Loose volume = bank volume × (1 + swell percent / 100). A 20% swell on 1,000 cubic yards of bank cut yields 1,200 cubic yards in the truck bed. Estimators apply swell to translate bid-grade bank volumes into trucking quantities.
Soil type drives the factor: clay swells 20-40%, silt 25-35%, sand 5-15%, gravel 10-25%, rock 50-80%. Moisture conditions matter — wet clays swell more than dry clays. Use a 20-25% default for ordinary site soil; bump to 35-40% for cohesive clays.
Don't confuse swell with shrink. Swell is bank → loose. Shrink is bank → compacted (the soil is denser after placement and rolling than it was in its undisturbed state).