Max weight for a heavy load is Str * 10 lb. at 10 Str and lower. From 11 through 15, it's an exponential curve, doubling every +5 Str, rounded to the nearest 5 lb (100 * 2 ^ ((Str - 10) / 5)). Str of 16 and higher continues that curve, although for simplicity the rounding part of the equation is dropped in favor of just being 2x what the carrying capacity of -5 Str was. Light load and medium load limits are 1/3 and 2/3 of the heavy load limit, respectively, rounded to the nearest lb.