The Rule of 78s: Why Paying Off a Business Loan Early Can Cost You Full Interest

You take a two-year business loan, and eight months in a good quarter lets you pay it off. You call to get the payoff amount, expecting to owe roughly the remaining principal plus a little interest. Instead the number barely moves. You’ve been paying for months, but almost none of your balance is gone. The likely culprit is a financing method most owners have never heard of: the Rule of 78s, applied to a precomputed loan.

This isn’t fraud, and it isn’t hidden in most contracts — it’s a specific, legal interest-allocation formula. But it changes the math of early payoff so completely that the whole reason you rushed to pay early can evaporate. Here is what it is, how to spot it before you sign, and how to protect yourself.

Simple interest vs. precomputed interest

Most modern business loans use simple interest. Interest accrues on your outstanding balance, day by day. Pay the balance down and the interest clock slows with it. Pay the whole thing off in month eight and you owe the principal plus interest accrued up to that day — nothing more. Early payoff genuinely saves you money.

A precomputed loan works differently. The lender calculates the entire finance charge for the full term at signing, adds it to the principal, and divides the total into equal payments. The interest for all 24 months is baked in from day one. If you pay off early, the lender is supposed to refund the portion of that pre-baked interest you haven’t “earned” the right to be charged — the unearned interest. The Rule of 78s is one formula for deciding how much that refund is. And it’s deliberately stingy in the borrower’s early months.

Where the name comes from

On a 12-month loan, add the digits of the months: 1 + 2 + 3 … + 12 = 78. That sum is the denominator. The formula assigns interest to each month in reverse order. Month one is charged 12/78 of the total interest, month two 11/78, and so on down to 1/78 in the final month.

Translated into plain terms: the first month of a one-year loan carries more than 15% of the entire finance charge, and the first half of the loan absorbs roughly two-thirds of it. On longer terms the front-loading is even more severe, because the denominator grows and the early fractions stay large. A 24-month loan uses a denominator of 300 (the sum of 1 through 24), and the earliest months carry an outsized slice.

So when you pay off that 24-month loan at month eight, the Rule of 78s says you’ve already “used up” a large share of the total interest — far more than a simple-interest loan would say you owe for the same eight months. The refund of unearned interest is small, your payoff is high, and your early payment bought you very little.

A concrete comparison

Imagine two loans, identical on the surface: same amount, same stated rate, same 24 equal monthly payments. You pay both off at the exact midpoint, month 12.

• Simple-interest loan: interest stopped accruing as you paid down principal. You owe the remaining principal plus interest through payoff day. Roughly half the total interest, or a bit less, has been charged.
• Rule of 78s loan: by month 12 you’ve been allocated interest for months 24 through 13 in reverse — the largest fractions. Depending on the term, you may have already been charged around two-thirds of the total finance charge, even though you borrowed the money for only half the time.

The gap between those two payoff figures is money that stays in the lender’s pocket purely because of which formula was written into your note. Nothing about your behavior changed — you paid early in both cases.

What federal law does and doesn’t protect

For consumer loans, Congress limited this decades ago. Under the Truth in Lending Act, a creditor generally may not use the Rule of 78s to compute a rebate of unearned interest on a consumer credit transaction with a term longer than 61 months — refunds on those longer consumer loans must use the actuarial (simple-interest) method (15 U.S.C. § 1615). Many states go further and restrict or ban precomputed interest on shorter consumer loans too.

Here is the trap: those protections are built around consumer credit. A loan taken out for business or commercial purposes generally falls outside the Truth in Lending Act’s core protections, which apply to credit extended primarily for personal, family, or household purposes (see the scope of Regulation Z, 12 C.F.R. Part 1026). So the very method restricted on a long consumer loan can still appear, legally, on the business loan sitting on your desk. Whether it’s allowed on your specific deal depends heavily on your state’s commercial-lending rules — and those vary widely.

How to spot it before you sign

The Rule of 78s rarely announces itself in bold type. Read for these signals in the note and the payoff/rebate section:

• The words “precomputed,” “add-on interest,” or “Rule of 78s” anywhere in the agreement — often in a “prepayment” or “rebate of unearned charges” clause.
• A single fixed “finance charge” or “total of payments” figure stated up front, rather than a rate that accrues on the balance. If the total interest is a fixed dollar amount baked into equal payments, ask directly whether it’s precomputed.
• A rebate or refund method named for early payoff. If the contract promises a rebate of unearned interest “computed under the Rule of 78s” or “the sum-of-the-digits method,” that is precisely this formula.
• Vague or missing prepayment language. If you can’t find a clear statement that early payoff stops interest on a daily basis, assume it might not.

The single most useful question you can ask a lender or broker is blunt: “If I pay this off in month eight of a 24-month term, what is my exact payoff, and how is the interest rebate calculated?” Get the answer in writing. A simple-interest lender will give you a number that clearly reflects only the months you held the money. A precomputed lender using the Rule of 78s will give you a much higher figure — and now you know why.

How it interacts with other early-payoff traps

The Rule of 78s often travels with other clauses that punish early payoff, and it’s worth understanding how they stack. A prepayment penalty is a separate charge on top of the payoff; the Rule of 78s instead shrinks your refund. You can face both. Some financing products don’t use interest at all — a merchant cash advance quotes a fixed factor rate where early repayment saves nothing by design. And origination fees are typically not refunded regardless of method. Reading these clauses together tells you the real cost of getting out early.

The plain-English test: on a healthy business loan, paying early should always save you meaningful money. If the payoff quote says otherwise, the interest method — not your math — is the reason.

What to do if you already have one

If you’re already in a precomputed loan, you still have moves. First, request a written payoff quote and a plain explanation of the rebate method — you’re entitled to understand what you owe. Second, compare the early-payoff cost against simply continuing to pay on schedule; with the Rule of 78s, once you’re past the midpoint, the front-loaded interest is largely already charged, so rushing to pay off may save almost nothing. Third, if you’re refinancing to escape it, run the payoff figure — not the remaining balance — into your new-loan math, or you’ll under-borrow.

The takeaway

The Rule of 78s isn’t a scam; it’s a formula. But it quietly rewrites the incentive that makes early payoff worthwhile, and because commercial loans sit outside most consumer protections, it can appear on a business note that looks perfectly ordinary. Before you sign anything with a fixed, baked-in finance charge, confirm in writing that interest accrues on your balance and that early payoff stops it. That one question protects the savings you’re working so hard to earn.

Important: MidBank is not a bank, a financial institution, or a financial advisor. We are an advocate and ISO affiliate that connects businesses to vetted third-party providers. This article is general information published on August 4, 2026, not legal, tax, or financial advice — rules and rates change, and your situation is specific to you. Confirm details with the primary sources linked above and with a qualified tax or legal professional before acting.