The relationship between average particle size ($D_p$) and fineness modulus (FM) is often represented by an empirical equation. This equation helps estimate the particle size based on the fineness modulus value.
The specific equation provided relates $D_p$ (in mm) to FM as follows:
$D_p = 0.135 (1.366)^{FM}$
This formula is used in material science and civil engineering to characterize granular materials.