A method of allocating the interest charge on a loan across its payment periods. Under the Rule of 78, periods are weighted by comparing their numerical values to the sum of all the digits of the periods. The weights are applied in reverse, applying large weights to early periods.




When paying off a loan, the repayments consist of two parts: the principal and the interest charge. The Rule of 78 weights earlier payments with more interest than later ones. If the loan is not terminated or prepaid early, the total interest paid between simple interest and the Rule of 78 will be equal. However, because the Rule of 78 weights the earlier payments with more interest than a simple interest method, paying off a loan early will result in the borrower paying more interest overall.

This method of allocating interest was commonplace in loans for consumer goods, such as automobiles. However, the U.S. government has outlawed the use of the Rule of 78 for loans longer than five years. This is largely because this method penalizes borrowers who pay off debts early.