Bitcoin fee estimate API (sat/vB) and mempool status

$0.002 per call · failed calls are never charged · no account or subscription: pay per call with a card key or USDC

Current Bitcoin network fee rates in sat/vB for next-block, half-hour, one-hour and economy confirmation, the estimated cost in sats of a typical payment, mempool size and total pending fees, and the latest block height. Use before sending a Bitcoin transaction or to decide whether to wait for lower fees.

When to use it

Guide: Bitcoin fee estimate API

Run it free (25 free calls) Buy credits by card (from $1) Example result (JSON)

Request

Over HTTP, with a card key from /credits:

curl -H "Authorization: Bearer adt_…" "https://tools.funwords.eu/btc-fees?format=json"

Or one of your 25 free calls, no key:

curl "https://tools.funwords.eu/btc-fees?format=json&trial=1"

Without a key, the same URL answers 402 Payment Required with x402 and MPP terms, so an agent with a wallet pays per call in USDC (Base, Polygon, Arbitrum or Solana over x402; Base or Tempo over MPP) and retries.

Example result

{
  "network": "bitcoin",
  "unit": "sat/vB",
  "fees": {
    "nextBlock": 4,
    "halfHour": 3,
    "hour": 1,
    "economy": 1,
    "minimum": 1
  },
  "typicalPaymentSats": {
    "nextBlock": 564,
    "hour": 141
  },
  "typicalPaymentVbytes": 141,
  "source": "https://mempool.space/api",
  "limitations": [
    "Estimates do not guarantee confirmation time",
    "141 vB is an illustrative one-input/two-output SegWit size; actual transaction size varies",
    "Source endpoints are fetched concurrently, not as an atomic snapshot"
  ],
  "mempool": {
    "transactions": 83832,
    "vsize": 42066150,
    "totalFeesBtc": 0.1093664
  },
  "blockHeight": 970000,
  "checkedAt": "2026-10-01T00:00:00.000Z"
}

From an AI agent (MCP)

Add the MCP server to Claude, Cursor or any MCP client. This tool is btc_fees, and the first 25 successful calls are free:

https://tools.funwords.eu/mcp

Questions

Where do the numbers come from?

The mempool.space API, at call time.

How do I pay?

Each successful call costs $0.002; failed calls are not charged. The first 25 calls are free. Then pay with card credits (from $1; the key goes in an Authorization header or, for clients that can only fetch a URL, as api_key= in the URL) or per call in USDC over x402 or MPP. No account is needed.

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