Particle size plays a vital role in determining the behavior and quality of materials, particularly in the pharmaceutical industry, where it affects dissolution rate, flow properties, uniformity, bioavailability, and stability.

The microscopy method is a direct technique that involves observing individual particles under an optical microscope. This method is particularly useful for particles in the size range of 1–100 μm.

A stage micrometer is used to calibrate the microscope, converting eyepiece divisions into micrometer units. After calibration, the particle size of a sufficient number of particles (usually 50–100) is measured. The data are grouped into size ranges (class intervals), and a frequency distribution curve or histogram is plotted. The mean particle diameter (Dₘ) can be determined using the formula:

Dₘ = Σ(nᵢ × dᵢ) / Σnᵢ

Where:
nᵢ = Number of particles in each size range
dᵢ = Midpoint of each size range
The shape of the distribution curve (narrow, broad, or skewed) helps interpret the uniformity of the sample.