The following formula is involved in the calculation of annual percentage rates for installment loans:

Annual percentage rate (Nratio method):
where M is the number of payments per year, N is the total number of loan payments, C is the total loan finance charges, and P is the total amount borrowed.

The following references can be used to cite this annual percentage rate (apr) calculator for an installment loan:
 Soper, D.S. (2017) "Annual Percentage Rate (APR) Calculator for an Installment Loan (Online Software)", http://www.danielsoper.com/fincalc. 
 Siegel, J. and Shim, J. (1997). "Schaum's Quick Guide to Business Formulas", New York, NY: McGrawHill. 