26 CFR 1.103(n)-6T - Determinations of population (temporary).
Q-1: What is the proper method for determining population?
A-1: All determinations of population must be made with respect to any calendar year on the basis of the most recent census estimate (whether final or provisional) of the resident population of the State or other governmental unit published by the Bureau of the Census in the “Current Population Reports” series before the beginning of the calendar year.
However, determinations of the population of a general purpose governmental unit (other than a State, territory, or possession) within a State, territory, or possession may not be based on estimates that do not contain estimates for all of the general purpose governmental units within such State, territory, or possession. Thus, a county may not determine its population on the basis of a census estimate that does not provide an estimate of the population of the other general purpose governmental units within the State (e.g., cities, towns). If no census estimate is available for all such general purpose governmental units, the most recent decennial census of population may be relied on.