The following formula is involved in the calculation of cash flow to longterm debt ratios:

Cash flow to longterm debt:
where CF is a company's cash flow over a specified time period and D_{LT} is the company's longterm debt.

The following references can be used to cite this cash flow to longterm debt calculator:
 Soper, D.S. (2017) "Cash Flow to LongTerm Debt Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Bragg, S. (2012). "Business Ratios and Formulas: A Comprehensive Guide", Hoboken, NJ: John Wiley & Sons. 