Tones. You order / buy by the ton. Or trailer full, if pick up. Drive vehicle / trailer over scale. Once loaded over scale to see how much material you received.
Or you have a load delivered. Typical landscape delivery is free if over 8T . So you always go bigger than needed. They also base material needed on sq footage and depth of material. I want 2" of driveway rock (1/2-2" mix) over X area. They say 35T, i'm getting 50t to have some for other projects.
Unlike some landscape places in CO that try telling you one of their buckets is X yards of material. Which if you weighed it always falls much shorter than you paid for.