The net migration rate is a quantity that tells us how many migrants have entered a country relative to its population in a fixed time period.
It tells us at what rate the net migration has augmented the population over the course of a year. It is defined as the ratio of the net migration in a year to the annual mean population multiplied by 1000.
It can be calculated using the formula,
Net Migration Rate = [(Number of Immigrants – Number of Emigrants)/(Annual Mean Population)] x 1000.
If the number of immigrants is greater than the number of emigrants then we have a positive net migration rate whereas, if the number of emigrants is greater than the number of immigrants then we have a negative net migration rate.
Suppose that in a given year, a particular country has a mean population of 25000000. If 200000 people immigrate into the country that year and 50000 depart overseas, then the net migration rate is given as,
Net Migration Rate = ((200000 – 50000)/25000000) x 1000 = 6.
This means that the population of the country is augmented by 6 people annually per 1000 people in the population.