# Grain size

Not to be confused with crystallite size, which is referred to as "grain size" by metallurgists.

File:Wentworth-Grain-Size-Chart.pdf

Beach cobbles at Nash Point, South Wales.

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 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 1937, is a logarithmic scale computed by the equation

$\phi=-\log_2{D/D_0},$

where

$\phi$ is the Krumbein phi scale,
$D$ is the diameter of the particle,[clarification needed] 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 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 14688-1:2002, 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.[2]

ISO 14688-1:2002
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

## Sorting

An accumulation of sediment can also be characterized by the grain size distribution, called sorting. According to a formula[3] the sorting can be quantified as

φ < 0.35 0.35 < φ < 0.50 0.50 < φ < 0.71 0.71 < φ < 1.00 1.00 < φ < 2.00 2.00 < φ
very well sorted well sorted moderately well sorted moderately sorted poorly sorted very poorly sorted