The original principle doesn't matter as much as the amount of money left to be paid does. In order to calculate reducible interest, you need to know this number as well as the number of payments you have left and the rate of interest that you pay on each payment.

Find out the total number of payments you have left on your loan. This information is on the loan statement you receive each month, as well as on any online statements you have access to. Add "1" to this number. Write the resulting number down.

Convert your current interest rate to a decimal point. Divide the percentage of interest you are paying by 100 to arrive at a decimal. For example, if you're paying 9.99 percent interest, 9.99 divided by 100 = 0.0999.

Multiply the number of payments you have left by the number you arrived at in "Step 2." If you have 15 payments left and your number from "Step 2" was 0.0999, 15 x 0.0999 = 1.499. Multiply this number by 2. For example, 1.499 x 2 = 2.997.

Divide your number from "Step 3" by the number from "Step 1" to find your reducible interest rate. For example, if you arrived at 2.997 in "Step 3" and you added "1" to 15 remaining payments in "Step 1" (to arrive at 16), 2.997 divided by 16 = 0.187. Multiply this by 100 to turn it into a percentage. For example, 0.187 x 100 = 18.7 percent as a reducible interest rate.