Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. As the term is used by geologists, sand particles range in diameter from 0.0625 (or 1⁄16 mm, or 62.5 micrometers) to 2 millimeters.

An individual particle in this range size is termed a sand grain. The next larger size class above sand is gravel, with particles ranging from 2 mm up to 64 mm (see particle size for standards in use). The next smaller size class in geology is silt: particles smaller than 0.0625 mm down to 0.004 mm in diameter. The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm.[1] Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour).

ISO 14688 grades sands as fine, medium and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In USA, sand is commonly divided into five sub-categories based on size: very fine sand (1⁄16 - 1⁄8 mm diameter), fine sand (1⁄8 mm - 1⁄4 mm), medium sand (1⁄4 mm - 1⁄2 mm), coarse sand (1⁄2 mm - 1 mm), and very coarse sand (1 mm - 2 mm). These sizes are based on the Krumbein phi scale, where size in ض = -log base 2 of size in mm. On this scale, for sand the value of ض varies from -1 to +4,

with the divisions between sub-categories at whole numbers.