Grain Size Phi Calculator

Compressing a huge size range onto one scale

Natural sediment grains range from meter-sized boulders down to clay particles a fraction of a micron across - a range spanning many orders of magnitude. The phi scale converts this into a compact logarithmic scale, where each whole-number step represents a doubling or halving of grain diameter.

Worked example

For a 0.25mm grain diameter:

phi = -log2(0.25) = 2.0 phi (Medium to fine sand)

Phi Range	Wentworth Classification
Below -6	Cobble and larger
-2 to -6	Gravel/Pebble
-1 to 4	Sand
4 to 8	Silt
Above 8	Clay

Note the inverse relationship: larger grains have smaller (or more negative) phi values, while finer grains have larger phi values - this can feel counterintuitive at first since it is the opposite direction from millimeter measurements.