Enter the 3 known loan terms in the appropiate entry fields and click on "Compute." Note that when trying to find the interest rate, please select a "guess" from the pull-down menu by the same name. The closer your guess is to the actual interest rate, the faster the calculator will arrive at a result. If you don't select a "guess" the calculator automatically starts out at a 10% guess and works up or down until it finds a close approximation. If it turns out the actual rate is 19.95% it could take the calculator as many 56,000 iterations to find the answer. On the other hand if you had "guessed" 19% the calculator's iterations will be reduced significantly.