For the record, it's pretty easy to do this by hand. The calculator assumes the attacker knows how many of each kind of character there is, which is a weird assumption so I'll not use that. Anyway you can take the base-2 log of the number of possibilities, or more easily add the entropies of each character (if they're not related). If you take e.g. the 64 symbols of Base64 as your allowed space you get: n*log_2(64)= 6n bits of entropy for an n-character password.