The number of tons of gravel in a cubic yard depends on the density of the gravel. One ton is 2,000 pounds.
Assuming the gravel has an average density of 100 pounds per cubic foot, there are 0.74 cubic yards of gravel in a ton. Divide this by the density to give the number of cubic feet, and then divide this figure by 27 to give the number of cubic yards. If the gravel has a density of 100 pounds per cubic foot, the calculations are 2,000/100 = 20 cubic feet per ton. 20/27 = 0.74 cubic yards per ton. The inverse (27/20) is 1.35 tons (2700 pounds) per cubic yard.
Weight density measures the compactness of a substance. It depends on particle size, moisture content and particle shape. Examples include cement with a weight density of 94 pounds per cubic foot, concrete at 150 pounds per cubic foot and limestone at 171 pounds per cubic foot.
Gravel is loose rock, bigger than sand and smaller than cobble. Gravel can be sub-categorized into granules, pebbles, and rocks. There are many different types of gravel, including fine, lag, pay, pea, bank and bench. Gravel density can vary based on particle size. For comparable particle sizes, the densities are fairly similar. The biggest variance is with mixed particle sizes. Mixed particle sizes will allow for packing and filling voids, whereas larger, more uniform sizes will leave larger gaps.
Gravel is used to help stabilize soil where heavy traffic or heavy drainage are needed. Smaller particles make for a good drainage base under foot-paver. Larger particles resist embedding into the subsoil when run over by vehicles. "RipRap", is used around creeks, drainage pipes, and where heavy machinery will operate. This is 6" or larger rocks, wrapped in wire. Effectively, it is jumbo-sized gravel which resists erosion, but allows water to flow through.
For crushed gravels, sizes range from "dust" to 4". Washed #2 gravel is 2" to 4" rocks, and is about 23CuFt per ton. Washed #4 gravel is 1"-2" rocks, and about 20CuFt per ton, which is comparable in density to medium river rock at 1"-2" sizes. For rounded gravel, such as might be mined from riverbeds, one might find Pea gravel (1/4") at 18CuFt/ton, vs 19CuFt per ton of crushed #9 (also 1/4").
Gravel deposits are a common geological feature and formed as a result of the erosion and weathering of rocks. Rivers and waves pile up gravel in large accumulations. Gravel is also produced by quarrying and crushing rocks, such as limestone, sandstone and basalt. The USA is the world’s largest consumer and producer of gravel.
Edited and corrected 2012-05-14 jdsd.