The 1820 United States census was the fourth census conducted in the United States. It was conducted on August 7, 1820. The 1820 census included six new states: Louisiana, Indiana, Mississippi, Illinois, Alabama and Maine. There has been a district wide loss of 1820 census records for Arkansas Territory, Missouri Territory,^[1] and New Jersey.

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1820 United States census
← 1810 August 7, 1820 (1820-08-07) 1830 →
Seal of the United States Marshals Service, which administered the census
General information
Country	United States
Authority	Office of the United States Marshal
Results
Total population	9,638,454 ( 33.1%)
Most populous ​state	New York 1,532,881
Least populous ​state	Illinois 55,211

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The total population was determined to be 9,638,453, of which 1,538,022 were slaves. The center of population was about 120 miles (193 km) west-northwest of Washington in Hardy County, Virginia (now in West Virginia).

This was the first census in which any states recorded a population of over one million—New York, Virginia, and Pennsylvania—as well as the first in which a city recorded a population of over 100,000—New York. It was also the first census in which Baltimore was ranked as the country's second-most populous city. Thirdly, in this census and the 14 subsequent ones, New York was the most populous state until being superseded by California in the 1970 census.

The 1820 census contains a great deal more information than previous censuses. Enumerators listed the following data in columns, left to right:

1. Name of the head of family
   1. of free white males under age 10
   2. of free white males age 10 to under 16
   3. of free white males age 16 to 18
   4. of free white males age 16 to under 26
   5. of free white males age 26 to under 45
   6. of free white males age 45 and up
   7. of free white females under age 10
   8. of free white females age 10 to under 16
   9. of free white females age 16 to under 26
   10. of free white females age 26 to under 45
   11. of free white females age 45 and up
   12. of foreigners not naturalized
   13. of persons engaged in agriculture
   14. of persons engaged in commerce
   15. of persons engaged in manufacture
   16. of male slaves under 14
   17. of male slaves age 14 to under 26
   18. of male slaves age 26 to under 45
   19. of male slaves age 45 and up
   20. of female slaves under 14
   21. of female slaves age 14 to under 26
   22. of female slaves age 26 to under 45
   23. of female slaves age 45 and up
   24. of free male colored persons under 14
   25. of free male colored persons age 14 to under 26
   26. of free male colored persons age 26 to under 45
   27. of free male colored persons age 45 and up
   28. of free female colored persons under 14
   29. of free female colored persons age 14 to under 26
   30. of free female colored persons age 26 to under 45
   31. of free female colored persons age 45 and up
   32. of all other persons except Indians not taxed

Several of these columns were for special counts, and not to be included in the aggregate total. Doing so would have resulted in counting some individuals twice. Census takers were asked to use double lines, red ink or some other method of distinguishing these columns so that double counting would not occur. For example, the count of free white males between 16 and 18 was a special count, because these individuals were also supposed to be tabulated in the column for free white males of age 16 and under 26.

The other special counts were foreigners not naturalized, persons engaged in agriculture, persons engaged in commerce, and persons engaged in manufacture.

Census takers were also instructed to count each individual in only one of the occupational columns. For example, if an individual was engaged in agriculture, commerce, and manufacture, the census taker had to judge which one the individual was primarily engaged in.

Censustaking was not yet an exact science. Before 1830, enumerators lacked pre-printed forms, and drew up their own, sometimes resulting in pages without headings, line tallies, or column totals. As a result, census records for many towns before 1830 are idiosyncratic. This is not to suggest that they are less reliable than subsequent censuses, but that they may require more work on the part of the researcher.