You just got a great quarter. Cash is sitting in the business checking account, and there's a $40,000 balance left on the loan you took out to buy a delivery van or a CNC machine. Paying it off early feels like the obvious move — kill the debt, stop the interest, simplify the books. So you call the lender for a payoff quote.
The number that comes back is bigger than you expected. Not because of a prepayment penalty — the contract doesn't have one. It's because of three words buried in the fine print: "Rule of 78."
If you've never heard of it, you're not alone. It's one of the oldest tricks in consumer and small-business lending, still legal in a lot of places, and it quietly determines whether paying off a loan early actually saves you money — or barely saves you anything at all.
What the Rule of 78 Actually Is
The Rule of 78 is a method for calculating how loan interest gets divided across the life of a fixed-term, fixed-payment loan. The name comes from a specific number: for a 12-month loan, if you add up the digits 1 through 12 (1+2+3...+12), you get 78.
Here's the mechanic. Instead of charging interest on the loan's actual outstanding balance each month (the way a standard amortizing loan works), the lender calculates the total interest you'll pay over the entire term up front, then splits that total unevenly across the months — weighted toward the beginning.
For a 12-month loan using Rule of 78:
- Month 1 gets 12/78 of the total interest
- Month 2 gets 11/78
- Month 3 gets 10/78
- ...down to Month 12, which gets just 1/78
Your monthly payment stays the same dollar amount every month, like any installment loan. But the mix inside that payment shifts hard toward interest early and toward principal late. That's the opposite of what most borrowers assume is happening.
Why It's Called "Precomputed" Interest
Loans that use the Rule of 78 are called precomputed or add-on interest loans. The lender doesn't calculate interest daily or monthly against your remaining balance — the total interest charge is fixed at origination, added to the principal, and then divided into equal payments. The Rule of 78 is just the formula used to figure out how much of each payment counts as "earned" interest versus principal if you pay off before the end.
This is fundamentally different from a simple interest (or "actuarial method") loan, where interest accrues only on whatever principal balance is actually outstanding that month. On a simple-interest loan, paying extra or paying off early always reduces future interest, dollar for dollar, because there's less balance left to charge interest on. On a Rule of 78 loan, the lender has already "front-loaded" its claim to a disproportionate share of your total interest — so an early payoff doesn't return nearly as much value.
A Real Example With Numbers
Say your business finances $40,000 in equipment over 48 months at an add-on rate that produces $8,000 in total interest — a $48,000 payback split into 48 equal payments of $1,000.
Under the Rule of 78, the sum of the digits 1 through 48 is 1,176. The lender's claim on interest each month is weighted by the digit sum, largest first:
- Month 1: 48/1,176 of $8,000 ≈ $326.53 in interest, ~$673 to principal
- Month 24 (halfway through the term): by this point, the lender has already "earned" roughly $5,959 of the $8,000 total interest — about 74% — even though you've only made half your payments and used half the loan term.
- Month 48: the final payment carries just 1/1,176 of the total interest, almost all principal.
Now compare that to a true simple-interest loan carrying the same $8,000 in total interest if held to term. Because simple interest accrues only on the declining balance, the lender's earned share at the halfway mark is meaningfully lower than 74% — closer to what you'd intuitively expect from "half the term, roughly half the interest." Pay off a simple-interest loan at month 24 and you owe the actual remaining balance, no more. Pay off a Rule of 78 loan at month 24, and you're settling up against a lender that's already booked most of its profit.
That's the whole story in one sentence: the earlier you pay off a Rule of 78 loan, the less benefit you get from paying it off early, because the lender structured the math to collect its interest up front regardless of how long you actually carry the balance.
Where This Still Shows Up
The Rule of 78 sounds like a relic, and for the most part it is — but it hasn't disappeared, especially in corners of small-business financing that don't get much scrutiny:
- Buy-here-pay-here and subprime auto/equipment dealers, who often finance vehicles or gear in-house rather than through a bank
- Some equipment-finance companies and leasing outfits, particularly for shorter-term contracts where federal restrictions don't apply
- Merchant cash advance-adjacent lenders and other non-bank small-business lenders that use precomputed, add-on-rate structures instead of a stated APR
Because it's precomputed rather than accruing daily like a credit card or bank term loan, the Rule of 78 is most common on shorter-term, fixed-payment financing — think 12-to-48-month equipment loans, not 10-year commercial mortgages.
The Legal Landscape: What's Actually Banned
Federal law does restrict the Rule of 78, but the restriction is narrower than most business owners assume. Under 15 U.S.C. § 1615, any precomputed consumer credit loan with a term longer than 61 months must use a refund method "at least as favorable to the consumer as the actuarial method" — which, in practice, bans the Rule of 78 outright for those longer loans. That statute took effect for loans originated after September 30, 1993.
The catch: that federal rule only kicks in past the 61-month mark, and it's written for consumer credit, which creates a gray area for business-purpose loans. Below 61 months, and outside strict consumer-lending definitions, it's a state-by-state patchwork. Roughly half of U.S. states have banned the Rule of 78 for loans of any length; others allow it below a certain dollar threshold or loan term, and a handful still permit it broadly. If your business is financing equipment through an out-of-state or non-bank lender, don't assume your state's ban protects you — check the choice-of-law clause in the contract, since many financing agreements specify which state's law governs.
How to Spot It Before You Sign
Lenders using precomputed interest generally aren't hiding it, but they also aren't leading with it in the sales conversation. A few things to check before signing an equipment loan or financing agreement:
- Look for the words "precomputed," "add-on interest," "Rule of 78," or "actuarial method" in the contract. They usually appear in a section about prepayment or interest refunds.
- Ask directly: "Is this a simple-interest loan or a precomputed loan?" A simple-interest lender will say yes immediately, because it's a selling point. A precomputed lender may hedge.
- Ask what happens if you pay it off in month 12 versus month 36. Have the lender show you, in writing, exactly how much you'd owe at each point. If they can't or won't, that's itself a red flag.
- Watch for "rebate of unearned interest" language. If a contract talks about refunding interest rather than simply charging interest on the declining balance, you're looking at a precomputed loan — and the Rule of 78 is the most common method used to calculate that refund.
- Compare the total cost, not just the monthly payment. Two loans with identical monthly payments and terms can have very different early-payoff economics depending on the interest method.
If early payoff flexibility matters to your business — and for most growing businesses, it does — a simple-interest loan is worth paying a slightly higher stated rate for on the same term. It's not that it's unusual, only that it doesn't come with a hidden penalty for treating debt the way most business owners assume it works: pay it off faster, owe less.
If You're Already Locked Into One
If you've already discovered — mid-payoff-quote — that your equipment loan uses the Rule of 78, you still have a few options worth working through before you write the check:
- Ask for the payoff calculation in writing, itemized. You want to see the unearned-interest rebate figure, not just a single lump-sum number. If your state bans or restricts the Rule of 78 for your loan's term and amount, the lender is required to use a more favorable method, and an itemized breakdown is how you'd catch a miscalculation.
- Check the loan's origination date and term against 15 U.S.C. § 1615. Anything precomputed, over 61 months, and originated after September 30, 1993 legally cannot use the Rule of 78 for its refund calculation — if yours does anyway, that's worth raising with the lender or your state's financial regulator.
- Run the math both ways before deciding to prepay. Because the early-payoff savings are smaller than they'd be on a simple-interest loan, it's worth comparing what that same cash would earn — or save in interest — if redirected somewhere else in the business, like a higher-rate line of credit or a revolving balance, instead of automatically assuming payoff is the better move.
- For your next equipment purchase, negotiate the interest method, not just the rate. Dealers and finance companies quote APR, but APR alone doesn't tell you whether interest is simple or precomputed. Two loans with the same APR and term can have very different early-payoff economics — ask the lender to confirm in writing which method applies.
None of this means precomputed loans are always a bad deal — for a business that has no intention of paying early and plans to carry the loan to term, the interest method is close to irrelevant. The risk is specifically for businesses that expect to have the option to pay down debt faster than scheduled, which is exactly the kind of flexibility a growing business wants to preserve.
Keep Your Loan Math as Clear as Your Books
Whether a loan uses simple interest or the Rule of 78, the only way to actually know what you're paying is to track it — not estimate it from the monthly payment on a statement. That's true of every liability on your balance sheet, not just equipment loans. Beancount.io gives you plain-text accounting that makes every interest charge, principal reduction, and loan balance fully transparent and auditable in your own ledger, instead of buried in a lender's amortization table. Get started for free and keep your debt schedule as precise as the rest of your books.