The following formula is involved in the calculation of sales breakeven points:

Breakeven point (sales):
where VC is a company's variable costs and FC is the company's fixed costs.

The following references can be used to cite this breakeven point calculator:
 Soper, D.S. (2015) "BreakEven Point 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. 