This involves a concept of bulk density, which refers to the mass of bulk coal loaded per unit volume; Another concept is the angle of repose, which is what we usually call the angle of slope. The engineering estimates are as follows: anthracite: bulk density (0.7- 1.0), angle of repose (27-45), bituminous coal: bulk density (0.8), angle of repose (35-45), brown coal (0.6-0.8), angle of repose (35-50), etc. But I suggest that if you buy it yourself, you'd better weigh it. After all, coal is so expensive now, as the old saying goes, it is better to buy than to sell.