Estimating the volume of intracranial hematoma on CT

The commonly used formula for estimating the volume of intracranial bleed on CT is


ABC/2 
where

A is the maximal diameter of the hematoma by CT
B is the diameter 90° to A, and
C is the approximate number of CT slices with hemorrhage multiplied by the slice thickness

For a number of years, I could not really grasp why the formula is as such. I thought that the formula is derived from the