Digging breaks up a soil’s structure, so the spoil pile is always bigger than the hole: the swell factor is that bulking, applied to the in-place (bank) volume to predict loose volume. Representative figures run about 10–15% for sand and gravel, 20–30% for common earth and clay, and 40–65% for blasted rock — but they are rules of thumb that shift with material, moisture, and handling, which is why a takeoff treats presets (15% rock, 30% Type A clay, 25% Type B, 12% Type C sand/gravel) as estimates and overrides them with the geotech report’s or local experience’s number when one exists. Swell is representative practice, not code.
The money is in the multiplication. A 50 ft utility trench, 6 ft deep with a 3 ft bottom in Type B soil, sloped at OSHA’s 1:1, holds 100 yd³ of bank volume; at 25% swell about 125 yd³ of loose material comes out — 13 truckloads at 10 yd³ each — and both the haul-off trucking and the stockpile footprint are sized by the swelled figure, not the neat geometric one.
Swell has a mirror image on the fill side: compacted embankment occupies less space than the bank material that made it, handled through a shrinkage factor (typically 0.80–0.95 compacted per bank volume) when cut is balanced against fill. Loose, bank, and compacted are three different cubic yards of the same dirt, and every earthwork number should say which one it is in.