Affirm calculates the annual percentage rate (APR) of a loan using simple interest, which equals the rate multiplied by the loan amount and by the number of months the loan is outstanding. 

This model differs from compound interest, in which the interest expense is calculated on the loan amount and the accumulated interest on the loan from previous periods. Think about compound interest as “interest on interest,” which can increase the loan amount. Credit cards, for example, use compound interest to calculate the interest expense on outstanding credit card debt.