How to find EXACT number of layers for coil wound on drum. Inner and outer diameter and thickness are known?

Question

How to find EXACT number of layers for coil wound on drum. Inner and outer diameter and thickness are known?

Answer 1

It seems to me that if you take the 1/2 of the Outside Diameter and subtract  1/2 the inside diameter, and divide the answer by the thickness of the material wound around it, you will know the number of layers (wraps) in the coil.  

Example: a coil of aluminum valley flashing is found to be 6" outside diameter.  The inside diameter is 5".  Subtract 1/2 the ID (2-1/2") from 1/2 the OD (3") for a thickness of 1/2".  Divide the thickness (1/2") by the thickness of the material (.019") yields 26 winds (layers).

The outside circumference (C= pi x D) 6" x 3.1416 =approx 18" x 26 wraps' = 468"/12 = approx. 39' in length.