Determine your principal. That’s the amount you’re borrowing before interest charges are added.Calculate your monthly interest rate by dividing the annual interest rate by 12. If your annual interest rate is 3%, your monthly rate would be 0.25%.Calculate the number of payments by multiplying the length of your mortgage in years by 12. A 25-year mortgage would consist of 300 payments.Once you have your figures, plug them into the following equation: PMT = (P)*(i/(1-((1+i)-n)))PMT = your mortgage paymentP = your principali = the monthly interest raten = number of months needed to pay back the loan