Simple interest means a finance charge calculated only on the original principal of a loan or deposit, never on interest that has already accrued. The formula is Interest = Principal × Rate × Time, usually written I = P × R × T. Because the rate always applies to the original balance rather than a growing one, you can predict exactly what a loan will cost or what a deposit will earn from the day the agreement is signed.
That definition carries real weight in a contract dispute. When a promissory note or loan agreement specifies simple interest, a court will enforce calculations based exclusively on the original principal. If a lender folds accrued interest back into the balance without contractual authority and charges interest on that larger figure, the borrower may have grounds to challenge the extra charges.
How to Calculate Simple Interest
You need three figures from your agreement:
- Principal (P): the original amount borrowed or deposited.
- Rate (R): the annual interest rate as a decimal. Divide the percentage by 100, so 5% becomes 0.05.
- Time (T): the length of the agreement in years. For months, divide by 12. For days, divide by 365 or 360 depending on the contract.
Multiply the three together to get the interest. Add that to the principal to find the total you will repay or receive.
A Three-Year Example
Suppose you borrow $10,000 at 6% annual interest for three years under a simple interest agreement.
- P = $10,000
- R = 0.06
- T = 3
I = $10,000 × 0.06 × 3 = $1,800. Total repayment is $11,800. That $1,800 is the full cost of borrowing, and it stays the same regardless of how the payments are structured, because the interest is never added back to the principal.
A Shorter-Term Example
Borrow $5,000 at 8% for nine months. Convert the term: 9 ÷ 12 = 0.75 years. Then I = $5,000 × 0.08 × 0.75 = $300. Total repayment is $5,300.
Simple Interest vs. Compound Interest
The difference between the two methods is what the rate applies to. Simple interest applies the rate only to the original principal for the entire life of the agreement. Compound interest applies the rate to the principal plus any interest already accumulated, so you pay (or earn) interest on interest.
For borrowers, compounding raises the total cost of a loan compared with simple interest at the same stated rate and term. For savers, compounding raises earnings. The gap widens over longer horizons: a 30-year loan under compound interest costs considerably more than the same loan at simple interest, because every compounding period enlarges the base the next period’s interest is calculated on.
How Simple Interest Works on an Installment Loan
The I = P × R × T formula gives the total interest on a lump sum held for a fixed period, but most consumer loans are paid back in monthly installments. On those loans, the lender applies simple interest to the remaining balance each month rather than to the original principal across the whole term.
Each month, the lender multiplies the current balance by the monthly rate (the annual rate divided by 12). That figure is the interest portion of the payment. Whatever is left of your fixed payment goes to principal, which lowers the balance for the next month. As the balance falls, the interest portion of each payment shrinks and the principal portion grows.
Take a $250,000 loan at 5% with a fixed monthly payment of $1,342. The first month’s interest is $250,000 × (0.05 ÷ 12) = $1,041.67. The remaining $300.33 reduces the balance to $249,699.67. Next month, interest is recalculated on that smaller balance: $249,699.67 × (0.05 ÷ 12) = $1,040.42, and $301.58 goes to principal. The pattern continues until the loan is paid off.
Day Count: 365-Day vs. 360-Day Year
Lenders don’t all define a “year” the same way when they calculate daily interest. Two conventions are common in the United States.
- Actual/365: the lender counts the actual days in each month and divides by 365. This is the standard for most consumer loans, including federal student loans.
- Actual/360: the lender counts actual days but divides by 360. Because the divisor is smaller, each day’s interest charge is slightly higher. This convention is common in commercial lending and some mortgage products.
The difference is not trivial. On a $200,000 balance at 6%, daily interest under Actual/365 is about $32.88. Under Actual/360 it is roughly $33.33. Across a full 365-day year, the 360-day convention produces about $167 more in interest. Which method applies to your loan should be stated in the interest calculation or definitions section of your agreement.
Where You’ll See Simple Interest
Several common financial products rely on simple interest calculations:
- Auto loans and other retail installment contracts, which calculate finance charges on the declining balance.
- Personal installment loans from banks and credit unions.
- Federal student loans, where interest accrues daily on the current balance using the loan’s stated rate.
- Short-term business loans and lines of credit, often with an Actual/360 convention.
- Some certificates of deposit, which pay interest on the original deposit at the end of the term.
What the Lender Has to Tell You
The Truth in Lending Act requires lenders to give you standardized information about the cost of credit before you commit, so you can compare offers on equal terms.1Office of the Law Revision Counsel. 15 U.S. Code 1601 – Congressional Findings and Declaration of Purpose Under Regulation Z, every closed-end credit agreement must disclose:
- the annual percentage rate (APR), the yearly cost of credit as a percentage;
- the finance charge, the total dollar cost of credit over the life of the loan;
- the amount financed, the credit extended after any prepaid finance charges are subtracted; and
- the total of payments you will have made once the schedule is complete.
These figures have to appear together so the full cost is visible at a glance.2Consumer Financial Protection Bureau. Regulation Z 1026.18 – Content of Disclosures For a simple interest loan, the finance charge and total of payments should line up with what I = P × R × T produces, adjusted for any fees the lender has rolled into the finance charge.
Paying Early Reduces What You Owe
Because simple interest is calculated on the remaining principal, every extra dollar you put toward principal immediately shrinks future interest. If you are halfway through a five-year auto loan and make an additional $1,000 principal payment, every remaining monthly interest calculation uses a balance $1,000 lower.
Some agreements include prepayment penalties, which charge a fee for paying off the balance ahead of schedule. Check your loan documents for a prepayment clause before making extra payments; state laws vary on when such penalties are allowed. For high-cost mortgages, federal law prohibits them: a high-cost mortgage cannot contain any term requiring the borrower to pay a penalty for early repayment of all or part of the principal.3Office of the Law Revision Counsel. 15 USC 1639 – Requirements for Certain Mortgages