The present value of an annuity formula is used to calculate the present value of a series of periodic payments. The payments are for the same amount, made at the end of each period, and a discount rate i% is applied.
The debt constant sometimes referred to as the loan constant or mortgage loan constant is the ratio of the constant periodic payment on a loan to the original loan.
The debt service constant is only relevant to loans that have a fixed interest rate over the term of the loan, and is used to make quick calculations of the amount needed to repay a loan over its term, and the balance outstanding on the loan at any point in time.
An annuity is a series of annual payments made at the end of each year for a fixed number of years. Annuities with payments at the end of each year are sometimes referred to as regular annuities.
Annuity formulas are use to calculate annuity values. The formula to use will depend on which components of the annuity are already known.