Aggregate is quoted by the tonne, spread by the cubic metre, and it shrinks when you compact it. All three are handled here, so the number you order is the number that finishes the job.
Notes
Why compaction matters
Loose gravel loses 15-25% of its volume under a plate compactor. Order the compacted depth and you finish 20 mm short across the whole area, which on a driveway is a lot of missing material.
Volume to tonnes
Multiply cubic metres by the bulk density. Roughly 1.5 t/m3 for gravel and hardcore, 1.6 for sharp sand, 1.4 for topsoil, 2.4 for solid stone. Merchants publish their own figures and they do vary.
Depths that actually work
Decorative gravel 40-50 mm, pedestrian paths 50 mm over a base, driveways 150-200 mm of sub-base plus a wearing course. Too thin and it rutts, too thick and it never locks up.
What this does not do
It does not design a build-up or tell you whether you need a membrane and a sub-base. On anything a car drives over, you usually do.