One simple deffinition of roundness is the maximum ferret minus the minimum ferret of the particle. For a circle this is equal to "0". Another measure is the projected area of the particle divided by the area of the smallest enclosing (circumscribing) circle. A perfect circle would result in a value of "1". The sum of all the inclosed radii for each corner on a particle divided by the number of corners (the average radius for the sum of radii for all corners) divided by the radius of the maximum enclosed circle can result in a number of shapes being equal to a circle though they don't look like a circle. They would be neccessarily rounded. There are a number of other ways to calculate a value for roundness. The six subjective classes generally suffice but some measurements are provided below.