Reference

API documentation

All endpoints are static JSON files. No authentication, no rate limiting, no SDK required. Fetch them with anything that speaks HTTP.

Endpoints

Path	Returns	Sample
/constants/<SLUG>.json	One constant record.	/constants/speed-of-light-in-vacuum.json
/index.json	Compact array of every slug. Use for autocomplete.	/index.json
/all.json	Every full record in one file.	/all.json
/categories.json	Array of categories with counts.	/categories.json

Base URL: https://constants.oriz.in · also available at https://cdn.jsdelivr.net/gh/oriz-org/constants-api@main

Record schema

Each /constants/<SLUG>.json file returns one record:

type Constant = {
  slug:               string;           // e.g. "planck-constant"
  name:               string;           // CODATA-canonical name
  value:              number | string;  // parsed numeric value (string if too large for Number)
  value_string:       string;           // human-readable spaced form, e.g. "6.626 070 15 e-34"
  uncertainty:        number | null;    // null when the value is exact
  uncertainty_string: string;           // raw uncertainty, or "exact"
  unit:               string | null;    // SI unit, e.g. "J Hz^-1"
  exact:              boolean;          // true when CODATA defines an exact value
  category:           string;           // "universal" | "electromagnetic" | "atomic-and-nuclear" | "physico-chemical" | "conversions-and-adopted" | "photometric"
  source:             "CODATA 2022 (NIST)";
  source_url:         string;
}

Exact vs measured

Since the 2019 SI redefinition, seven constants have fixed exact numeric values by definition: speed of light, Planck constant, elementary charge, Boltzmann constant, Avogadro constant, caesium hyperfine frequency, and the luminous efficacy of 540 THz radiation. These records have exact: true and uncertainty: null. Everything else is a measured value carrying a finite uncertainty.

Slugs

Slugs are derived from the CODATA name: lowercased, dots stripped, non-alphanumerics collapsed to dashes. Examples:

speed-of-light-in-vacuum — speed of light in vacuum
planck-constant — Planck constant
fine-structure-constant — fine-structure constant
electron-mass — electron mass
avogadro-constant — Avogadro constant

Get the full list via /index.json.

Errors

The only possible error is 404 — unknown slug. There is no authentication, no rate limit, and no quota, so 401/403/429 cannot occur. CORS is wide open on both origins (Cloudflare Pages and jsDelivr).

License & source

Code (scripts/, build tooling): MIT
Data (constants/, *.json): NIST works are public domain in the United States

Source: 2022 CODATA recommended values, published by NIST at physics.nist.gov/cuu/Constants/. Raw ASCII listing: allascii.txt.