The following formula is involved in the calculation of capitalization rates (income yields) for real estate investments:

Capitalization rate (income yield):
where NOI is the net operating income for an investment property and P is the purchase price for the property.

The following references can be used to cite this capitalization rate calculator:
 Soper, D.S. (2014) "Capitalization Rate Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Schram, J.F. (2006). "Real Estate Appraisal", Bellevue, WA: Rockwell Publishing. 