Hardy-Weinberg Equilibrium Calculator

How to use this calculator

1. Enter the frequency of allele A as a percentage.
2. Enter the number of individuals for optional expected genotype counts.
3. Compare the calculated expectations with the assumptions of the population model.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Allele Frequency and Punnett Square.

A baseline for population genetics

Hardy–Weinberg equilibrium supplies a simple reference distribution for genotypes. If the frequency of one allele is p and the other is q, their sum is one. Randomly pairing alleles gives p² for two A copies, q² for two a copies, and 2pq for one of each.

The heterozygote term has a factor of two because A can come from the first parent and a from the second, or the reverse. Adding the three genotype terms expands (p + q)², which equals one.

The conditions behind the model

The familiar equilibrium model assumes a large population, random mating, and no effective selection, migration, or mutation changing allele frequencies. The two-allele calculation also assumes the locus and population have been defined consistently.

Departures between observed and expected genotype frequencies can have many explanations. Population structure, nonrandom mating, selection, sampling variation, and data-quality problems are possibilities. The calculator does not test those explanations or perform a statistical equilibrium test.

Expected counts are not observed counts

Multiplying a proportion by population size gives an expected count. It can be fractional even though actual individuals are counted in whole numbers. Keep the unrounded expectations when performing a separate statistical analysis; rounding all categories independently can distort the total.

This tool handles two alleles. A locus with additional alleles requires a fuller set of allele frequencies and pairwise genotype terms. Do not merge several biologically different alleles into one category unless that grouping is justified for the question being asked.