Gravel /ÃËárævÃâ¢l/ is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel 2 to 4 mm or 0.079 to 0.157 in and pebble gravel.