US Census Bureau API Reference

Overview

The US Census Bureau API provides access to hundreds of datasets including the American Community Survey (ACS), Decennial Census, Economic Census, Population Estimates, and more. It is the primary source for US demographic, social, economic, and housing data.

Base URL

https://api.census.gov/data

Authentication

API Key: REQUIRED (free). Register at https://api.census.gov/data/key_signup.html
Pass as query parameter: &key=YOUR_KEY
Requests without a key are throttled to ~500/day. With a key, limits are much higher.

Rate Limits

Without key: approximately 500 requests per day.
With key: up to 500 requests per day per IP is the documented soft limit, but in practice the key grants significantly more.
No formal per-minute rate limit documented; keep automated requests to a few per second.

Key Datasets and URL Patterns

The general URL pattern is:

https://api.census.gov/data/{year}/{dataset}?get={variables}&for={geography}&key=YOUR_KEY

Major Dataset Paths

Dataset	Path Segment	Description
ACS 5-Year Detailed Tables	`acs/acs5`	5-year estimates, most geographies (2009-present)
ACS 1-Year Detailed Tables	`acs/acs1`	1-year estimates, areas 65k+ pop (2005-present)
ACS 5-Year Subject Tables	`acs/acs5/subject`	Precomputed subject tables
ACS 5-Year Data Profiles	`acs/acs5/profile`	Social/economic/housing profiles
Decennial Census (2020)	`dec/dhc`	Demographic and Housing Characteristics
Decennial Census (2020 PL)	`dec/pl`	Redistricting data (PL 94-171)
Decennial Census (2010)	`dec/sf1`	Summary File 1
Population Estimates	`pep/population`	Annual population estimates
Economic Census	`ecnbasic`	Economic Census (2017, 2022)
County Business Patterns	`cbp`	Business establishment counts
Annual Business Survey	`abscs`	Business characteristics

Key Endpoints

1. ACS 5-Year Estimates (Most Common)

GET /data/{year}/acs/acs5?get={variables}&for={geography}&key=YOUR_KEY

Parameter	Required	Description
`get`	Yes	Comma-separated variable names (e.g., `NAME,B01001_001E`)
`for`	Yes	Target geography (e.g., `state:`, `county:`, `tract:*`)
`in`	Sometimes	Parent geography for sub-state levels
`key`	Yes	Your API key

Example (total population for all states, 2022 ACS 5-year):

https://api.census.gov/data/2022/acs/acs5?get=NAME,B01001_001E&for=state:*&key=YOUR_KEY

Example (median household income for all counties in California):

https://api.census.gov/data/2022/acs/acs5?get=NAME,B19013_001E&for=county:*&in=state:06&key=YOUR_KEY

Example (population by race for a specific tract):

https://api.census.gov/data/2022/acs/acs5?get=NAME,B02001_001E,B02001_002E,B02001_003E&for=tract:000100&in=state:06&in=county:075&key=YOUR_KEY

Response (JSON array of arrays, first row is headers):

json

[
  ["NAME", "B01001_001E", "state"],
  ["Alabama", "5024279", "01"],
  ["Alaska", "733391", "02"],
  ["Arizona", "7151502", "04"]
]

2. ACS 1-Year Estimates

GET /data/{year}/acs/acs1?get={variables}&for={geography}&key=YOUR_KEY

Same parameters as ACS 5-year. Only available for geographies with 65,000+ population.

Example (poverty rate for all states):

https://api.census.gov/data/2022/acs/acs1?get=NAME,B17001_001E,B17001_002E&for=state:*&key=YOUR_KEY

3. ACS Data Profiles

GET /data/{year}/acs/acs5/profile?get={variables}&for={geography}&key=YOUR_KEY

Uses DP prefix variables with precomputed percentages.

Example (educational attainment profile):

https://api.census.gov/data/2022/acs/acs5/profile?get=NAME,DP02_0068PE&for=state:*&key=YOUR_KEY

4. Decennial Census 2020

GET /data/2020/dec/dhc?get={variables}&for={geography}&key=YOUR_KEY

Example (total population by state, 2020 Census):

https://api.census.gov/data/2020/dec/dhc?get=NAME,P1_001N&for=state:*&key=YOUR_KEY

5. Decennial Census 2010

GET /data/2010/dec/sf1?get={variables}&for={geography}&key=YOUR_KEY

Example:

https://api.census.gov/data/2010/dec/sf1?get=NAME,P001001&for=state:*&key=YOUR_KEY

6. Discover Available Variables

GET /data/{year}/{dataset}/variables.json

Example:

https://api.census.gov/data/2022/acs/acs5/variables.json

Returns a large JSON object listing all available variables with labels and concepts.

7. Discover Available Geographies

GET /data/{year}/{dataset}/geography.json

Example:

https://api.census.gov/data/2022/acs/acs5/geography.json

8. List Available Datasets

GET /data.json

Returns all available datasets with their titles, years, and API endpoints.

Geography Syntax

Level	`for` syntax	`in` requirement
Nation	`us:1` or `us:*`	None
State	`state:06` or `state:*`	None
County	`county:075` or `county:*`	`in=state:06` (optional for all)
County Subdivision	`county subdivision:*`	`in=state:XX&in=county:YYY`
Census Tract	`tract:*`	`in=state:XX&in=county:YYY`
Block Group	`block group:*`	`in=state:XX&in=county:YYY&in=tract:ZZZZZZ`
Place (city)	`place:*`	`in=state:XX`
Metro Area (CBSA)	`metropolitan statistical area/micropolitan statistical area:*`	None
ZIP Code Tab Area	`zip code tabulation area:*`	None

State FIPS codes: 01=AL, 02=AK, 04=AZ, 05=AR, 06=CA, 08=CO, 09=CT, 10=DE, 11=DC, 12=FL, 13=GA, 15=HI, 16=ID, 17=IL, 18=IN, 19=IA, 20=KS, 21=KY, 22=LA, 23=ME, 24=MD, 25=MA, 26=MI, 27=MN, 28=MS, 29=MO, 30=MT, 31=NE, 32=NV, 33=NH, 34=NJ, 35=NM, 36=NY, 37=NC, 38=ND, 39=OH, 40=OK, 41=OR, 42=PA, 44=RI, 45=SC, 46=SD, 47=TN, 48=TX, 49=UT, 50=VT, 51=VA, 53=WA, 54=WV, 55=WI, 56=WY

Common Variable Codes

ACS Detailed Tables (B-tables)

Variable	Description
`B01001_001E`	Total population
`B01002_001E`	Median age
`B02001_001E`	Total (race)
`B02001_002E`	White alone
`B02001_003E`	Black or African American alone
`B03001_003E`	Hispanic or Latino
`B19013_001E`	Median household income
`B19001_001E`	Household income (total, for distribution)
`B25077_001E`	Median home value
`B25064_001E`	Median gross rent
`B17001_001E`	Poverty status (total)
`B17001_002E`	Poverty status (below poverty)
`B15003_022E`	Bachelor's degree
`B15003_023E`	Master's degree
`B15003_025E`	Doctorate degree
`B23025_005E`	Unemployed (civilian labor force)
`B25001_001E`	Total housing units
`B08301_001E`	Means of transportation to work (total)

Variable naming: B{table}_{seq}E for estimates, B{table}_{seq}M for margins of error.

ACS Data Profile Variables (DP-tables)

Variable	Description
`DP02_0068PE`	% with bachelor's degree or higher
`DP03_0062E`	Median household income
`DP03_0128PE`	% below poverty level
`DP04_0089E`	Median home value
`DP05_0001E`	Total population

Decennial 2020 (DHC)

Variable	Description
`P1_001N`	Total population
`P1_003N`	White alone
`P1_004N`	Black or African American alone
`H1_001N`	Total housing units
`H1_002N`	Occupied housing units

Response Format

All data responses are JSON arrays of arrays. The first array is always the column headers; subsequent arrays are data rows.

json

[
  ["NAME", "B01001_001E", "B19013_001E", "state", "county"],
  ["Los Angeles County, California", "10014009", "73538", "06", "037"],
  ["San Diego County, California", "3298634", "85750", "06", "073"]
]

Values are strings (even numeric ones).
Missing or unavailable data may appear as null, "-", or "N".
Annotation values: "-" (too few sample cases), "N" (not available), "(X)" (not applicable).

Notes

Always include NAME in your get parameter to get human-readable geography labels.
The E suffix means "Estimate"; use M suffix for Margin of Error (e.g., B19013_001M).
Variable discovery: browse https://api.census.gov/data/{year}/acs/acs5/variables.html for a searchable table.
For ACS, 5-year estimates cover all geographies but are less current; 1-year covers only larger geographies but is more recent.
Group endpoint: ?get=group(B01001) retrieves all variables in a table group.