Estimate how much material you'll excavate, how much it swells once dug loose, and how many truckloads you'll need to haul it away.
In the ground, soil is compacted. Once dug up, it becomes loose and full of air pockets, so the same weight of soil occupies 15–30% more volume — this is called 'swell'.
Divide your loose (swelled) volume by your truck's capacity in cubic yards, then round up — this calculator does that automatically.
Depths vary by frost line and soil-bearing capacity, but 3–4 feet is common for residential strip footings — always confirm with a structural engineer or local code.