I've said this before - but this is just a reminder. I'll be breaking BOTL over time in order to get it into a far better place. Just be patient if you can't reach us! We'll be back
The usable capacity of a RAID 5 array is (N-1) * \min(S_1, S_2, \dots, S_N) = (N-1) * S_{min}, where N is the total number of drives in the array, Si is the capacity of the ith drive, and Smin is the capacity of the smallest drive in the array.