The following formula is involved in the calculation of gross income multiplier values:

Gross income multiplier (gross rent multiplier):
where AP is the asking price (or alternatively, the market price) of the investment property and GRI is the gross rental income produced by the property.

The following references can be used to cite this gross income multiplier (gim) calculator:
 Soper, D.S. (2017) "Gross Income Multiplier (GIM) Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Schram, J.F. (2006). "Real Estate Appraisal", Bellevue, WA: Rockwell Publishing. 