How to use Calculate the Interest Rate for a Loan in Excel. To determine the amount due for loan payment, given a loan duration/term, a Rate of Interest, and the initial loan amount, we can utilize the Excel PMT Operation Syntax.. In the example illustrated below, the operation syntax inserted into the formula bar of cell B6 is thus-=PMT (B3/12,B2,-B1) This calculates the monthly payment with interest for the loan. Figure 2. of Excel PMT Function. There is a formula in Excel which calculates simple interest by multiplying the principal, the rate, and the term. Calculate simple interest in Excel. Consider the example demonstrated below in which the formula in C5 is =C2*C3*C4. In order to do a simple interest calculation in Excel using the COUNTA function, follow the procedure below: Excel RATE Function. nper - The total number of payment periods. pmt - The payment made each period. pv - The present value, or total value of all loan payments now. fv - [optional] The future value, or desired cash balance after last payment. Default is 0. type - [optional] When payments are due. 0