Divide your student loan's interest rate by 12 to determine the interest rate per month. Use the interest rate on the loan, not the APR. The interest rate on the loan is used to calculate the accrued interest for the loan, while the APR takes into account fees and capitalization methods for the loan. For example, if you have a $10,000 loan at 8 percent interest payable over 25 years, or 300 months, then 0.08 / by 12 = 0.0067.
Add "1" to the interest rate per month, then raise the sum to the power of the number of months of payments. The number of months of payments is an exponent. In our example, 1 + 0.0067 = 1.0067, so 1.0067^300 = 7.4135.
Subtract "1" from the number calculated in Step 2. In our example, 7.4135 - 1 = 6.4135.
Divide the rate per month by the number calculated in Step 2. In our example, 0.0067 / 6.4135 = 0.001045.
Add the rate per month to the number calculated in Step 3. In the example, 0.0067 + 0.001045 = 0.007745.
Multiply the number calculated in Step 4 by the principal on the loan to determine the monthly payment. In the example, 0.007745 * $10,000 = $77.45.
The monthly loan payment is $77.45.
Repeat these steps for each student loan, then sum the total payments to come up with a monthly payment for all loans. In the example, if the student has two other loans with monthly payments of $80 a month and $90 a month, then the total monthly payment is $77.45 + $80 + $90 = $247.45.
The combined monthly payment totals $247.45 a month.
Multiply the combined monthly payment by the number of months payments are due to calculate the total cost to pay back loans. In the example, $247.45 per month times 300 months equals total repayment of $74,235.
$247.45 * 300 = $74,235