Source: U.S. Census Bureau  

Release: National Population Estimates  

Units:  Persons, Not Seasonally Adjusted

Frequency:  Annual, As of July 1

This historical series may be used in conjunction with other population data to construct long time series. Caution should be exercise as the series contains breaks.
National population data for the years 1900 to 1949 exclude the population residing in Alaska and Hawaii. National population data for the
years 1940 to 1979 cover the resident population plus Armed Forces overseas.
National population data for all other years cover only the resident population. Estimates of the population including Armed Forces
overseas are as follows:

1919 105,063,000
1918 104,550,000
1917 103,414,000

National population data for the years 1900 to 1929 are only available rounded to the nearest thousand.

For more information see https://www.census.gov/population/estimates/nation/popclockest.txt