[kragen]

You calculate password entropy of a password drawn from a set of equally likely passwords by taking the log_2 of the size of the set. (This is a convenient special case of the more general formula for entropy.) A convenient thing about this is that if you concatenate two things (in such a way that you can pull them apart again) the number of possibilities is the product of the possibilities for each of the things, so the entropy is the sum of the entropies.