Free currency conversion.
ExchangeRate-API is a free Currency Exchange API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Accurate & reliable Exchange Rates API trusted by tens of thousands of developers since 2010. Free access, all world currencies, helpful support & easy to integrate JSON API.
Details from exchangerate-api.com.

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Real-time foreign exchange rates for major currency pairs
The ExchangeRate-API is provided by ExchangeRate-API (exchangerate-api.com). Accurate & reliable Exchange Rates API trusted by tens of thousands of developers since 2010. Free access, all world currencies, helpful support & easy to integrate JSON API.
Yes. The ExchangeRate-API is listed in the Currency Exchange category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The ExchangeRate-API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The ExchangeRate-API supports CORS, so front-end JavaScript can call it directly.
Yes. The ExchangeRate-API is served over HTTPS, so it's safe to use in production.