Field	Type	Required	Notes
`activity`	string	yes	fuel \| electricity \| freight \| passenger-travel
`quantity`	number	yes	fuel: units of that fuel; electricity: kWh; freight: short ton-miles; passenger-travel: one-way miles
`fuel`	string	no	activity=fuel: natural-gas \| motor-gasoline \| diesel \| jet-fuel \| propane \| aviation-gasoline \| residual-fuel-oil \| biodiesel \| ethanol \| bituminous-coal \| wood
`region`	string	no	activity=electricity: eGRID subregion, default us
`mode`	string	no	activity=freight: truck \| rail \| water \| air; activity=passenger-travel: car \| light-truck \| motorcycle \| bus \| commuter-rail \| transit-rail \| intercity-rail \| air
`passengers`	number	no	passenger-travel only; ignored for vehicle-basis modes

Example input:

{
  "activity": "passenger-travel",
  "mode": "air",
  "quantity": 2500,
  "passengers": 2
}

Cost and provenance


Price	$0.003
Source	US EPA GHG Emission Factors Hub, January 2025 (public domain)
Licence	public-domain
Attribution	Emission factors: US EPA GHG Emission Factors Hub (January 2025)

Why call it rather than answer from memory

A procurement, logistics or ESG-reporting agent has to attach a defensible CO2e number to a shipment or an invoice line, and the factor tables are a spreadsheet it cannot read at runtime.

Calling it

A skill is loaded on its own, so this document has to be enough by itself. It does not assume the general fittings skill is also loaded. call.mjs ships alongside this file; if it is missing (this document reached you without the rest of the skill), fetch it once with curl -sO https://fittings.sh/skill/call.mjs.

Free, and needs no wallet. Read the schema and see what a call would cost:

curl -s https://fittings.sh/catalog/epa.carbon-estimate | jq

Paying. The endpoint answers 402 Payment Required with a signed quote, you sign an authorization, and a facilitator settles it — you never send a transaction and never pay gas. Before it spends anything, the client checks its own hash against the gateway and refuses rather than sign with logic the gateway has moved past:

# Dry run. Prints the quote and pays nothing, because no key is set.
node call.mjs epa.carbon-estimate activity=passenger-travel mode=air quantity=2500 passengers=2

# For real: a wallet holding USDC on Base, and nothing else.
export FITTINGS_PRIVATE_KEY=0x...
node call.mjs epa.carbon-estimate activity=passenger-travel mode=air quantity=2500 passengers=2

Spending is never a side effect of curiosity: with no FITTINGS_PRIVATE_KEY the client fetches the quote and stops. FITTINGS_MAX_PRICE caps what it will pay without asking, and defaults to $0.05.

Use a throwaway wallet holding a few dollars, not a main one. The key sits in plaintext wherever the agent runs, and the whole cost of entry is USDC on Base — no account, no API key, and no ETH, because the payer signs off-chain and the facilitator broadcasts and pays the gas.

Writing your own client instead of using this one? @x402/core and @x402/evm on npm, and the fittings skill records the two things that cost real debugging time: the challenge is in a response header rather than the body, and it must be re-signed from the exact request being retried.

If this is not the right service, https://fittings.sh/catalog/search?q=... searches all of them for free.

epa-carbon-estimate

CO2e emissions estimate

The call