phi grade scale

The phi grade scale is a logarithmic scale used in sedimentology to classify the size of particles in sediments and sedimentary rocks. It is a modification of the Udden-Wentworth scale, which uses millimeter measurements. The phi scale converts grain size diameters in millimeters to phi units using the formula: (\phi = -\log_2 S), where (\phi) is the size in phi units and (S) is the grain size in millimeters. Grains larger than 1 mm have negative phi values, while grains smaller than 1 mm have positive values.12

This scale helps standardize the description of particle sizes, which provides information about the transportation, deposition, and history of sediments. The phi scale is particularly useful because it gives equal significance to size ratios across different particle types, from gravel to clay.1