Grain size in geology refers to the physical dimensions of particles of rock or other solid. This is different from the crystallite size, which is the size of a single crystal inside the solid (a grain can be made of several single crystals). These can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders. Size ranges define limits of classes that are given names in the Wentworth scale used in the United States. The Krumbein phi (φ) scale is logarithmic and computed by the equation: φ = -log 2 (grain size in mm).
| Phi Scale | Size range (metric) |
Size range (approx. inches) |
Aggregate name (Wentworth Class) |
Other names |
| < -8 | > 256 mm | > 10 in | Boulder | |
| -6 to -8 | 64 - 256 mm | 2.5 - 10 in | Cobble | |
| to -6 | 32 - 64 mm | 1.2 - 2.5 in | Very coarse gravel | Pebble |
| 16 - 32 mm | 0.6 - 1.2 in | Course gravel | Pebble | |
| 8 - 16 mm | 0.3 - 0.6 in | Medium gravel | Pebble | |
| - 2 to | 4 - 8 mm | 0.15 - 0.3 in | Fine gravel | Pebble |
| -1 to -2 | 2 - 4 mm | 0.08 - 0.15 in | Very fine gravel | Granule |
| 0 - -1 | 1 - 2 mm | 0.04 - 0.08 in | Very coarse sand | |
| 1 - 0 | 0.5 - 1 mm | 0.02 - 0.04 in | Coarse sand | |
| 2 - 1 | 0.25 - 0.5 mm | 0.01 - 0.02 in | Medium sand | |
| 3 - 2 | 0.125 - 0.25 mm | 0.005 - 0.01 in | Fine sand | |
| 4 - 3 | 0.063 - 0.125 mm | 0.002 - 0.005 in | Very fine sand | |
| 8 - 4 | 0.004 - 0.063 mm | 0.00015 - 0.002 in | Silt | |
| > 8 | 0.001 - 0.004 mm | 0.0004 - 0.00015 in | Clay | |
| < 0.001 mm | < 0.0004 in | Colloid |
In some schemes "gravel" is anything larger than sand (>2.0 mm), and includes granule, pebble, cobble, and boulder. In this scheme pebble covers the size range 4 to 64 mm (-2 to -6 φ).
