You order gravel loose and place it compacted, and the difference is about a quarter. Getting that backwards is why the last load never finishes the driveway.
Depth is what you want when it is done. The order figure adds the compaction allowance on top.
Gravel arrives loose and gets placed compacted. Dense-graded material like crusher run loses roughly a fifth to a quarter of its volume as the plate drives the fines into the voids, so a six inch finished layer starts life as about seven and a half inches of loose stone.
That is why the last load never quite finishes the job when the order was figured off the finished depth.
Clean single-sized stone is the opposite case. There are no fines to migrate, so #57 washed stone barely compacts and needs little or no allowance. Selecting the material above sets a sensible default.
| Material | lb / yd³ | tons / yd³ |
|---|---|---|
| Crusher run / dense base | 2,900 | 1.45 |
| Pea gravel | 2,800 | 1.40 |
| #57 washed stone | 2,700 | 1.35 |
| River rock | 2,600 | 1.30 |
| Recycled concrete | 2,500 | 1.25 |
Wet material weighs more without occupying more space, which is worth knowing before arguing with a scale ticket after a rainy week.
Four inches of loose material per lift is the working maximum. A plate compactor transmits energy a limited distance, and below that it is simply not compacting — the bottom of a twelve inch dump stays loose and settles under the first heavy vehicle.
Two six inch lifts take longer than one twelve inch dump and are the difference between a driveway that holds and one that ruts.
Four inches compacted is a minimum on good subgrade, six to eight is normal, ten to twelve for trucks or soft ground.
On soft or clay subgrade, yes. Without separation the stone works down into the soil and the layer gradually disappears — the gravel has not gone anywhere, it is just underground.
Not meaningfully. It is single-sized with no fines, which is exactly why it drains, and also why it stays loose. It is a drainage layer, not a base course.