41,700,825 (likely average population of the U.S. in 2100 AD)

The U.S. population between 1870 and 1880 is about where we will again be by 2100 AD or around

41,700,825 

Here is a population chart by year of the U.S. 

Begin population quote from:

https://en.wikipedia.org/wiki/Demographic_history_of_the_United_States

1870	38,558,371	22.63%
1880	50,189,209	30.16%
1890	62,979,766	25.48%
1900	76,212,168	21.01%
1910	92,228,496	21.02%
1920	106,021,537	14.96%
1930	123,202,624	16.21%
1940	132,164,569	7.27%
1950	151,325,798	14.50%
1960	179,323,175	18.50%
1970	203,211,926	13.32%
1980	226,545,805	11.48%
1990	248,709,873	9.78%
2000	281,421,906	13.15%
2010	308,745,538	9.71%
2020	331,449,281	7.35%