Effective Population Size Calculator

You can use this handy calculator to compute an effective population size through the application of the sex-ratio equation.

Calculate effective population size in two simple steps:

  1. Enter the two known quantities in the fields provided.
  2. Click on the "Calculate" button to generate the missing field.
Effective Population Size Calculator

You may set the number of decimal places in the online calculator. By default there are only two decimal places.

Results
Effective Size = 0
Number of Males = 0
Number of Females = 0

How Effective Population Size is Calculated

In the fields of ecology and biology, the effective population size, which is denoted by Ne, is the scale of the ideal population that would be expected to exhibit the same degree of loss of heterozygosity, inbreeding, and genetic drift as the population of interest. The effective population size will always be smaller than the true census count of a population. In fact, it is often significantly lower if strong forces are at work that decrease genetic diversity.

Various formulas that take multiple dimensions into account can be used to calculate the effective size of a population. One of the most basic methods involves the application of the gender-ratio equation, as it is relatively simple to compute the number of subjects of each gender. Alternative equations depend on the use of mutation rates or allele frequencies as inputs, data that can be challenging and time-consuming to acquire.

The gender-ratio equation for the effective population size is as follows:

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

where Nf and Nm are the respective numbers of females and males within a given population. If the census count is denoted by P and the number of males and females is equal; that is, Nf = Nm = P/2, the effective population size would be as follows:

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

As such, the census size and effective population size are equal if the gender ratio is 50/50. In real terms, in the presence of an alternative gender ratio, the effective size is lower than the census count.

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