The following formula is involved in the calculation of direct factory overhead to sales ratios:

Factory overhead to sales ratio:
where DFO is a company's direct factory overhead costs and S is the company's total sales.

The following references can be used to cite this factory overhead to sales calculator:
 Soper, D.S. (2017) "Factory Overhead to Sales Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Siegel, J. and Shim, J. (1997). "Schaum's Quick Guide to Business Formulas", New York, NY: McGrawHill. 