A verbal formula for change in population over a time period (disregarding immigration and emigration) would be as follows:
Population Change equals Number of Live Births minus Number of Deaths (the births and deaths occurring in the stated time period).
If births outnumber deaths, the population change is a gain. If deaths outnumber births, the change is negative, or a loss.
Population growth rate is defined as the percentage of population change over time, expressed as a fraction of the initial population.
Example of Population Growth Rate Calculation:
World Population in 2006 = 6.7 Billion
World Population in 2011 = 7.0 Billion
Population Growth Rate = (7.0 – 6.7)/ 6.7 x 100 = .3/6.7 x 100 = 4.47% (in 5 years) = .89% per year
As an interesting aside: the world population is projected to reach 7 billion by this coming Halloween!