Roofing material price per square calculator

Roofing Price Per Square

The Roof Price Per Square Material Tab

In Enter how much per roll for felt and how many squares per roll. The shaded areas are areas than can be changed. The fields on this page that are not shaded cannot be changed.

How are these prices calculator?
Plywood: $14 per sheet x 3.125 = $43.75 per sq. Felt: $12 per roll divided by 2 = $6 per sq etc.

Why change ? If you use OSB instead plywood, or if you used 25lb felt, 3 sq per roll etc.