Converting Between 1920 and 1930 Census Eds in One Step
Free tool from Steve Morse lets you obtain an Enumeration District (ED) for an address in selected large cities in the 1920 US census if you know the ED for that address in the 1930 census, and vice versa.

FamilySearch - United States Census Districts
Arranged alphabetically by state and within each state by supervisor's district and enumeration district, these microfilms contain census descriptions of geographic subdivisions and enumeration districts for the United States census 1830-1950. Available through your local Family History Center, these are copies of National Archives microfilm publication T1224.

NARA 1930 Census Locator - Geographic Search
This free search tool allows you to search for enumeration district descriptions by state, county, city, and/or place name to determine the ED and schedule microfilm roll number. Acceptable place names include names of town, streets, post offices, rural townships, election precinct and ward numbers, schools, institutions, and rivers and lakes. Select a US state to begin your search.

Obtaining EDs for the 1930 Census in One Step
Free, interactive search tool from Steve Morse allows you to search for an Enumeration District in the US 1930 census by state, city, and street address. Covers only large U.S. cities.

Untangling Townships
County and/or land township designations were often used to determine enumeration districts in the U.S. Census, especially in rural areas. Learn more in this helpful article by Linda Haas Davenport.