SONIA Rate API from JSON

A SONIA rate API is a REST endpoint that returns the Sterling Overnight Index Average as JSON. On Interest Rates API the field is SONIA (percent, GBP). Call GET /api/v1/latest with symbols=SONIA. Auth is the api_key query parameter.

The official Latest sample is dated 2026-04-20 with SONIA at 4.70 and dates.SONIA 2026-04-20 (currency GBP). That is a docs fixture — copy the curl with your key for today’s print. Starter allows 1 symbol per request, so Starter curls must pass symbols=SONIA only. The four-symbol JSON below is what Professional and above may request in one call. Docs: interestratesapi.com/docs. Starter is $19/mo (250 requests, 60/min, 90-day max range, 7-day trial). Professional is $49/mo (750 calls) and can pass multiple symbols.

Request: SONIA on Starter

curl "https://interestratesapi.com/api/v1/latest?symbols=SONIA&api_key=YOUR_KEY" \
-H "Accept: application/json"

Professional and above can pass symbols=FED_FUNDS,SOFR,ECB_MRO,SONIA. Optional category: central_bank, interbank, treasury, reference. Optional base filters by currency and drops everything else — with base=USD, SONIA (GBP) will not come back. Do not pass base=USD when you want SONIA.

Response: rates, dates, and currencies

Envelope from the official Latest docs (sample date 2026-04-20 — four symbols, Professional+). Starter’s symbols=SONIA call returns the same SONIA fields from this fixture (rates.SONIA 4.70, dates.SONIA 2026-04-20, currencies.SONIA GBP) without the other three symbols:

{
"success": true,
"date": "2026-04-20",
"base": "MIXED",
"rates": {
"FED_FUNDS": 4.33,
"SOFR": 4.31,
"ECB_MRO": 2.65,
"SONIA": 4.70
},
"dates": {
"FED_FUNDS": "2026-04-20",
"SOFR": "2026-04-19",
"ECB_MRO": "2026-03-31",
"SONIA": "2026-04-20"
},
"currencies": {
"FED_FUNDS": "USD",
"SOFR": "USD",
"ECB_MRO": "EUR",
"SONIA": "GBP"
}
}

The top-level date is the newest observation in the payload. Use dates.SONIA for the series observation date. SONIA is a daily overnight benchmark in the interbank category.

What SONIA is for (vs SOFR)

Sterling loans and facilities. Label the rate with dates.SONIA and currencies.SONIA (GBP). Do not pass base=USD or SONIA is dropped.
Cross-currency screens. SOFR is the USD cousin. Starter cannot request both in one call (1 symbol). Professional can pass symbols=SOFR,SONIA.
Not a policy rate. Bank of England Bank Rate is a different symbol family than overnight SONIA. Keep the field name on the chart so users do not mix them.

Go live with SONIA

1. Register and start the 7-day trial (Starter is $19/mo after).

2. List symbols with GET /api/v1/symbols?category=interbank.

3. Call https://interestratesapi.com/api/v1/latest?symbols=SONIA&api_key=YOUR_KEY

4. Render rates.SONIA and label it with dates.SONIA. Do not add base=USD.

Support: [email protected].

FAQ: SONIA rate API

What is SONIA vs SOFR?

SONIA is the GBP overnight unsecured rate. SOFR is the USD secured overnight financing rate. Both are interbank symbols. Starter must request them in separate calls.