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Dirt gets bigger when you dig it — order trucks for the bigger number.
The most expensive excavation mistake is ordering trucks for the in-ground volume. Soil swells when dug — 25% for common earth, 40% for clay, 65% for blasted rock — so loose volume, not bank volume, sets the truck count. This calculator runs the full chain: dimensions to bank yards, bank to loose with the US Army Corps of Engineers FM 5-434 Table 1-2 swell factors, loose yards to truckloads, and loads to haul-away cost.
Anyone pricing a dig — basement, pool, pond, foundation, drainage — uses this before calling the hauler: the in-ground yards are never the hauled yards, and the 25–40% swell difference is real money at $150 a load. Excavators use it as a five-second sanity check on truck counts before the iron shows up.
Per US Army FM 5-434 Table 1-2: sand and gravel about 10–15%, common earth and loam about 25%, clay about 40%, and blasted rock 50–65%. One bank cubic yard of loam becomes roughly 1.25 loose yards in the truck — that 25% is the line item that blows up DIY haul budgets.
Bank volume is 30 × 12 × 1.5 ÷ 27 = 20 bank yards. In common earth at 25% swell that is 25 loose yards — three loads in a 12-yard dump truck (25 ÷ 12 = 2.08, round up; the last load is always partial). In clay at 40% it becomes 28 loose yards — still three loads, but barely.
Subtract the backfill bank volume from the total before computing loads — but remember compacted backfill shrinks below its bank volume, so a dig rarely backfills itself with its own spoil. Whatever does get hauled is priced on loose volume, which is why the swell factor matters.