Grain size

Grain size, also called particle size, is the diameter of tiny pieces of sediment or the particles found in clastic rocks. It can also describe other materials made of small grains. This is different from crystallite size, which is the size of a single crystal inside a grain. A single grain may actually contain several crystals.

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

Granular materials can vary in size from very tiny colloidal particles all the way up to large boulders. These materials include clay, silt, sand, gravel, cobbles, and boulders. Understanding grain size helps scientists and engineers study how rocks form and how soil behaves in different environments.

Krumbein phi scale

The Krumbein phi scale is a way to measure the size of tiny particles like sand or gravel. It was created by a scientist named W. C. Krumbein in 1934. Instead of using regular numbers, this scale uses a special math formula that works like a logarithm.

This scale helps scientists give names to different sizes of particles. For example, they can tell if something is sand or a bigger piece like a pebble. The scale uses a reference size of 1 millimeter, which is a common way to measure very small things.

φ scale	Size range (metric)	Size range (approx. inches)	Aggregate name (Wentworth class)	Other names
	>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

International scale

ISO 14688-1:2017 sets the basic rules for identifying and classifying soils. These rules focus on the material and mass features most often used when studying soils for engineering. This standard applies to natural soils found in their original place, similar man-made materials, and soils that people have moved to new locations.

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

Sorting

When we look at a pile of sand or mud, the pieces can be different sizes. Scientists study how these sizes are spread out, which they call "sorting." Sometimes, nature sorts these pieces, like when a river carries away smaller grains, leaving bigger ones behind.

To measure how well sorted the grains are, scientists use a special formula called the Inclusive Graphic Standard Deviation. This formula looks at different parts of the grain size range to give a number that shows how mixed or separated the sizes are.

Diameter (phi units)	Description
σ I {\displaystyle \sigma _{I}}	very well sorted
0.35	well sorted
0.50	moderately sorted
1.00	poorly sorted
2.00	very poorly sorted
4.00	extremely poorly sorted

Krumbein phi scale

International scale

Sorting

Images