The number of population resultant from the balance of the number of persons (in total or according to specific features) residing at a given moment in a given territory.

In most cases, the following population sizes are used: 1) as of each year end, i.e. as of 31st December, 2) as of mid-year, i.e. as of 30th June. The size as of mid-year is assumed as the average size of population in the surveyed year. Average size of population can also be calculated as the arithmetic mean of the population size at the beginning and population size at the end of the surveyed period (practically the size from end of the previous period is assumed to be the beginning of the period).