Estimating the amount of gravel you need to build a foundation for a concrete slab is not complicated, but it does require some planning. The gravel foundation prevents the slab from sinking under stress and improves the site's drainage among other purposes. The depth of the foundation depends upon your soil type, and the thickness of the slab itself.
Consider the purpose of the slab in figuring the thickness of the concrete and gravel base. A driveway slab should be 6 inches thick with a 6- to 8-inch gravel base. A walkway or patio slab is typically 4 inches thick with a 4- to 6-inch gravel base. If the area has poor drainage, increase the depth of the foundation to twice the thickness of the slab.
To figure the amount of gravel you need for a concrete slab, multiply the length and width of the site to get the square footage, and then multiple the square footage by the depth of the foundation. Divide this number by 27 to convert it to cubic yards, and then multiply it by 1.5 tons to figure the amount of gravel needed.
Loose gravel is used in a variety of ways so it comes in different sizes ranging from 3/8 inch to 1 1/4 inches. The smallest gravel contains broken down rocks that create a sand-like consistency. When compacted, this material is too dense for underground moisture to escape. A mix of 3/4 inch gravel and larger is dense enough to support the concrete, but it still has enough flexibility to allow water to escape and absorb tension when the ground freezes.
The most difficult part of building the gravel foundation is excavating the site. Once the site is dug out, compact the subsoil and lay geotextile fabric over the bottom to reinforce the gravel. Pour the gravel over the site in 2-inch layers and compact it before adding more. Spread the gravel even with a rake and keep tamping it until you can't compact it any tighter.