# Effective Population Size Calculator

 Enter exactly two known quantities. Hit calculate to find the missing quantity. Ne (Effective Size) Nf (Number of Females) Nm (Number of Males)

In population biology and ecology, the effective population size, Ne is the size of an ideal population that would display the same amount of genetic drift, inbreeding, and loss of heterozygosity as the observed population. It is almost always smaller than the actual census count of a population, sometimes much smaller if there are strong forces working to reduce genetic diversity.

Biologists can estimate effective population size using a number of formulas that take different factors into account. One of the easiest to apply is the sex-ratio equation, since it is fairly straight forward to count the number of individuals of each sex. Other formulas require allele frequencies or mutation rates as inputs, factors which require more work to measure. The sex-ratio equation for effective population size is

Ne = 4*Nf*Nm/(Nf + Nm)

where Nf and Nm are the respective numbers of females and males in the population. If the census count is P and there are an equal number of males and females, i.e., Nf = Nm = P/2, then the effective population size is

Ne = 4(P/2)(P/2)/(P/2 + P/2) = P²/P = P

Thus, the effective population size equals the census size when the sex ratio is 50/50. In fact, under any other sex ratio the effective size is strictly less than the census count.

You can use the calculator above to compute Ne, Nf, or Nf given the values of the other two quantities. Results are rounded to the nearest hundredth.