Compacted Depth
gravel
The thickness of a material after compaction, which is less than the loose depth it was spread at — typically by 15% to 25% for crushed aggregate.
Specifications are written in compacted depth because that is what carries load, but you order material by loose volume. The gap between them means a 4-inch compacted base needs roughly 5 inches of loose stone spread before compaction. Aggregate must also be compacted in lifts of 3 to 4 inches — a plate compactor cannot densify 8 inches of loose rock in one pass, and a base that looks finished but is loose underneath fails in the first freeze-thaw cycle. This is the most common reason a driveway or patio settles unevenly within a couple of seasons.
Example
A driveway specified at 6 inches compacted needs about 7.5 inches of loose stone, placed and compacted in two lifts.
Common mistake
Ordering material for the compacted depth and spreading it once. You end up 20% short of the specified thickness, and the shortfall is invisible until the surface starts to move.
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