30L is 0.03m3. So you could fit approx 33.3 buckets in a 1m3 bag, so approx 1620Kg. Making the 1600Kg above seem the correct value to use.
The volume of a cylinder is the base time height, so (pi)*r2*height, or 0.425^2 * 3.14 * 0.59 = 0.33m3 of gravel, if a cylinder.