Phi scale
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Grain size is often measured in millimeters, which canbe converted to the phi scale, where the grain size in phi = –log2 × grain size in mm. Mean grain size and sorting (standard deviation of grain size measurements) are then calculated. Mean grain size can be expressed in either the millimeter or phi scale, but sorting must be expressed in the phi scale to maintain a sorting index that is useful across a wide range of grain sizes.