What Is the Rule of 78 and How Does It Affect Loans?

The Rule of 78 is a method some lenders use to divide up the interest on a fixed-term installment loan, weighting the early months much more heavily than the later ones. It only matters if you pay the loan off ahead of schedule: because so much of the interest is treated as “earned” in the first few months, the refund of unearned interest you get at payoff is smaller than it would be under a standard simple-interest calculation. Federal law bans the method on consumer loans longer than 61 months, and about half the states restrict it further, but shorter precomputed loans can still use it legally.

Where the Name Comes From

Add the numbers 1 through 12 and you get 78. That sum is the denominator the method uses for a 12-month loan. Each month gets a weight equal to the number of months remaining at its start: month one is weighted 12, month two 11, and so on down to 1 in the final month. The first month’s share of the total interest is 12/78, the second month’s is 11/78, and the last month’s is 1/78.

Longer loans use bigger denominators built the same way. A 24-month loan sums to 300. A 36-month loan sums to 666. The general formula is n × (n + 1) ÷ 2, where n is the number of months in the term. Whatever the length, the logic is identical: heavy weights up front, light weights at the end.

Why It Costs You Money to Pay Off Early

If you make every payment on schedule and never pay early, the Rule of 78 changes nothing about your total cost. The finance charge was fixed when you signed. The allocation across months only matters when you try to exit the loan early, because at that point the lender owes you a refund of the unearned portion of the finance charge, and the Rule of 78 defines “unearned” using those same digit weights.

The numbers are lopsided. After six months of a 12-month loan, you’ve already paid 57/78 of the total interest, which is roughly 73 percent. You’re halfway through the term, but nearly three-quarters of the interest is gone. Pay the loan off at that point and you save only about 27 percent of the total finance charge, not the half you might expect.

A concrete example makes the gap clear. Borrow $5,000 for 36 months at 17.5 percent interest and the total finance charge is about $1,462. Pay off after six months and a simple-interest method would refund roughly $1,049 of unearned interest. Under the Rule of 78, the refund is about $1,021. That’s $28 more in the lender’s pocket for the same early payoff. The gap grows through the middle of the loan; paying off at 18 months produces a refund about $32 smaller under the Rule of 78 than under simple interest. Near the very end of the term, the two methods converge again because little interest is left either way.

How Simple Interest and the Actuarial Method Compare

A simple-interest loan charges interest on whatever principal you still owe, recalculated each period. Pay extra, and the balance that generates interest drops right away. There’s no refund calculation at payoff because interest was never charged in advance. Most mortgages, most bank and credit union auto loans, and most personal loans today work this way.

A precomputed loan is different. The lender calculates all the interest you’ll owe over the full term and adds it to the principal at origination. Your monthly payment covers a slice of that combined total. The Rule of 78 is one way to decide, at payoff, how much of the precomputed interest counts as already earned.

The actuarial method is the alternative that federal law requires on longer loans. It figures the refund based on the interest that would have actually accrued on the declining principal balance, treating each payment as covering built-up interest first and principal second. The refund under the actuarial method is always at least as large as under the Rule of 78, and usually larger.

How to Tell If Your Loan Uses the Rule of 78

Federal law requires lenders to state upfront whether you’ll get a rebate of finance charges if you prepay. Under Regulation Z, every closed-end loan disclosure has to say one of three things: that a rebate applies, that no rebate applies, or that a prepayment penalty applies. Silence isn’t allowed. Where a rebate applies, the disclosure will name the method used to calculate it.¹

Check two documents. First, the federal Truth in Lending disclosure you received at closing, in the prepayment section. Second, the loan agreement itself, which usually has a clause describing how an early payoff is figured. Phrases like “sum of the digits,” “Rule of 78s,” or “precomputed finance charge” point to this method. Language about daily interest accrual or the “unpaid principal balance” points to a simple-interest loan, in which case the Rule of 78 doesn’t apply to you.

Precomputed loans are less common than they once were, but they still show up in some subprime auto financing and in smaller installment loans from consumer finance companies.

What Federal Law Restricts

The main federal protection is 15 U.S.C. § 1615. It does two things. First, any lender on a consumer credit transaction has to promptly refund unearned interest whenever a borrower prepays in full, whether the payoff comes from a refinance, a sale, a debt consolidation, or the lender’s own acceleration after default. Refunds under $1 are the only carve-out.²

Second, on any precomputed consumer loan with a term longer than 61 months, originated after September 30, 1993, the refund has to be calculated using a method at least as favorable to the borrower as the actuarial method. That effectively bans the Rule of 78 on those longer loans. Loans of 61 months or shorter aren’t covered by that ban, which is why some auto loans and shorter installment products can still use it under federal law.²

State law fills in a lot of the gap. Roughly half the states have enacted their own restrictions. Some ban the Rule of 78 outright on consumer loans, some prohibit it above a certain dollar amount, and some require the actuarial method on any loan above a shorter term threshold. The rules vary enough that the only reliable check is your state’s consumer lending statutes or your state financial regulator.

Getting a Payoff Figure Before You Decide

If you have a precomputed loan and you’re thinking about paying it off, federal law lets you get the actual number in writing. Within five business days of your request, the lender has to provide a statement showing the total needed to prepay in full. If the amount includes a refund of unearned interest, the statement has to break out the refund separately. The request can be oral or written, though a written request entitles you to a written response.²

You get one free payoff statement each year. After that the lender can charge a reasonable fee for more, but only if the fee was disclosed beforehand. Ask for that statement before you commit to a refinance or a payoff. Compare it against what the balance would be under simple-interest amortization, and you’ll see in dollars what the Rule of 78 is costing you.²

What You Can Do If a Lender Breaks the Rules

A lender that uses the Rule of 78 where it’s prohibited, or that fails to refund unearned interest after a prepayment, is exposed to civil liability under the Truth in Lending Act. On a closed-end consumer loan, you can sue for actual damages plus twice the finance charge on the transaction. A successful plaintiff also recovers attorney’s fees and court costs, which makes smaller cases economically viable.³

Class actions are also available, with damages capped at the lesser of $1,000,000 or 1 percent of the creditor’s net worth. State law may add remedies on top, including its own statutory damages and administrative penalties from state banking regulators.³

1
Consumer Financial Protection Bureau. Regulation Z 1026.18 – Content of Disclosures
2
Office of the Law Revision Counsel. 15 USC 1615 – Prohibition on Use of Rule of 78s in Connection With Mortgage Refinancings and Other Consumer Loans
3
Office of the Law Revision Counsel. 15 USC 1640 – Civil Liability