# Grain size

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 named after geologists Chester K. Wentworth and Johan A. Udden) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a logarithmic scale computed by the equation
    $\varphi =-\log _{2}{\frac {D}{D_{0}}},$
where
    $\varphi$ is the Krumbein phi scale,
    $D$ is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation) 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:
    $D=D_{0}\cdot 2^{-\varphi }\,$
| φ scale | Size range (metric) | 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 identifying and classifying soils based on those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 applies to natural soils in situ, similar man-made materials in situ and soils redeposited by people.
ISO 14688-1:2017 :
| Name | Name | Name | Name | Size range (mm) | Size range (approx. in) |
| Very coarse soil | Very coarse soil | Large boulder | lBo | >630 | >24.8031 |
| Very coarse soil | Very coarse soil | Boulder | Bo | 200–630 | 7.8740–24.803 |
| Very coarse soil | Very coarse soil | Cobble | Co | 63–200 | 2.4803–7.8740 |
| Coarse soil | Gravel | Coarse gravel | cGr | 20–63 | 0.78740–2.4803 |
| Coarse soil | Gravel | Medium gravel | mGr | 6.3–20 | 0.24803–0.78740 |
| Coarse soil | Gravel | Fine gravel | fGr | 2.0–6.3 | 0.078740–0.24803 |
| Coarse soil | Sand | Coarse sand | cSa | 0.63–2.0 | 0.024803–0.078740 |
| Coarse soil | Sand | Medium sand | mSa | 0.2–0.63 | 0.0078740–0.024803 |
| Coarse soil | Sand | 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 |
| Fine soil | Silt | Medium silt | mSi | 0.0063–0.02 | 0.00024803–0.00078740 |
| Fine soil | Silt | Fine silt | fSi | 0.002–0.0063 | 0.000078740–0.00024803 |
| Fine soil | Clay | Clay | Cl | ≤0.002 | ≤0.000078740 |

## Sorting

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:
    $\sigma _{I}={\frac {\phi 84-\phi 16}{4}}+{\frac {\phi 95-\phi 5}{6.6}}$
where
    $\sigma _{I}$ is the Inclusive Graphic Standard Deviation in phi units
    $\phi 84$ 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 |
| $\sigma _{I}$ < 0.35 | very well sorted |
| 0.35 < $\sigma _{I}$ < 0.50 | well sorted |
| 0.50 < $\sigma _{I}$ < 1.00 | moderately sorted |
| 1.00 < $\sigma _{I}$ < 2.00 | poorly sorted |
| 2.00 < $\sigma _{I}$ < 4.00 | very poorly sorted |
| 4.00 < $\sigma _{I}$ | extremely poorly sorted |