Grain size converter
Convert between millimeters and the phi scale that sedimentologists use, and see what the material is called.
That's 1.43× as coarse as medium sand, while gravel is 8× coarser. That is sand: individual grains you can see.
What it means
What is that material?
Class boundaries follow the Wentworth scale. Mesh numbers are approximate: sieve standards differ between countries.
Every unit
Your grain size in every unit
Learn
Three things worth knowing
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Each phi step halves the grain
0 phi is 1 mm, 1 phi is 0.5 mm, 2 phi is 0.25 mm. Sand runs from −1 to 4 phi.
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Wentworth and Krumbein
Wentworth set the size classes in 1922; Krumbein turned them into the phi scale in 1934, so statistics could be done on sediment samples.
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Phi runs backwards
A bigger phi number means a smaller grain. Boulders have negative phi, which surprises everyone at first.
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Popular grain size conversions
Background
About the phi scale
Grain sizes span from thousandths of a millimeter to meters, so geologists use a logarithmic scale: phi = −log₂(diameter in mm). Each whole step halves the grain, and the scale runs backwards, with negative phi for gravel.