Rule of 78s

Also known as the "Sum of the Digits" method, the Rule of 78s is a term used in lending that refers to a method of yearly interest calculation. The name comes from the total number of months' interest that is being calculated in a year (the first month is 1 month's interest, whereas the second month contains 2 months' interest, etc.). This is an accurate interest model only based on the assumption that the borrower pays only the amount due each month. The outcome is that more of the interest is apportioned to the first part or early repayments than the later repayments. As such, the borrower pays a larger part of the total interest earlier in the term.

If the borrower pays off the loan early, this method maximizes the interest paid by applying funds to the interest before principal. The Rule of 78 is designed so that borrowers pay the same interest charges over the life of a loan as they would with a loan that uses the simple interest method. But because of some mathematical quirks, they end up paying a greater share of the interest upfront. That means if they pay off the loan early, they would end up paying more overall for a Rule of 78s loan compared with a simple-interest loan.^[1][2]