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Basic concepts
Particle size, Grain size, Size distribution, Morphology
Methods and techniques
Mesh scale, Optical granulometry, Sieve analysis, Soil gradation

Related concepts
Granulation, Granular material, Mineral dust, Pattern recognition, Dynamic light scattering
Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594
Beach cobbles at Nash Point, South Wales

Grain size (or particle size) is 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 refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

Krumbein phi scale

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein[1] in 1934, is a logarithmic scale computed by the equation


is the Krumbein phi scale,
is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)[2] and
is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

φ scale Size range
Size range
(approx. inches)
Aggregate name
(Wentworth class)
Other names
<−8 >256 mm >10.1 in Boulder
−6 to −8 64–256 mm 2.5–10.1 in Cobble
−5 to −6 32–64 mm 1.26–2.5 in Very coarse gravel Pebble
−4 to −5 16–32 mm 0.63–1.26 in Coarse gravel Pebble
−3 to −4 8–16 mm 0.31–0.63 in Medium gravel Pebble
−2 to −3 4–8 mm 0.157–0.31 in Fine gravel Pebble
−1 to −2 2–4 mm 0.079–0.157 in Very fine gravel Granule
0 to −1 1–2 mm 0.039–0.079 in Very coarse sand
1 to 0 0.5–1 mm 0.020–0.039 in Coarse sand
2 to 1 0.25–0.5 mm 0.010–0.020 in Medium sand
3 to 2 125–250 μm 0.0049–0.010 in Fine sand
4 to 3 62.5–125 μm 0.0025–0.0049 in Very fine sand
8 to 4 3.9–62.5 μm 0.00015–0.0025 in Silt Mud
10 to 8 0.98–3.9 μm 3.8×10−5–0.00015 in Clay Mud
20 to 10 0.95–977 nm 3.8×10−8–3.8×10−5 in Colloid Mud

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

International scale

ISO 14688-1:2017, 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 people.[3]

ISO 14688-1:2017
Name Size range (mm) Size range (approx. in)
Very coarse soil Large boulder lBo >630 >24.8031
Boulder Bo 200–630 7.8740–24.803
Cobble Co 63–200 2.4803–7.8740
Coarse soil Gravel Coarse gravel cGr 20–63 0.78740–2.4803
Medium gravel mGr 6.3–20 0.24803–0.78740
Fine gravel fGr 2.0–6.3 0.078740–0.24803
Sand Coarse sand cSa 0.63–2.0 0.024803–0.078740
Medium sand mSa 0.2–0.63 0.0078740–0.024803
Fine sand fSa 0.063–0.2 0.0024803–0.0078740
Fine soil Silt Coarse silt cSi 0.02–0.063 0.00078740–0.0024803
Medium silt mSi 0.0063–0.02 0.00024803–0.00078740
Fine silt fSi 0.002–0.0063 0.000078740–0.00024803
Clay Cl ≤0.002 ≤0.000078740


An accumulation of sediment can also be characterized by the grain size distribution. A sediment deposit can undergo sorting when a particle size range is removed by an agency such as a river or the wind. The sorting can be quantified using the Inclusive Graphic Standard Deviation:[4]


is the Inclusive Graphic Standard Deviation in phi units
is the 84th percentile of the grain size distribution in phi units, etc.

The result of this can be described using the following terms:

Diameter (phi units) Description
< 0.35 very well sorted
0.35 < < 0.50 well sorted
0.50 < < 1.00 moderately sorted
1.00 < < 2.00 poorly sorted
2.00 < < 4.00 very poorly sorted
4.00 < extremely poorly sorted

See also


  1. ^ Krumbein, W. C. (1934). "Size frequency distributions of sediments". Journal of Sedimentary Petrology. 2 (4). doi:10.1306/D4268EB9-2B26-11D7-8648000102C1865D.
  2. ^ PetroWiki: Estimating permeability based on grain size
  3. ^ "ISO 14688-1:2017 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description". International Organization for Standardization (ISO).
  4. ^ Folk, Robert L.; Ward, William C. (1957). "Brazos River bar: a study in the significance of grain-size parameters" (PDF). Journal of Sedimentary Petrology. 27 (1): 3–26. Bibcode:1957JSedR..27....3F. doi:10.1306/74d70646-2b21-11d7-8648000102c1865d. Archived from the original (PDF) on 12 May 2014. Retrieved 11 May 2014.