Decode may throw if the string is not valid percent-encoding.
Encode uses encodeURIComponent (UTF-8 percent-encoding, reserved characters encoded). Decode uses decodeURIComponent; malformed sequences raise an error message. This is for quick checks, not a full URL parser.
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.