Physical constants (CODATA) API

Resolve a physical constant by name or symbol (c, h, G, Na, e, k, R, ...) to its CODATA 2018 recommended value, SI units, standard uncertainty, and relative uncertainty — flagging exact (defined) constants. LLMs misremember digits and units; this is authoritative. Answers 'what is the speed of light', 'value of Planck constant', 'Avogadro number with units', 'gravitational constant uncertainty'.

Price$0.01per request

MethodPOST

Route/v1/ref/physical-constants

StatusLive

MIME typeapplication/json

Rate limit60/minute

CacheNo cache

refconstantsphysicscodatasciencephysical-constantsunitssi

API URLhttps://x402.hexl.dev/v1/ref/physical-constants

Integration docs

Example request

{
  "query": "c"
}

Example response

{
  "query": "c",
  "found": true,
  "symbol": "c",
  "name": "Speed of light in vacuum",
  "value": 299792458,
  "unit": "m/s",
  "uncertainty": 0,
  "relativeUncertainty": 0,
  "exact": true
}

Input schema

{
  "type": "object",
  "required": [
    "query"
  ],
  "properties": {
    "query": {
      "type": "string",
      "examples": [
        "c",
        "planck",
        "avogadro"
      ]
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}