Layers of a gravel driveway
Maine’s gravel road manual for private and camp roads gives a compacted base layer of 12 to 18 inches, of well-graded gravel mostly smaller than 3 inches, under a compacted surface layer of about 4 to 6 inches with stones no larger than 2 inches. It is the closest public guidance to a home driveway. A supplier, River Sand, names #57 stone and crusher run as the two most common driveway gravels.
The Federal Highway Administration’s rule for gravel roads is that each layer should be at least twice as thick as its largest stone.
Allow for compaction
The depths are compacted. Maine’s manual shows multiplying the volume by about 1.11 to end up with the compacted depth you want, which is the 11% allowance in the example. Ask your supplier for its own figure for the stone you buy.
Topping up an existing driveway
To refresh a driveway that already has a sound base, enter only the surface layer’s depth and set the base to zero.