Grain size

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Krumbein phi scale

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale named after geologists Chester K. Wentworth and Johan A. Udden) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a logarithmic scale computed by the equation

φ = - \log_{2} \frac{D}{D_{0}},

where

φ

is the Krumbein phi scale,

D

is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation) and

D_{0}

is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

D = D_{0} \cdot 2^{- φ}

In some schemes, gravel is anything larger than sand (comprising granule, pebble, cobble, and boulder in the table above).

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Krumbein phi scale

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