Gravel is rock that is of a certain particle size range. In geology, gravel is any loose rock that is larger than two millimeters (2mm) in its largest imension (about 1/12 of an inch) and no more than 63 millimeters (about 2.5 inches). Sometimes gravel is restricted to rock in the 2-4 millimeter range, with pebble being reserved for rock >4-63 millimeters. The next smaller size class in geology is sand, which is >0.063 mm to 2 mm in size. The next larger size is cobble, which is >63 millimeters to 256 millimeters (about 2.5 to ten inches). One cubic foot (28.32 dm3) of gravel typically weighs about 100 pounds (45 kg) .