What is quadratic mean diameter?

The quadratic mean diameter is defined as the square root of the sum of the squares of all the diameters of a population divided by the total number of diameters. It is a measure of the average diameter of particles in a population or sample and is often used in particle size analysis. Unlike the arithmetic mean diameter, the quadratic mean diameter gives more weightage to the larger particles in the population, as their diameters contribute more to the sum of the squares. The quadratic mean diameter is also known as the root mean square diameter.