A simple interest loan charges interest only on the principal you still owe, never on interest that has already built up. That one rule makes your total borrowing cost predictable from the day you sign, and it means every extra dollar you put toward the balance cuts what you’ll pay. Auto loans, most personal loans, standard mortgages, and federal student loans all work this way.
How the Interest Is Calculated
The formula is one line: Interest = Principal × Rate × Time. Principal is what you currently owe, Rate is your annual interest rate as a decimal, and Time is measured in years. Because the rate only ever applies to principal, you never pay interest on interest.
Borrow $10,000 at 5% for three years with no payments until the end, and your interest is $10,000 × 0.05 × 3 = $1,500. Payoff is $11,500. You could calculate that the day you sign, and it wouldn’t change.
Real loans don’t wait for one lump payment. You pay monthly, and each payment covers that month’s accrued interest first, then chips away at principal. As the principal falls, the next month’s interest charge falls with it. That’s why an amortization schedule shows early payments going mostly to interest and later payments going mostly to principal.
How It Differs From Compound Interest
Compound interest charges you on principal plus any interest that has already accumulated. Credit cards are the familiar example: today’s interest gets added to the balance, and tomorrow’s interest is calculated on that larger number.
Take the same $10,000 at 5% for three years, compounded annually. Year one: $500 in interest, the same as simple interest. Year two: the rate applies to $10,500, so $525. Year three: it applies to $11,025, so $551.25. Total interest is $1,576.25 instead of $1,500. The $76.25 difference is entirely interest charged on earlier interest.
On a small, short loan the gap looks minor. On a $300,000 balance over 30 years, the same effect produces tens of thousands of dollars in additional interest. The distinction matters most when the balance is large and the term is long.
Where You’ll See Simple Interest Loans
Simple interest is the default structure for most installment debt a consumer runs into.
- Auto loans. Nearly all of them calculate interest daily on the outstanding balance. The Consumer Financial Protection Bureau describes simple interest as “far more common” than precomputed interest in auto financing, and because interest accrues daily, the exact day you make your payment affects that month’s charge.1Consumer Financial Protection Bureau. What’s the Difference Between a Simple Interest Rate and Precomputed Interest on an Auto Loan?
- Personal loans. Most fixed-rate personal loans with one- to five-year terms use simple interest. The predictable schedule is part of why they’re pitched as an alternative to revolving credit.
- Mortgages. A standard 30-year fixed-rate mortgage calculates interest monthly on the remaining principal. The monthly interest charge is your current balance multiplied by one-twelfth of your annual rate.
- Federal student loans. All federal Direct Loans use a simple daily interest formula: outstanding principal multiplied by a daily interest rate factor (your annual rate divided by the days in the year), times the days since your last payment.2Federal Student Aid. Interest Rates and Fees for Federal Student Loans
How Payment Timing Changes What You Pay
Because interest accrues daily on whatever principal is still outstanding, paying even a few days early reduces the principal sooner, and that smaller principal generates less interest the next day and every day after. Over months and years, those small savings stack.
Anything you pay beyond the minimum goes to principal after covering accrued interest. An extra $1,000 payment on a $50,000 auto loan at 6% cuts roughly $60 per year in interest going forward, and the savings keep compounding as you continue making regular payments on the smaller balance. One well-timed extra payment from a bonus or tax refund can shave months off a loan term.
The reverse is just as true. A payment four days late means four more days of interest on the pre-payment balance. That extra interest comes out of your payment first, leaving less to reduce principal. Habitual late payments can quietly extend your effective loan term and cost hundreds or thousands more than the original schedule projected.
Watch-Outs That Undo the Benefit
Precomputed Interest and the Rule of 78s
Not every loan that looks like simple interest actually is one. Some lenders, particularly in subprime auto and consumer finance, use precomputed interest. The lender calculates the total interest for the full term up front and bakes it into the schedule. Pay on time for the whole term and you pay the same total as a simple interest loan. Pay early, and the picture changes.
On a true simple interest loan, early payoff saves you all the interest that would have accrued on the remaining term. On a precomputed loan, the lender may use the Rule of 78s to figure your refund. That method front-loads interest: on a 12-month loan you’d owe 12/78 of the total interest in month one, 11/78 in month two, and so on. By the halfway mark you’ve already paid about 75% of the total interest, so paying off at six months saves far less than you’d expect.
Federal law prohibits using the Rule of 78s on any precomputed consumer loan with a term over 61 months.3Office of the Law Revision Counsel. 15 USC 1615 – Prohibition on Use of Rule of 78s in Connection With Mortgage Refinancings and Other Consumer Loans Shorter loans can still use it in many states. Before signing, ask the lender directly whether interest is simple or precomputed. If it’s precomputed under the Rule of 78s and you might pay early, look elsewhere.
Capitalization on Student Loans
Federal student loans accrue simple interest daily, with a twist. If you go through deferment or forbearance and don’t pay the interest as it accrues, that unpaid interest can capitalize: it gets added to your principal. Once capitalized, future interest is calculated on the higher balance, which creates a compounding-like effect on what started as a simple interest loan. The Department of Education specifies the exact circumstances when capitalization occurs, and they vary by loan type and repayment plan.2Federal Student Aid. Interest Rates and Fees for Federal Student Loans Paying at least the accruing interest each month during deferment prevents capitalization.
Negative Amortization
If your payments don’t cover the interest that accrues each period, the unpaid interest may be added to your principal, a process called negative amortization. Your balance grows even though you’re paying.4Consumer Financial Protection Bureau. What Is Negative Amortization? It shows up most often on adjustable-rate mortgages where the minimum payment is set below the full interest charge, and on student loans in income-driven repayment plans where the calculated payment can fall below the monthly interest. If your loan allows a minimum payment that doesn’t cover interest, you’re trading short-term affordability for a larger balance later.
What the Lender Must Tell You
Under Regulation Z, which implements the Truth in Lending Act, your lender has to give you written disclosures before you commit, using specific terms so you can compare offers directly. The required figures include the finance charge (the total dollar cost of credit in interest and certain fees), the annual percentage rate, the amount financed, the total of payments, and the payment schedule.5eCFR. 12 CFR Part 1026 Subpart C – Closed-End Credit A lender that won’t put those numbers in front of you up front is a reason to walk away.
Pay particular attention to APR rather than the stated interest rate when you compare offers. The interest rate is what’s charged on principal; the APR folds in origination fees and other upfront charges to show the loan’s real annual cost.6Consumer Financial Protection Bureau. What Is the Difference Between a Loan Interest Rate and the APR? Two loans at the same interest rate can have meaningfully different APRs.
Paying Off Early
Extra payments on a simple interest loan translate directly into interest savings, but check for a prepayment penalty before you start sending extra money. Most personal and auto loans don’t have one. Mortgages are more regulated: federal law bans prepayment penalties entirely on non-qualified mortgages, and for qualified mortgages that do include one, the law caps the penalty at 3% of the outstanding balance in year one, 2% in year two, and 1% in year three, with no penalty allowed after that.7GovInfo. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans
When you’re ready to close out a loan, request a payoff statement. For a home loan, the lender has to provide an accurate payoff balance within seven business days of your written request.8Office of the Law Revision Counsel. 15 USC 1639g – Requests for Payoff Amounts of Home Loan The payoff figure is slightly higher than your current balance because it includes interest accrued through the anticipated payoff date. On a simple interest loan, that daily accrual means the exact date you send the money matters: paying off on a Monday instead of a Friday saves four days of interest.