FST Calculator

The FST calculator measures genetic differentiation between two or more populations by computing Wright's fixation index (FST), one of the most widely used statistics in population genetics. FST quantifies how much of the total genetic variation in a species is partitioned among populations rather than residing within them. A value near zero means the populations share almost identical allele frequencies and behave as one interbreeding unit, while a value near one means the populations are strongly differentiated, perhaps approaching fixation for different alleles.

This FST calculator accepts data in two convenient forms. In allele-frequency mode you enter the frequency of a single allele in each population (two or three populations), and the tool derives the expected heterozygosities and the fixation index for you. In heterozygosity mode you enter the total heterozygosity (Ht) and the mean within-subpopulation heterozygosity (Hs) directly, which is handy when those values already come from software such as Arlequin, GenAlEx, or a published table. Either way the calculator returns FST, the percentage of differentiation, an estimate of gene flow (Nm), and a plain-language interpretation following Wright's qualitative guidelines.

Because the fixation index is foundational to conservation genetics, phylogeography, evolutionary biology, and human population studies, the FST calculator is useful to students learning Hardy-Weinberg theory, researchers screening microsatellite or SNP panels, and anyone who needs a fast, transparent FST estimate without setting up a full analysis pipeline.

The core of every FST calculation in this tool is the heterozygosity-based definition introduced by Sewall Wright and refined by Masatoshi Nei. The fixation index is the relative reduction in heterozygosity caused by population subdivision:

FST = (Ht − Hs) / Ht

Here Ht is the expected heterozygosity of the total (pooled) population and Hs is the average expected heterozygosity within the individual subpopulations. When you work in allele-frequency mode, the calculator first finds the mean allele frequency across populations, p̄, then builds both heterozygosities from it:

• Mean frequency: p̄ = (p₁ + p₂ + … + p_n) / n
• Total heterozygosity: Ht = 2 · p̄ · (1 − p̄)
• Within heterozygosity: Hs = average of 2 · p_i · (1 − p_i) across all n populations

Because the within-population heterozygosity Hs is always less than or equal to the total Ht whenever frequencies differ, FST ranges from 0 (no differentiation) to 1 (complete differentiation). The calculator also reports an equivalent variance-based estimate, FST = Var(p) / [p̄(1 − p̄)], which gives the identical answer and underscores that FST is fundamentally a standardized variance in allele frequencies among populations.

One of the most powerful uses of the FST calculator is indirectly estimating gene flow, the number of migrants exchanged between populations each generation. Under Wright's classic island model of migration-drift equilibrium, the fixation index and the effective number of migrants (Nm) are linked by a simple expression:

FST ≈ 1 / (4Nm + 1), which rearranges to Nm = (1/FST − 1) / 4

The calculator solves this rearranged equation automatically and reports Nm in the "Gene Flow Estimate" panel. The intuition is straightforward: when many migrants move between populations every generation, their allele frequencies are homogenized, FST stays low, and Nm is large. When migration is rare, drift drives the populations apart, FST climbs, and Nm shrinks. As a rule of thumb from the island model, more than one migrant per generation (Nm > 1) is usually enough to prevent populations from diverging by drift alone, whereas Nm below 1 allows substantial differentiation to accumulate.

Keep in mind that the Nm estimate assumes an idealized equilibrium model with symmetric migration, equal population sizes, and selective neutrality. Real systems violate these assumptions, so treat Nm as a comparative index rather than a literal migrant count. Even so, it remains a valuable summary that lets researchers rank populations by their connectivity and flag isolated demes that may need conservation attention.

Raw FST numbers become meaningful when placed against Wright's qualitative benchmarks, which the calculator applies to label every result. These thresholds are the standard reference in textbooks and the primary literature for describing the strength of population structure:

For human populations, global FST across continents typically falls around 0.10 to 0.15, confirming that most human genetic variation lies within rather than between groups. In contrast, isolated island endemics, cave-dwelling species, or fragmented landscapes can produce FST values well above 0.25. The calculator also reports GST (Nei's gene-diversity analogue, numerically equal to FST in this two-allele model) and D (Jost's differentiation), which corrects for the way heterozygosity caps FST when within-population diversity is high.

The fixation index computed by this FST calculator appears in nearly every subfield that studies how genes are distributed across space. In conservation genetics, FST helps identify distinct management units and evolutionarily significant units, guiding decisions about translocation, captive breeding, and habitat corridors. A high FST between two remnant populations warns that they have diverged enough to warrant separate protection, while a low FST may justify treating them as a single connected unit.

In phylogeography and evolutionary biology, scanning FST across many loci reveals which genomic regions are diverging faster than the neutral background, a hallmark of local adaptation and incipient speciation. So-called FST outlier scans use exactly this logic to detect selection. In human genetics, FST underpins studies of ancestry, admixture, and disease-allele distribution, and it is a routine quality-control metric in genome-wide association studies. Agricultural and fisheries scientists rely on FST to monitor the genetic integrity of breeds, stocks, and wild relatives. Because this online FST calculator returns results instantly from simple allele-frequency or heterozygosity inputs, it is well suited to classroom exercises, exam preparation, and quick checks of values produced by larger analysis programs before committing to a full study.