Yeah, it's dead easy. Yesterday I had a guy laying a 9m x 4m x 100mm concrete slab. To order the concrete we calculated;
9 x 4 x 0.1 = 3.6 cubic metres of concrete. SImples. The metric system is a doddle.
A square of 10cm x 10cm x 10cm filled with water is 1 litre and weighs 1 kg. Makes calculations for anything so easy.
For calculating the weight of steel, for example, the relative density is 7.8 times that of water, so a litre of steel is 7.8kg. For your aggregates, just find out from Professor Google what your material's relative density is and insert that. So to find out your volume, calculate length by breadth by height (in metres) then multiply by relative density to find out the weight in Kg.