The Sauter diameter (SD, also denoted D[3,2] or d_{32}) for a given particle is defined as:
where d is the so-called surface diameter and d is the volume diameter, defined as:
The quantities A and V are the ordinary surface area and volume of the particle, respectively.
The equation may be simplified further as:
This is usually taken as the mean of several measurements, to obtain the Sauter mean diameter (SMD):
This provides intrinsic data that help determine the particle size for fluid problems.