How to Use a Financial Calculator. The main function of a financial calculator is to calculate payments, determine interest rates and to solve for the present or future value of a loan or annuity. There are many different financial calculators, but they all have certain functions in common. Five keys represent the variables used most frequently on a financial calculator. They are "N" - number of periods, "I" - periodic interest rate, "PV" - present value, "PMT" - payments and "FV" - future value. Using a financial calculator, you can solve for any of these five functions.

Determine which variable you would like to solve for. Keep in mind that the present value is usually the part of the loan or annuity that you are starting with. For example if you have a 30 year mortgage on a home, the present value is the amount of the loan. You will need to have four out of the five variables to start, keeping in mind that the PMT, PV or FV may be zero.

Know when you have to use "zero." PMT is zero when you either invest a lump sum or owe all of your money at the end of a term, PV is zero when you are receiving or making payments and FV is zero when a loan or annuity is paid off or finished paying out.