Add-on interest is a loan pricing method that calculates the entire finance charge on the original principal for the full term, adds that lump sum to the balance at signing, and splits the total into equal payments. Because the interest never adjusts as you pay the loan down, a stated 5% add-on rate on a five-year loan produces roughly the same cost as a 9% simple-interest loan. The method still shows up in some subprime auto loans, retail installment contracts, and short-term personal loans, and it consistently costs more than the advertised rate suggests.
How the Calculation Works
The math is deliberately simple. Multiply the principal by the annual rate, then by the number of years. That figure is the total interest for the life of the loan, fixed at origination.
Total Interest = Principal × Annual Rate × Term in Years.
On a $10,000 loan at a 5% add-on rate for five years, the interest is $10,000 × 0.05 × 5 = $2,500. Add that to the $10,000 principal and the borrower owes $12,500. Divided across 60 months, the payment is $208.33.
That payment stays flat, and so does the composition of every installment. A standard amortized loan front-loads interest and gradually shifts payments toward principal; an add-on loan splits interest and principal in the same ratio every month. By month 30 you have handed back roughly half the principal, but the interest charge is unchanged. You are paying interest on money the lender no longer has out.
Why It Costs More Than Simple Interest
A simple interest loan charges interest only on the remaining balance. Each payment reduces the principal, and next month’s interest is calculated on the smaller number. An add-on loan ignores that entirely and charges interest on the full original amount for every month of the term.
The gap between the two methods is large. On that $10,000 loan at 5% for five years, a simple interest calculation produces a $188.71 monthly payment and $1,322.74 in total interest. The add-on version charges $2,500 in interest and a $208.33 payment. Same stated rate, same term, roughly 89% more interest.
The APR Is the Number That Matters
The annual percentage rate is what exposes the difference. For a simple interest loan with no fees, the APR and the stated rate are essentially equal. For the 5% add-on example above, the effective APR is about 9.06%. A borrower comparing a “5% add-on” offer against a “7% simple interest” offer might assume the add-on is cheaper. It isn’t. The 7% simple interest loan costs less.
Paying Early Won’t Save What You’d Expect
With a simple interest loan, every extra dollar reduces the balance and tomorrow’s interest recalculates on the smaller number. Add-on interest doesn’t work that way. The finance charge was baked into the note at origination, so extra payments may shorten the term but they do not automatically shrink the interest you owe. Getting any of that back requires a formal interest rebate, and the rebate method controls how much you actually recover.
What Happens If You Pay Off Early
Federal law requires the lender to refund unearned interest when a precomputed loan is paid off ahead of schedule. What counts as “unearned” depends on which method the contract uses, and the two common methods produce very different refunds.
The Rule of 78s
The Rule of 78s, also called the sum-of-the-digits method, treats interest as earned faster at the start of the loan. On a 12-month loan, the first month carries a weight of 12, the second month 11, down to 1 in the final month. Those weights sum to 78. The lender counts 12/78 of the total interest as earned in the first month, 11/78 in the second, and so on.
Pay off that 12-month loan at the halfway mark and the lender has not earned half the interest. Under the Rule of 78s, they have earned 57/78, or about 73%. The rebate covers only the remaining 27%. An actuarial calculation, which allocates interest more evenly against the actual outstanding balance, would return substantially more.
The 61-Month Federal Limit
Federal law bars lenders from using the Rule of 78s on any precomputed consumer loan with a term longer than 61 months. On those longer loans, the rebate has to be at least as favorable to the borrower as an actuarial calculation, and the lender has to refund the unearned interest promptly whether the prepayment is voluntary, part of a refinancing, or the result of acceleration. A refund under $1 doesn’t have to be returned.1Office of the Law Revision Counsel. 15 U.S. Code 1615 – Prohibition on Use of Rule of 78s in Connection With Mortgage Refinancings and Other Consumer Loans
For loans of 61 months or less, the Rule of 78s remains legal under federal law, though some states restrict it further. If you’re thinking about paying off a short-term add-on loan early, ask the lender which rebate method your contract uses before assuming the savings will be meaningful.
How to Recognize It in a Contract
Loan documents rarely label the product “add-on interest.” The more common term is precomputed interest or precomputed finance charge. Either phrase means the interest was calculated on the full principal for the full term and folded into the balance at signing.2Consumer Financial Protection Bureau. What’s the Difference Between a Simple Interest Rate and Precomputed Interest on an Auto Loan?
A few practical signals:
- The stated rate looks low but the disclosed APR is much higher. A wide spread between the nominal rate and the APR almost always means precomputed interest.
- The contract says extra payments shorten the term but do not reduce the total finance charge.2Consumer Financial Protection Bureau. What’s the Difference Between a Simple Interest Rate and Precomputed Interest on an Auto Loan?
- The paperwork references a “refund of unearned finance charge” or the “sum of the digits.” Both point to a precomputed loan with a Rule of 78s rebate provision.
- The total of payments equals the loan amount plus a round interest figure, such as exactly $2,500 on a $10,000 loan. Amortized loans rarely produce such clean totals.
The CFPB has flagged precomputed auto loans specifically. On a simple interest auto loan, paying a few days early each month saves money. On a precomputed one, it doesn’t. That difference matters when you’re comparing dealership financing offers.
What Federal Disclosure Rules Require
Every closed-end consumer loan has to disclose four numbers in standardized terms so borrowers can compare products directly:3Office of the Law Revision Counsel. 15 U.S. Code 1638 – Transactions Other Than Under an Open End Credit Plan
- The annual percentage rate, which reflects the true annualized cost of the credit.
- The finance charge, the total dollar cost of the credit.
- The amount financed, the credit the borrower actually receives.
- The total of payments, the amount financed plus the finance charge.
For add-on loans, Regulation Z treats the precomputed interest as separate from the principal, so the disclosed amount financed reflects only the money the borrower actually got.4Consumer Financial Protection Bureau. Regulation Z 1026.18 – Content of Disclosures Those requirements are why add-on interest has retreated from most mainstream lending. Once a lender has to disclose that a “5%” loan actually carries a 9% APR, the marketing edge disappears. It still surfaces in subprime auto financing, retail installment contracts for furniture and appliances, and short-term personal loans, where borrowers may not scrutinize the APR line. When you review any loan offer, the APR is the number to read first.