Overview

Particle size denotes a representative linear dimension of an individual solid particle. For irregular shapes it is usually reported as an equivalent spherical diameter that reproduces a chosen property (volume, surface area or settling behaviour). The range of interest extends from nanometres for engineered nanoparticles and ultrafine aerosols to millimetres or larger for sands and gravels. General definitions and formal terminology are available in technical references (general definitions).

Definitions and common metrics

Several metrics are used: geometric diameter, volume-equivalent diameter, surface-equivalent diameter and aerodynamic diameter. Particle size distribution (PSD) characterises the fraction of material across sizes and is commonly summarised by percentiles such as D10, D50 (median) and D90. Mean values may be number-, area-, volume- or mass-weighted; each emphasises different aspects of a sample.

Measurement methods

Measurement choice depends on size range, concentration and material state. Typical methods include:

  • Sieving for coarse granular materials and powders.
  • Sedimentation and hydrometer techniques that infer size from settling behaviour.
  • Laser diffraction and optical particle counters that estimate distributions by scattering or counting.
  • Dynamic light scattering (DLS), electron microscopy and atomic-force microscopy for fine nanoparticles, offering mean sizes and images.
  • Electrical sensing (Coulter principle) and aerodynamic sizing for aerosols and polydisperse flows.

Particle size distributions and statistics

Distributions may be unimodal or multimodal and are often approximately log-normal. Reporting should state whether the distribution is presented by number, surface, volume or mass and include sample preparation details. Percentiles, span and standard deviation are typical descriptors used to characterise breadth and central tendency.

Standards, reporting and preparation

Standards from national and international organisations guide test procedures, instrument calibration and reporting conventions. Important reporting elements include measurement method, dispersion medium (wet or dry), pre-treatment to break agglomerates, and the weighting basis of the distribution; these affect comparability between datasets.

Applications and practical importance

Particle size controls bulk properties such as flowability, packing density, surface area, reactivity and optical behaviour. In geology and sedimentology it informs transport and depositional processes; in industry it affects milling, blending, filtration, catalysis and drug delivery. For airborne particles, aerodynamic diameter determines deposition in the respiratory tract and is central to exposure assessment and regulatory limits.

Notes and distinctions

Size is only one attribute: shape, porosity, surface chemistry and density also influence performance. Agglomeration state can change apparent size; therefore measurement and interpretation must consider the intended application and conditions under which the particles will be used or encountered.