Uses TextEncoder + btoa. Large pastes may slow the tab.
Your text is encoded as UTF-8 bytes, then converted with the browser’s btoa helper (binary string). Characters outside what that path supports can throw an error. The second line shows the Base64 string length in characters.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.