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Effective Population Size Calculator

Effective population size (Ne) from unequal sex ratios and from fluctuating census sizes over generations, with the Ne/N ratio and loss of heterozygosity per generation.

Breeding males

Breeding females

Census sizes over past generations (optional)

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How it works

Ne = 4·Nm·Nf ÷ (Nm + Nf) · Ne (over t generations) = harmonic mean of Nₜ · ΔH = 1 ÷ 2Ne

Genetic drift depends not on head count but on how many individuals actually contribute genes equally. A skewed sex ratio wastes genetic capacity — 20 males with 180 females act like 72 individuals — and population crashes dominate the long-run average because the harmonic mean is pulled toward the smallest generation. Conservation targets Ne above 50 short-term and 500 long-term.

Worked example

20 males, 180 females: 4 × 20 × 180 ÷ 200 = 72. Census history 500, 60, 400, 800 → harmonic mean 183.

Frequently asked questions

Why is Ne usually much smaller than N?

Unequal sex ratios, variance in family size, overlapping generations and past bottlenecks all reduce it; typical Ne/N ratios are 0.1–0.3.

How is Ne estimated in the wild?

From temporal changes in allele frequencies or linkage disequilibrium in genetic samples, or demographically as here.

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For teaching and planning — verify against your lab protocol and instrument documentation.