Dirt, gravel, mulch, sand and ready-mixed concrete are all sold by the cubic yard, and a cubic yard is 27 cubic feet: a box three feet on every side. Spread an inch deep it covers 324 square feet; four inches deep, 81.
Most jobs are not one box. A patio and the path to it are two rectangles at different depths, a round bed is a circle, the corner of a yard is a triangle, and a room measured on the calculator is an area already known. Add each as a section with its own depth and the page adds them up, section by section, so the order is for the whole job and you can see where it went.
If the material is gravel or topsoil, choose it and the page weighs the load at the published figure the gravel and topsoil pages use — 1.62 tons a loose cubic yard of road aggregate, 1.12 tons a cubic yard of dry silt loam — or at your supplier’s figure, which beats both. For anything else, type the weight from the supplier’s ticket. It counts bags too, in the sizes a garden centre sells.
Two things the arithmetic cannot know: a base that will be compacted is ordered deeper than it finishes — the gravel page works that out — and how your supplier rounds a delivery. Round the answer up to what they sell, never down.