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Table T1. Comparison of millimeter, phi, and Wentworth classification of grain size.

Grain size
(mm)
Phi (ϕ) Wentworth grain size classification (ϕ)
(1 km) –20  
4096 –12  
1024 –10 Boulder (–8 to –12)
256 –8 Cobble (–5 to –8)
64 –6  
16 –4 Pebble (–2 to –5)
4 –2  
3.36 –1.75  
2.83 –1.5 Granule (–1 to –2)
2.38 –1.25  
2    
1.68 –0.75  
1.41 –0.5 Very coarse sand (0 to –1)
1.19 –0.25  
1 0  
0.84 0.25  
0.71 0. 5 Coarse sand (1 to 0)
0.59 0.75  
0.5 1  
0.42 1.25  
0.35 1.5 Medium sand (2 to 1)
0.3 1.75  
0.25 2  
0.21 2.25  
0.177 2.5 Fine sand (3 to 2)
0.149 2.75  
0.125 3  
0.105 3.25  
0.088 3.5 Very fine sand (4 to 3)
0.074 3.75  
0.0625 4  
0.053 4.25  
0.044 4.5 Coarse silt (5 to 4)
0.037 4.75  
0.031 5  
0.0156 6 Medium silt (6 to 5)
0.0078 7 Fine silt (7 to 6)
0.0039 8 Very fine silt (8 to 7)
0.002 9  
0.00098 10 Clay
0.00049 11 (Some use 2 µm or 9 as the clay boundary)
0.00024 12  
0.00012 13  

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