You buy by weight not volume. Stuff weighs different amounts per m3 as others have said, plannings are heavy. The ticket you sign with each load should say how much it is loaded with. Certainly when I use Days Aggregates they have this info on the ticket. Usually varies by 100kg either way but around 18t. Days have a couple of Alloy tippers that they only run with topsoil and you get neigh on 20t out of them but they wont let you put muck on for a return load!