Grain size

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Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195

Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which is the size of a single crystal inside the particles or grains. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.

Krumbein Phi Scale-United States[edit]

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden-Wentworth) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein[1] in 1937, is a logarithmic scale computed by the equation



\phi is the Krumbein phi scale,
D is the diameter of the particle, 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 \times 2^{-\phi}\,
φ scaleSize range
Size range
(approx. inches)
Aggregate name
(Wentworth Class)
Other names
−8 <256 mm <10.1 in <Boulder
−6 to −864–256 mm2.5–10.1 inCobble
−5 to −632–64 mm1.26–2.5 inVery coarse gravelPebble
−4 to −516–32 mm0.63–1.26 inCoarse gravelPebble
−3 to −48–16 mm0.31–0.63 inMedium gravelPebble
−2 to −34–8 mm0.157–0.31 inFine gravelPebble
−1 to −22–4 mm0.079–0.157 inVery fine gravelGranule
0 to −11–2 mm0.039–0.079 inVery coarse sand
1 to 0½–1 mm0.020–0.039 inCoarse sand
2 to 1¼–½ mm0.010–0.020 inMedium sand
3 to 2125–250 µm0.0049–0.010 inFine sand
4 to 362.5–125 µm0.0025–0.0049 inVery fine sand
8 to 43.90625–62.5 µm0.00015–0.0025 inSiltMud
> 8< 3.90625 µm< 0.00015 inClayMud
> 10< 1 µm< 0.000039 inColloidMud

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

International scale[edit]

ISO 14688-1, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 is applicable to natural soils in situ, similar man-made materials in situ and soils redeposited by man.[2]

ISO 14688-1[3]
namesize range
Very coarse soilLarge boulder, LBo>630 mm
Boulder, Bo200 – 630 mm
Cobble, Co63 – 200 mm
Coarse soilGravelCoarse gravel, CGr20 – 63 mm
Medium gravel, MGr6.3 – 20 mm
Fine gravel, FGr2.0 - 6.3 mm
SandCoarse sand, CSa0.63 - 2.0 mm
Medium sand, MSa0.2 - 0.63 mm
Fine sand, FSa0.063 - 0.2 mm
Fine soilSiltCoarse silt, CSi0.02 - 0.063 mm
Medium silt, MSi0.0063 - 0.02 mm
Fine silt, FSi0.002 - 0.0063 mm
Clay, Cl≤0.002 mm


An accumulation of sediment can also be characterized by the grain size distribution, called sorting. According to a formula by [4] the sorting can be quantified as

φ < 0.350.35 < φ < 0.500.50 < φ < 0.710.71 < φ < 1.001.00 < φ < 2.002.00 < φ
very well sortedwell sortedmoderately well sortedmoderately sortedpoorly sortedvery poorly sorted

See also[edit]


  1. ^
  2. ^
  3. ^ ISO 14688-1:2002 at
  4. ^ Folk, R.L., & Ward, W. (1957), Brazos River bar: a study in the significance of grain-size parameters. Journal of Sedimentary Petrology 27, 3-26,

External links[edit]