The particle size of a spherical object can be unambiguously and quantitatively defined by its diameter. However, a typical material object is likely to be irregular in shape and non-spherical. The above quantitative definition of particle size cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition to apply to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary sphere that has one of the properties identical with the particle.
;Volume-based particle size: Volume-based particle size equals the diameter of the sphere that has the same volume as a given particle. Typically used in sieve analysis, as shape hypothesis (sieve's mesh size as the sphere diameter).
:where
::: diameter of representative sphere
::: volume of particle
;Area-based particle size: Area-based particle size equals the diameter of the sphere that has the same surface area as a given particle.  Typically used in optical granulometry techniques.
:where
::: diameter of representative sphere
::: surface area of particle