I'm sure this has already been answered plently of times, but rather than say "Use the search" I'll just answer.
Manufacturers sell the drive based on 1GB=1000MB, 1MB=1000kb 1KB=1000bytes, etc.
Hard drives are formatted like this: 1GB=1024MB, 1MB=1024kb, 1KB=1024bytes, etc.
So you're not really losing anything, it's just deceptive marketing. So I think 1GB actually equals 1073.741824MB, but I probably did the math wrong (1024(b)x1024(k)x1024(m)). So for every GB you lose ~74MB. Works out for me, I've got 2 120GB drives, using 74MB/GB "loss" I should have 8880MB less when formatted, Windows shows it as (roughly) 111GB.