500 cM reference. If the two inputs match, subtracting them gives zero, so the percentage difference is 0%. If the observed amount is below the reference, the subtraction produces a negative number and therefore a negative percentage.
This formula describes only the relative size of two entered values. It has no population data, relationship ranges, pedigree information, or genetic model built into it. It also does not account for whether a service reports total shared DNA, a segment value, or another summary. The output is useful only when those values are comparable and the reference is appropriate to the question.
Use cases for the Centimorgan Relationship Calculator
Genealogy research often involves recording match amounts alongside names, dates, places, and documentary evidence. A percentage comparison can provide a compact way to describe how a result differs from a particular reference in a research log. It can also help when reviewing a list of match amounts and wanting to express the size of a gap proportionally rather than only in raw cM.
Possible uses include:
- Comparing one match amount with a working reference recorded in a family-history spreadsheet.
- Describing how far a newly observed value differs from an earlier value in your notes.
- Checking arithmetic before adding a comparison to a research summary.
- Teaching the distinction between an absolute difference in cM and a percentage difference.
For example, a family historian might have two match records and use one amount as a temporary baseline for explaining the other. The resulting percentage can make a note more readable, provided the writer labels the reference clearly. It should not be used alone to rank matches, decide which documentary lead is strongest, or infer a particular relationship. A percentage comparison is descriptive arithmetic, while genealogical interpretation requires context and supporting evidence.
Other factors to consider when calculating cM differences
Centimorgan amounts are only one piece of genetic genealogy evidence. Shared DNA can be consistent with more than one possible relationship, and the same amount may occur among relatives in different branches or generations. The amount by itself does not tell you which ancestor contributed the shared segments or how a match connects to your documented family tree.
Before interpreting a comparison, consider what each number represents. Confirm whether the values refer to total shared DNA or a single segment, and whether they came from the same reporting method. A reference taken from a general chart, a particular match, or a personal case study serves a different purpose. Label its source and scope so that a later reader understands why it was selected.
Combine DNA findings with records such as civil registrations, census entries, church registers, immigration documents, and family correspondence where available. Review shared matches and known relatives as additional clues, while recognizing that these clues may still need careful evaluation. Preserve uncertainty in research notes and avoid treating a numerical resemblance as proof. When a conclusion has significant personal or research consequences, seek guidance from a qualified genetic genealogist and consult the relevant testing service's documentation for how it reports cM.
FAQ
Does a centimorgan amount identify a specific relationship?
No. This calculator reports a percentage difference from a reference amount. It does not identify a cousin degree or any other relationship. Shared cM is one clue, and interpretation depends on the family tree, match context, and other evidence.
What reference amount should I enter?
Use a number that is relevant to the comparison you want to describe, and note where it came from. The output changes when the reference changes. There is no universal reference value supplied by this calculator.
Why can the result be negative?
The formula subtracts the reference from the observed amount. When the observed amount is smaller, the difference is negative, so the percentage is negative as well. It means “below this reference” and does not indicate a problem with the data.
Can I enter zero as the reference?
No. The reference field starts at 1 cM because the formula divides by that value. A zero denominator would make the percentage difference undefined. An observed value of zero is permitted and can be compared with a positive reference.
Does a larger percentage mean a closer relative?
Not by itself. The percentage expresses a relative difference from the number you entered as the reference. It is not a closeness score or a relationship probability, so it should not be used to order relatives without appropriate genealogical analysis.