We source our currency exchange rates from reputable sources, and update them every 5 minutes (sometimes even just 3 minutes, but never more than 5). These rates are provided for +140 currencies, and you can easily switch base currency for conversion by providing a ?base=[currencyCode] parameter in the URL. We return the conversion rate from USD by default.
Disclaimer: while the rates are updated at high frequency, we do not recommend using them to drive forex trading software and similar, while the numbers provided are highly accurate, and sourced from multiple reputable sources, they are not updated at high enough frequency (5 minutes instead of <=1 minute) to reliably use for forex trading. If you need updates at 1 minute or below, please reach out to us at hello@tellusdb.com