DNA Match Probability Calculator
Calculate random match probability and likelihood ratio for DNA evidence based on loci matched, population size, and allele frequencies.
About this calculator
This calculator estimates how strong a DNA match really is by computing the random match probability (RMP): the chance that a randomly chosen, unrelated person would happen to share the same profile at every tested locus. It applies Hardy-Weinberg genetics at each locus, treating the average allele frequency p as governing both homozygote genotypes (frequency p²) and heterozygote genotypes (frequency 2p(1-p)), then combines the per-locus probability 2p²(1-p)² + p⁴ across all matched loci by multiplication — the product rule assumes each locus is inherited independently, which is the standard forensic assumption once loci are chosen from different chromosomes.
Raising that combined probability to the power of loci matched (13 for older CODIS profiles, 20 for current ones) is what makes DNA evidence so powerful: each additional locus multiplies the discriminating power, driving RMP toward vanishingly small numbers. The calculator also reports the likelihood ratio (simply 1 divided by RMP) and the expected number of coincidental matches in a population of your chosen size, which helps translate an abstract probability into something like "roughly this many other people on Earth could also match." Treat every result here as illustrative rather than case-ready: this tool uses a single simplified average allele frequency across all loci, while real forensic laboratories use locus-specific frequencies from validated population databases, apply corrections for population substructure, and account for relatedness — all of which shift real-world RMPs by orders of magnitude from this simplified estimate.
Inputs
Results
Random Match Probability
5.734e-24
Likelihood Ratio
1.744e+23
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 ParametersLoci Matched = 13, Population Size = 330000000, Avg Allele Frequency = 0.1 = 3 input(s) provided
- Calculate Random Match Probability5.734e-24 = 5.734e-24
- Calculate Likelihood Ratio1.744e+23 = 1.744e+23
- Calculate RMP ExponentRMP Exponent-23.2 = -23.2
- Calculate Expected Matches in Pop.1.892e-15 = 1.892e-15
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does matching more loci make the random match probability so much smaller?
Each locus contributes its own per-locus match probability (roughly 2p²(1-p)² + p⁴ under this calculator's Hardy-Weinberg model), and the calculator multiplies those probabilities together across every matched locus, treating each as an independent event. Multiplying a small fraction by itself repeatedly shrinks the result exponentially, so going from 13 to 20 loci doesn't add a modest amount of certainty — it can move the probability by many additional orders of magnitude, which is why modern CODIS profiles use 20 loci instead of the older 13.
What does the likelihood ratio actually tell me that the match probability doesn't?
The likelihood ratio is simply 1 divided by the random match probability, so it reframes the same number as "how many times more likely this evidence is if the suspect is the true source versus a random unrelated person." A likelihood ratio of one million means the observed match is a million times more probable under the hypothesis that the suspect is the source than under the hypothesis of a coincidental match, which is often the more intuitive way courts and juries interpret DNA evidence.
Why does the calculator use a single average allele frequency instead of frequencies for each locus?
Real STR loci vary in how common their alleles are in a given population, so an actual forensic report pulls locus-specific frequencies from a validated population database rather than one shared value. This calculator simplifies that by applying one avgAlleleFreq (typically 0.05–0.15) uniformly to every locus, which makes the math tractable for a quick estimate but means the result is illustrative — a real case-ready RMP would differ because some loci are rarer or more common than the average you enter.
What does the 'Expected Matches in Pop.' number mean?
It multiplies the random match probability by the population size you enter, giving a rough estimate of how many other people in that population could be expected to coincidentally share the same profile at every matched locus. A value well below 1 (shown in scientific notation) means the profile is expected to be unique in that population, while a value approaching or exceeding 1 signals the match is statistically closer to something that could occur by chance.
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