A currency API is a web service that returns exchange rate data to your application over HTTPS, in a machine-readable format (usually JSON). Instead of scraping a website or maintaining your own pipeline of bank feeds, you make a REST request โ GET /rates?source=USD&target=EUR โ and get back the current rate.
This guide explains what a currency API does, how it works, common endpoints, real-world use cases, and how to pick the right one.
What a Currency API Does
A currency API sits between your application and the global FX market. It handles:
- Data aggregation โ pulling rates from interbank feeds, central banks, and aggregators.
- Normalization โ converting varied source formats into a consistent JSON response.
- Hosting โ serving rates from infrastructure designed for high availability and low latency.
- Access control โ issuing API keys, enforcing rate limits, tracking usage.
You call the API, you get a number. The provider handles everything else.
How a Currency API Works
A typical request-response cycle:
- Your app builds an HTTPS request with:
- A source currency (e.g.
USD) - One or more target currencies (e.g.
EUR,GBP) -
An API key (usually in an
Authorization: Bearer ...header) -
The provider's server:
- Authenticates the key
- Looks up the latest rates from its cache or live feed
-
Returns a JSON response with the rate and a timestamp
-
Your app:
- Parses the JSON
- Multiplies the amount by the rate
- Displays or stores the result
Example request:
curl -X GET "https://allratestoday.com/api/v1/rates?source=USD&target=EUR" -H "Authorization: Bearer YOUR_API_KEY"
Example response:
{
"source": "USD",
"target": "EUR",
"rate": 0.9234,
"time": "2026-04-14T12:00:00Z"
}
Common Currency API Endpoints
A mature currency API provides several endpoint types, each for a specific job:
Latest Rates
Returns the most recent exchange rates. The bread-and-butter endpoint.
GET /api/v1/rates?source=USD
Historical Rates
Returns the rate on a specific past date. Critical for accounting, tax reporting, and backtesting.
GET /api/historical-rates?source=USD&target=EUR&date=2024-03-15
Time Series
Returns a range of daily rates between two dates. Used for trend charts and analysis.
GET /api/time-series?source=USD&target=EUR&from=2026-01-01&to=2026-03-31
Convert
Does the multiplication for you โ give it an amount and it returns the converted amount.
GET /api/convert?source=USD&target=EUR&amount=100
Fluctuation
Returns the change, percentage change, and high/low over a period. Useful for rate alerts.
GET /api/fluctuation?source=USD&target=EUR&from=2026-04-01&to=2026-04-14
Where Currency APIs Get Their Data
Not all currency APIs are built on the same data. Understanding the source matters because it determines accuracy and update frequency.
- Interbank feeds (Reuters/Refinitiv, Bloomberg) โ live rates from the actual FX market. Highest accuracy and freshness. Used by AllRatesToday, XE, and institutional providers.
- Central banks (European Central Bank, Federal Reserve) โ one reference rate published per day. Free to use, but daily and limited to ~30 currencies. Used by Fixer and ExchangeRate-API free tiers.
- Aggregators โ blends of multiple sources, with a provider-specific methodology.
- Crypto exchanges โ order-book data from Binance, Coinbase, etc. Used for crypto pairs.
An API serving daily ECB data is not "wrong," but it cannot deliver real-time rates no matter how fast its servers are.
Common Use Cases
E-Commerce
Display prices in the visitor's local currency. Most carts check the rate at cart load and at checkout to avoid surprises.
SaaS Billing
Invoice customers in their own currency while reporting revenue in USD or EUR. Usually calls the API once per invoice generation, not per page load.
Financial Reporting
Convert foreign transactions to a reporting currency at historical rates. Historical endpoint + accounting software.
Trading and Dashboards
Live or near-live rates for charts, screeners, and portfolio trackers. Requires 60-second updates or faster.
Cross-Border Payouts
Marketplaces and payroll systems calculating settlement amounts. Usually combines a currency API with a payment processor.
Travel and Fintech Apps
Consumer-facing converters for travelers, remittance apps, and neobanks.
What Makes a Good Currency API
- Accurate data source โ interbank feeds beat daily ECB snapshots.
- Update frequency โ real-time (60s) for anything touching money; hourly or daily is fine for reporting.
- Coverage โ at least 150+ fiat currencies, plus metals and crypto if relevant.
- Flat JSON responses โ easy to parse, no nested quotes-object weirdness.
- Official SDKs โ save you writing HTTP plumbing.
- HTTPS and CORS โ table stakes in 2026.
- Transparent pricing โ a real free tier, no hidden fees.
- Good documentation โ working code samples in your language.
Currency API Pricing
Free tiers exist, usually with trade-offs (daily updates, low quotas, restricted base currency). Paid plans run from $10โ$100/month for most SaaS needs. Enterprise plans (XE, OANDA) run into thousands per year.
For most developers, a free tier is enough to prototype, and a $10โ$20/month plan covers production.
Choosing a Currency API
Match the API to the job:
- Indie project / prototype โ AllRatesToday free tier, Frankfurter (no key).
- Production e-commerce / SaaS โ AllRatesToday, Open Exchange Rates, ExchangeRate-API paid plans.
- Trading / fintech โ AllRatesToday (60s) or XE Enterprise.
- Enterprise / regulated โ XE, OANDA, Bloomberg.
FAQ
Is there a free currency API?
Yes. AllRatesToday, ExchangeRate-API, Open Exchange Rates, and Fixer all offer free tiers. Frankfurter is completely free with no key required.
What's the difference between a currency API and an exchange rate API?
Nothing โ they're the same thing. "Currency API," "exchange rate API," "FX API," and "forex API" are used interchangeably.
Do currency APIs support crypto?
Some do (CurrencyFreaks, CurrencyAPI, XE for select coins). Most are fiat-focused. For crypto-heavy work, use a dedicated crypto API like CoinGecko.
How often do rates update?
Depends on the provider and plan. Daily (ECB snapshots) at the low end, 60 seconds on real-time plans, microseconds on institutional feeds.
Can I use a currency API in the browser?
Only if it supports CORS. AllRatesToday and ExchangeRate-API do. Fixer, CurrencyLayer, and XE don't โ those require a server-side proxy.
Do I need a paid plan?
For prototypes and low-volume apps, a free tier is fine. For production with live pricing, you'll want a paid plan with at least hourly updates.
Ready to integrate? Get a free AllRatesToday API key โ 300 requests/month, real-time rates, historical data, and official SDKs for JavaScript, Python, and PHP.