How deep should crushed stone be for a drainage layer?

51 to 102mm, which is 2 to 4 inches. Cummin Landscape Supply: #57 stone covers 100 square feet at 2 inches.

How deep should a drainage layer be?

Drainage stone works by the voids between the stones, not by the stone itself, so a clean single-size aggregate matters more than depth alone.

Published laid depths for a drainage layer, by material
Material	Depth	In inches	Where the depth comes from
Crushed stone	51 to 102mm	2 to 4 in	Cummin Landscape Supply: #57 stone covers 100 square feet at 2 inches.

Material

Depth

In inches

Where the depth comes from

Crushed stone

51 to 102mm

2 to 4 in

Cummin Landscape Supply: #57 stone covers 100 square feet at 2 inches.

Working out the quantity once you have the depth

Multiply the area by the depth for a volume, then by the material's bulk density for a weight. Each calculator below does it in the units that material is sold in, and each shows the density range its figure came from.

Crushed stone calculator1424 to 1602 kg/m3, 10 percent waste
Aggregate calculator1500 to 1700 kg/m3, 10 percent waste
Landscape rock calculator1424 to 1602 kg/m3, 10 percent waste
Blue metal calculator1450 to 1600 kg/m3, 10 percent waste
Scoria calculator850 to 1000 kg/m3, 10 percent waste

Materials without a published depth for this use

No supplier read for this site publishes a laid depth for aggregate, river rock, blue metal and scoria on a drainage layer, so none is stated here. Their calculators take whatever depth you enter.