Pull demographic data from the US Census Bureau API

Verified steps

1. Register for a free Census API key at api.census.gov/data/key_signup.html; all data queries to the Census Data API require an api_key parameter.
2. Identify the dataset and vintage year: for the American Community Survey 5-year estimates use the base URL https://api.census.gov/data/{year}/acs/acs5; for 1-year estimates use acs1 — 5-year estimates provide data down to census tracts and block groups, while 1-year estimates require populations of 65,000 or more.
3. Construct a GET request with a get parameter listing the variable codes you need (e.g., B01003_001E for total population) and a for parameter specifying the geographic unit (e.g., county:* or tract:* within state:06 for California tracts).
4. Parse the JSON response: the first array in the response contains column headers and each subsequent array is a data row; join on the geographic FIPS codes to match rows to your spatial data.
5. To discover available variables, fetch the variables JSON at https://api.census.gov/data/{year}/acs/acs5/variables.json or use the subject-tables endpoint (acs5/subject) for pre-computed topic summaries.
6. Handle the Census API's 50-variable-per-request limit by splitting large variable lists across multiple requests and joining results on the geographic identifier.