The workshop below uses the Journeyman Rigger's Reference Card panel 6, to help provide the solution.
The area of a circle can be calculated with either (Pi x radius squared) or (dia. x dia. x .80), and both arrive at very similar values for Area of a Circle. For riggers in the field, we use the dxdx.80 method since it is faster and simpler.
If circumference is desired, a person can use Pi x diameter, officially 3.1416 x d, or for quick figuring in the field, 3.2 x diameter.