Add-on interest is a loan pricing method in which the lender calculates the entire interest charge on the original principal for the full term, adds that lump sum to the amount you borrowed, and then divides the combined total into equal payments. Because the interest is fixed at origination and never recalculated against a shrinking balance, an add-on loan costs far more than its stated rate suggests. On a $10,000 loan at a 5% add-on rate for five years, you pay $2,500 in interest — nearly double the $1,323 you would owe on a standard simple interest loan at the same rate.
How the Calculation Works
The math is deliberately simple. Multiply the principal by the annual rate, then by the number of years in the term. That is your total interest for the life of the loan, locked in on day one.
Total Interest = Principal × Annual Rate × Term in Years.
On a $10,000 loan at 5% for five years: $10,000 × 0.05 × 5 = $2,500. Add that to the principal and the total obligation becomes $12,500. Divide by 60 months and every payment is $208.33.
That payment never changes, and neither does its makeup. On a standard amortized loan, early payments are mostly interest and later payments are mostly principal. On an add-on loan, every payment carries the same proportion of interest and principal. By month 30 you have repaid roughly half the principal, but the interest charge has not moved. You are paying interest on money you already gave back.
How It Compares to Simple Interest
A simple interest loan charges interest only on the remaining balance. Each payment reduces the principal, and next month’s interest is computed on the smaller number. Over time, a shrinking share of each payment goes to interest.
Add-on interest ignores that declining balance. It charges interest on the full original amount for every month of the term, as though you never made a payment.
The cost gap is dramatic. On the same $10,000 loan at 5% for five years, simple interest produces a monthly payment of $188.71 and total interest of $1,322.74. The add-on method charges $2,500 in interest and a monthly payment of $208.33. That is roughly 89% more interest for an identical stated rate and term.
The APR Exposes the Gap
The annual percentage rate is the number that shows what the loan actually costs. For a simple interest loan with no fees, the APR and the stated rate are essentially identical: a 5% simple interest loan carries an APR of about 5%.
For the 5% add-on loan above, the effective APR is approximately 9.06%. A borrower comparing a “5% add-on” loan against a “7% simple interest” loan might reach for the add-on product thinking it is cheaper. It is not. The 7% simple interest loan has a lower true cost.
Extra Payments Don’t Save Interest
With a simple interest loan, paying ahead reduces principal immediately, and tomorrow’s interest is calculated on the smaller balance. Add-on interest offers no such benefit. The total interest was baked into the balance at origination, so extra payments do not reduce what you owe in interest. You may shorten the loan, but the lender has already booked the $2,500. Recovering any of it requires a formal interest rebate.
Paying Off Early and the Rule of 78s
When you pay off a precomputed loan early, federal law requires the lender to refund any unearned interest. How “unearned” gets defined determines how much you get back.
The Rule of 78s, also called the sum-of-the-digits method, front-loads the interest allocation. Each month gets a weight equal to its reverse position in the schedule. On a 12-month loan, month one is weighted 12, month two 11, down to 1 for the final month. The weights add to 78. The lender treats 12/78 of the total interest as earned in month one, 11/78 in month two, and so on.
The effect is that the lender considers a disproportionate share of the interest earned early. Pay off a 12-month add-on loan after six months and the lender has earned 57/78, or about 73%, of the total interest. Your rebate covers only the remaining 27%. Under a straight actuarial calculation you would receive a larger refund because interest would be allocated more evenly against the outstanding balance.
Federal law prohibits the Rule of 78s on any precomputed consumer loan with a term longer than 61 months; on those longer loans the refund must use a method at least as favorable to the borrower as the actuarial method, and the refund must be paid promptly whether the payoff is voluntary, a refinancing, or an acceleration.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 shorter, the Rule of 78s remains legal at the federal level, though some states impose tighter restrictions. Before you prepay a short-term add-on loan, ask the lender which rebate method your contract uses.
How to Spot Add-On Interest in a Contract
Loan documents rarely say “add-on interest” in bold. The more common label 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 your 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 giveaways:
- The stated rate looks low but the disclosed APR is nearly double. A wide spread between the nominal rate and the APR almost always signals precomputed interest.
- The contract says extra payments shorten the term but do not change 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 contract references a “refund of unearned finance charge” or the “sum of the digits” method.
- The total of payments equals the principal plus a suspiciously round interest figure, such as exactly $2,500 on a $10,000 loan.
The CFPB has specifically flagged precomputed auto loans as an area where borrowers misjudge their costs. With a simple interest auto loan, paying a few days early each month saves money. With a precomputed loan, it does not.
What Lenders Must Disclose
The Truth in Lending Act exists to prevent the confusion add-on interest creates by requiring standardized cost disclosures on every closed-end consumer loan.3Office of the Law Revision Counsel. 15 U.S. Code 1601 – Congressional Findings and Declaration of Purpose Four figures must appear, using those exact terms so products can be compared side by side:4Office of the Law Revision Counsel. 15 U.S. Code 1638 – Transactions Other Than Under an Open End Credit Plan
- The annual percentage rate, which on an add-on loan will be materially higher than the stated nominal rate.
- The finance charge, meaning the total dollar cost of the credit.
- The amount financed, meaning the actual credit you receive. On an add-on loan this excludes the precomputed interest folded into the note.
- The total of payments, meaning the amount financed plus the finance charge.
Regulation Z, which implements TILA, specifies that for add-on loans the disclosed amount financed reflects only the money the borrower actually received, not the inflated note balance.5Consumer Financial Protection Bureau. Regulation Z 1026.18 – Content of Disclosures
These rules are why add-on interest has largely retreated from mainstream lending. Once a lender has to disclose that a “5% loan” carries a 9% APR, the marketing advantage disappears. The method still turns up in some subprime auto lending, retail installment contracts for furniture and appliances, and short-term personal loans. When you review any loan offer, the single most important number on the page is the APR, not the rate printed in the promotional materials.