Sized from 1/4" to 2", an approximate weight per loose cubic yard of gravel (as loaded in a dump truck) is 3400 pounds wet, 2850 pounds dry, anywhere in between and can be somewhat outside those weight boundaries.
That's a long answer for an approximation but weight to volume factors differ because of different rock densities, moisture content, velocities, etc.. An individual gravel pit will have a more accurate volume to weight factor for the product they produce but even that will change slightly from run to run.
Why add water? Dust control is the main reason. Few pits have "dry" gravel. Even expensive, oven treated material has some moisture content. The higher the moisture content, the higher the weight.
If you use a weight of 3125 pounds per loose cubic yard of gravel, you'll be close. That gives you a cubic yard to ton weight factor of 1.5625.
1.5625 tons/yd^3 * 180 loose cubic yards of gravel will weigh approximately 281 tons, plus or minus.
Last edited by Dirtman
on Thu May 07, 2009 6:04 am, edited 1 time in total.