DNA Match Probability Calculator

Calculate random match probability and likelihood ratio for DNA evidence based on loci matched, population size, and allele frequencies.

Inputs

Loci Matched

Population Size

Avg Allele Frequency

Tab to move between fields

Results

Random Match Probability

5.734e-24

Likelihood Ratio

1.744e+23

RMP Exponent (10^x)-23.2

Expected Matches in Pop.1.892e-15

How to Use This Calculator

Enter Loci Matched, Population Size, and Avg Allele Frequency.
Review Random Match Probability and Likelihood Ratio.
Use RMP Exponent (10^x) and Expected Matches in Pop. to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

What each input means

Loci Matched: Number of STR loci with matching alleles (CODIS uses 20, older profiles use 13).
Population Size: Relevant population for the random match estimate.
Avg Allele Frequency: Average frequency of the matching alleles in the population (typically 0.05–0.15).

What each result means

Random Match Probability: Probability that a random person matches the profile.
Likelihood Ratio: How many times more likely the evidence is if the suspect is the source.
RMP Exponent (10^x): Order of magnitude of the match probability.
Expected Matches in Pop.: Expected number of random matches in the given population.

How this is calculated

Worked example, using the default values

Identify Input Parameters
Loci Matched = 13, Population Size = 330000000, Avg Allele Frequency = 0.1 = 3 input(s) provided
Calculate Random Match Probability
5.734e-24 = 5.734e-24
Calculate Likelihood Ratio
1.744e+23 = 1.744e+23
Calculate RMP Exponent
RMP Exponent
-23.2 = -23.2
Calculate Expected Matches in Pop.
1.892e-15 = 1.892e-15

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