HEX to binary and Binary to hex is pretty easy by itself; each group of two digits in hexadecimal is 8 binary digits. in fact I imagine that is hald the reason it was decided that hex would be used for things like dumps of unknown data- (hex dumps). each byte can be represented by two hex digits.
of course converting either into an actual decimal number isn't always strictly necessary, and when when it is it isn't always intuitive, either.