Particle size, also called grain size, refers to the diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of individual grains of sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

, or the lithified particles in clastic rocks. The term may also be applied to other granular material

Granular material

A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...

s. This is different from the crystallite

Crystallite

Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...

 size, which is the size of a single crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 inside the particles or grains. A single grain can be composed of several crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s. Granular material

Granular material

A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...

 can range from very small colloidal particles

Colloid

A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...

, through clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

, silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

, sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, and gravel

Gravel

Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

, to boulder

Boulder

In geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....

s.

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden-Wentworth) used in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein, (Krumbein & Sloss 1963) is a logarithmic scale

Logarithmic scale

A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...

 computed by the equation
where is the Krumbein phi scale, is the diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of the particle, and is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent

Dimensional analysis

In physics and all science, dimensional analysis is a tool to find or check relations among physical quantities by using their dimensions. The dimension of a physical quantity is the combination of the basic physical dimensions which describe it; for example, speed has the dimension length per...

).
This equation can be rearranged to find diameter using φ:

φ scale	Size range (metric)	Size range (approx. inches)	Aggregate name (Wentworth Class)	Other names
−8 <	256 mm <	10.1 in <	Boulder Boulder In geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....
−6 to −8	64–256 mm	2.5–10.1 in	Cobble Cobble (geology) A cobble is a clast of rock with a particle size of to based on the Krumbein phi scale of sedimentology. Cobbles are generally considered to be larger than pebbles and smaller than boulders . A rock made predominantly of cobbles is termed a conglomerate....
−5 to −6	32–64 mm	1.26–2.5 in	Very coarse gravel Gravel Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...	Pebble Pebble A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
−4 to −5	16–32 mm	0.63–1.26 in	Coarse gravel Gravel Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...	Pebble Pebble A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
−3 to −4	8–16 mm	0.31–0.63 in	Medium gravel Gravel Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...	Pebble Pebble A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
−2 to −3	4–8 mm	0.157–0.31 in	Fine gravel Gravel Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...	Pebble Pebble A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
−1 to −2	2–4 mm	0.079–0.157 in	Very fine gravel Gravel Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...	Granule Granule (geology) A granule is a clast of rock with a particle size of 2 to 4 millimetres based on the Krumbein phi scale of sedimentology. Granules are generally considered to be larger than sand and smaller than pebbles . A rock made predominantly of granules is termed a conglomerate....
0 to −1	1–2 mm	0.039–0.079 in	Very coarse sand Sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
1 to 0	½–1 mm	0.020–0.039 in	Coarse sand Sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
2 to 1	¼–½ mm	0.010–0.020 in	Medium sand Sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
3 to 2	125–250 µm Micrometre A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...	0.0049–0.010 in	Fine sand Sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
4 to 3	62.5–125 µm	0.0025–0.0049 in	Very fine sand Sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
8 to 4	3.90625–62.5 µm	0.00015–0.0025 in	Silt Silt Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...	Mud Mud Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
< 8	< 3.90625 µm	< 0.00015 in	Clay Clay Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...	Mud Mud Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
< 10	< 1 µm	< 0.000039 in	Colloid Colloid A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...	Mud Mud Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

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

See also

• Orders of magnitude (volume)
  Orders of magnitude (volume)
  The pages linked in the right-hand column contain lists of volumes that are of the same order of magnitude . Rows in the table represent increasing powers of a thousand. -Notes:...
  
  
• Soil texture
  Soil texture
  Soil texture is a qualitative classification tool used in both the field and laboratory to determine classes for agricultural soils based on their physical texture. The classes are distinguished in the field by the 'textural feel' which can be further clarified by separating the relative...
  
  
• Substrate (biology)
  Substrate (biology)
  In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
  
  
• Unified Soil Classification System
  Unified Soil Classification System
  The Unified Soil Classification System is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol...
  
  (USCS)