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 optional for up to 500 queries per IP per day; required above that allowance (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.
• Use a key when exceeding the anonymous 500-query daily allowance; do not infer an unlimited service guarantee.
• 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

Key Endpoints

1. ACS 5-Year Estimates (Most Common)

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

[
  ["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

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

ACS Data Profile Variables (DP-tables)

Decennial 2020 (DHC)

Response Format

Tabular dataset query responses are JSON arrays of arrays; discovery endpoints such as variables.json return objects. The first array is always the column headers; subsequent arrays are data rows.

[
  ["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 dataset-specific numeric sentinel codes (for example ACS -666666666). Read the variable annotations and special-value documentation; never treat sentinels as measured values.
• Annotation values: "-" (too few sample cases), "N" (not available), "(X)" (not applicable).

Notes

• Dataset paths and available geographies are vintage-specific (especially Population Estimates); discover them in data.json before changing a year.
• 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.