An interest rate convert API prices the same principal and term on two benchmarks and returns the spread and interest difference in one call. On Interest Rates API that is GET https://interestratesapi.com/api/v1/convert with from, to, amount, optional term_months (1–360, default 12), and query api_key. Valid symbols include FED_FUNDS and SONIA (and SOFR, EURIBOR tenors). Starter is $19/mo (normally 1 symbol per request on latest; /convert is the exception that allows two), 250 calls, 7-day trial. Pro is $49 with multi-symbol latest.
Do not pass base=USD when you need SONIA or ECB prints. FED_FUNDS is the FRED monthly average (day 1); daily is FED_FUNDS_DAILY. MCP: https://interestratesapi.com/mcp. Docs: interestratesapi.com/docs (convert).
Request: pair two symbols
You can call from=FED_FUNDS&to=SONIA the same way. The published numeric fixture is SOFR vs EURIBOR_3M — that is the JSON below. Do not invent a FED vs SONIA result.
curl "https://interestratesapi.com/api/v1/convert?from=SOFR&to=EURIBOR_3M&amount=500000&term_months=24&api_key=YOUR_API_KEY"
curl "https://interestratesapi.com/api/v1/convert?from=FED_FUNDS&to=SONIA&amount=500000&term_months=24&api_key=YOUR_API_KEY"
Response: official convert fixture
{
"success": true,
"amount": 500000,
"term_months": 24,
"from": {
"symbol": "SOFR",
"rate": 4.31,
"date": "2026-04-19",
"total_interest": 43100.00,
"total_payment": 543100.00
},
"to": {
"symbol": "EURIBOR_3M",
"rate": 2.41,
"date": "2026-03-31",
"total_interest": 24100.00,
"total_payment": 524100.00
},
"difference": {
"rate_spread": 1.9,
"interest_saved": 19000.00
}
}
SOFR 4.31 (2026-04-19) vs EURIBOR_3M 2.41 (2026-03-31): spread 1.9, interest_saved 19000.00 on 500000 over 24 months. Dates can differ per symbol.
Python: read the spread
import json, urllib.request
url = ("https://interestratesapi.com/api/v1/convert"
"?from=SOFR&to=EURIBOR_3M&amount=500000&term_months=24"
"&api_key=YOUR_API_KEY")
with urllib.request.urlopen(url) as r:
data = json.load(r)
spread = data["difference"]["rate_spread"]
Go live
1. Register — 7-day trial.
2. Copy api_key.
3. GET convert with two symbols, amount, and term_months.
4. Store difference.interest_saved.
