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Particle's shape and size is one of the most essential measurements for the particle technologist, however, unlike in the macro world where it is easy to understand, in the micro world it is less simple to define, and even harder to measure. Particles are seldom simple geometric shapes, and describing an irregular shape and converting or limiting this in to a size is a challenging task. So, particle size analysis becomes a crucial method. There is no single definition of particle size as it is dependent on the measurement method used, for example microscopy makes use of area projection, electroresistance makes use of conductivity, sedimentation makes use of hydrodynamic size and laser diffraction makes use of the optical properties of the material being measured. So, particle size varies according to the mode of measurement.

Keep in mind that a particle's shape or sphericity is independent of its size. Sphericity is the ratio of the surface area of a sphere with the same volume as the given particle to the surface area of the particle. Depending on the application, the size as well as its shape may be important. For example, spherical particles are needed for toner powders whereas polishing powders need sharp edges to produce an abrasive action. Of course, particles are far from identical whether in solid or liquid form, and therefore the size distribution and average size become the important measurements. Manufactured materials are usually polydisperse or polysized, that is, composed of particles of varied size and mass. As it is not usually feasible in a commercial surrounding to measure every individual particle in a sample it is important to design the correct sampling devices and methods. Adhering to statistical sampling theory all particles must be given the same probability of being selected.

There is the internationally agreed standard. It is the international standard ISO 9276 - Representation of results of particle size analysis - which is broken down in to six features, are, manipulation of an experimental curve to a reference model, characterization of a classification method, methods of calculation relating to particle size analyses using logarithmic normal probability distribution, descriptive and quantitative representation of particle shape and morphology, calculation of average particle sizes/diameters and moments from particle size distributions and graphical representation. One time the theory of defining size and distribution is agreed, there is then the practical method of measuring the sample to be thought about. In practice, obtaining reliable and correct size and distribution measurement is complex.