United States Census Bureau Population Estimates API

openapi: 3.1.0
info:
  title: Census Data American Community Survey Population Estimates API
  description: The United States Census Bureau Data API provides programmatic access to statistical data from Census Bureau surveys and datasets. The API covers demographic, economic, housing, and social data through a query-based interface. Key datasets include the American Community Survey (ACS), Decennial Census, Population Estimates, County Business Patterns, and many more. Users can query data by geographic level (national, state, county, tract, block group) and filter by hundreds of statistical variables. An API key is optional (up to 500 requests/day without one) but required for higher usage.
  version: 2.0.0
  contact:
    name: Census Developer Forum
    url: https://www.census.gov/data/developers.html
    email: cnmp.developers.list@census.gov
  x-tags:
  - Demographics
  - Federal Government
  - Open Data
  - Statistics
servers:
- url: https://api.census.gov/data
  description: Census Data API
tags:
- name: Population Estimates
  description: Annual population estimates and projections
paths:
  /{year}/pep/population:
    get:
      operationId: getPopulationEstimates
      summary: Get Population Estimates
      description: Returns annual population estimates for states, counties, and metropolitan areas. Uses births, deaths, and migration to estimate population change between decennial censuses.
      tags:
      - Population Estimates
      parameters:
      - name: year
        in: path
        required: true
        description: Vintage year for the estimates
        schema:
          type: integer
      - name: get
        in: query
        required: true
        description: Variable names (e.g., NAME,POP,DATE_DESC for name, population, and date description)
        schema:
          type: string
      - name: for
        in: query
        required: false
        description: Geographic level and FIPS filter
        schema:
          type: string
      - name: in
        in: query
        required: false
        description: Parent geography constraint
        schema:
          type: string
      - name: key
        in: query
        required: false
        description: Census API key
        schema:
          type: string
      responses:
        '200':
          description: 2D JSON array of population estimates
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/DataAPIResponse'
        '400':
          $ref: '#/components/responses/BadRequest'
components:
  responses:
    BadRequest:
      description: Bad request — invalid parameters, missing required fields, or variable not found
      content:
        text/html:
          schema:
            type: string
  schemas:
    DataAPIResponse:
      type: array
      description: A 2D JSON array where the first element is an array of variable names (headers) and each subsequent element is an array of corresponding data values. The last elements in each row are typically geography codes.
      items:
        type: array
        description: A single data row (header row or data row)
        items:
          type: string
          nullable: true
      example:
      - - NAME
        - B01003_001E
        - state
      - - Alabama
        - '5024279'
        - '01'
      - - Alaska
        - '733391'
        - '02'
      - - Arizona
        - '7151502'
        - '04'
externalDocs:
  description: Census Data API User Guide
  url: https://www.census.gov/data/developers/guidance/api-user-guide.html