grainsize

Grainsize, or particle size, is a measure of the linear dimension of the individual grains that compose a bulk material. It is a key descriptor in geology, soil science, materials engineering, and powder technology because size influences properties such as porosity, permeability, strength, density, and rheology. Grainsize is typically not described by a single number but by a grain-size distribution, often summarized with D10, D50, and D90 values (the sizes that bisect 10%, 50%, and 90% of the sample by mass), and by measures of spread such as sorting and skewness.

In geology and sedimentology, grain size helps infer the energy of transport and depositional environment. Standard coarse classifications include gravel (>2 mm), sand (0.0625–2 mm), silt (0.004–0.0625 mm), and clay (<0.004 mm). In civil engineering and soil classification, soils are described by grain-size distribution, typically using sieve analyses for coarser fractions and hydrometer or laser diffraction methods for finer fractions, with results often presented on a phi-scale (phi = -log2(d in mm)).

Methods for determining grainsize vary with the material. Sieve analysis separates coarse particles; for finer material, sedimentation-based methods like hydrometry or laser diffraction are common, sometimes complemented by image analysis or microscopy to assess particle shape and texture.

The grain-size distribution influences physical behavior: well-sorted sands flow more easily and drain well, while poorly sorted materials exhibit variable strength and permeability. In metals and ceramics, finer grain sizes generally increase yield strength but can reduce toughness, a relationship described by grain-size strengthening mechanisms.