The measurement of the size distribution of a particles mixture is utilised in industrial and biomedical fields. In the current method, the laser (or monochromatic) light scattered by the sample at various scattering angles is measured; then, a simplified analysis uses the diffraction theory in order to calculate the distribution of the particle size. In particular, a set of n equations is generated: n is the number of measurements that differ by the scattering angle (it acts as control parameter), and the unknown quantities are the ratios between the number of particles that fall in a specified size range and the total number of particles (obviously n must be not less than the number of the predefined size ranges). That requires the use of a dedicated setup and accurate angular measurements. Our method requires only two measurements on the sample, carried out by a spectrophotometer equipped with an integrating sphere. The diffuse spectral transmittance (or reflectance) and the total spectral transmittance (or reflectance) are measured and their ratio is calculated, then it is generated an equations set similar to the previous, but integrated on the angular range of the diffuse spectral transmittance measurement: now we utilise as control parameter the wavelength (then, each equation has a different value of wavelength), while the unknown quantities are the same of the previous method. Due to the fact that both methods use the same equations set, they have the same applicability limits, but our method has the advantage that we can use a standard commercial spectrophotometer.