Effective Period Rate = Nominal Annual Rate / n. Effective annual interest rate calculation. The effective interest rate is equal to 1 plus the nominal interest rate in percent divided by the number of compounding persiods per year n, to the power of n, minus 1. Effective Rate = (1 + Nominal Rate / n) n - 1 . Effective interest rate calculation To convert an annual interest rate to monthly, use the formula "i" divided by "n," or interest divided by payment periods. For example, to determine the monthly rate on a $1,200 loan with one year You can use the effective annual rate (EAR) calculator to compare the annual effective interest among loans with different nominal interest rates and/or different compounding intervals such as monthly, quarterly or daily. Effective annual rate (EAR), is also called the effective annual interest rate or the annual equivalent rate (AER).