Simple interest means interest charged only on the original amount of money borrowed or deposited, never on interest that has already accrued. The formula is Interest = Principal × Rate × Time, usually written I = P × R × T. Because the calculation always uses the original balance rather than a growing one, the cost of the loan (or the yield on the deposit) is predictable from day one.
The Formula and What Each Piece Means
Three numbers from your agreement are all you need.
- Principal (P): the original amount borrowed or deposited.
- Rate (R): the annual interest rate written as a decimal. Divide the percentage by 100, so 5% becomes 0.05.
- Time (T): the length of the agreement in years. Months divided by 12, days divided by 365 (or sometimes 360, discussed below).
Multiply the three together to get the interest. Add that to the principal for the total you’ll repay or receive: Total = P + I.
A Worked Example
Borrow $10,000 at 6% annual simple interest for 3 years.
I = $10,000 × 0.06 × 3 = $1,800. Total repayment is $11,800. That $1,800 is the entire cost of borrowing. It does not grow, because it is never folded back into the balance for the next calculation.
A shorter term works the same way. Borrow $5,000 at 8% for 9 months: convert months to years (9 ÷ 12 = 0.75), then I = $5,000 × 0.08 × 0.75 = $300. Total repayment is $5,300.
How Simple Interest Differs From Compound Interest
The difference comes down to what the rate is applied to. With simple interest, the rate is applied only to the original principal for the life of the agreement. With compound interest, the rate is applied to the principal plus any interest that has already accumulated, so you end up paying (or earning) interest on interest.
For a borrower, compounding raises the total cost of a loan compared with simple interest at the same rate and term. For a saver, compounding raises the yield. The gap widens with time. Over a 30-year horizon, a loan calculated with compound interest costs significantly more than the same loan at simple interest, because each compounding period enlarges the base that future interest is charged against.
Legal Weight of the Term
Courts treat simple interest as periodic compensation for the temporary use of money, and they enforce it literally. When a promissory note or loan agreement says “simple interest,” calculations must run against the original principal only. If a lender adds accrued interest back into the balance without contractual authority to do so, a borrower may have grounds to challenge the extra charges.
How Simple Interest Works on an Installment Loan
Most consumer loans aren’t lump-sum arrangements. They’re paid down monthly, and the interest is recalculated each month against whatever principal is left. This is still simple interest — the rate never applies to accrued interest — but the base shrinks as you pay.
Each month, the lender multiplies the remaining principal by the monthly rate (annual rate ÷ 12). That number is the month’s interest charge. Whatever’s left of your payment reduces the principal. Next month, the calculation runs against the lower balance, so the interest portion drops 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 goes to principal, bringing the balance to $249,699.67. The second month, interest is $249,699.67 × (0.05 ÷ 12) = $1,040.42, and $301.58 goes to principal. Each payment pushes a little more toward principal and a little less toward interest, all the way to payoff.
Where You’ll Encounter Simple Interest
Several common products calculate finance charges or yield this way:
- Auto loans and retail installment contracts, which apply the rate to the declining balance.
- Personal installment loans from banks and credit unions.
- Federal student loans, which accrue interest daily against the current balance.
- Short-term business loans and lines of credit, often paired with a 360-day count convention.
- Some certificates of deposit that pay interest on the original deposit at the end of the term.
Why the Day Count Convention Can Change Your Bill
Not every lender defines a year the same way when interest accrues daily. Two conventions are common in the United States.
Actual/365 counts the real number of days and divides by 365. It’s the more intuitive method and the standard for most consumer loans, including federal student loans. Actual/360 counts the real number of days but divides by 360. Because you’re dividing by a smaller number, each day’s interest charge is slightly higher. This convention shows up in commercial lending and some mortgage products.
The difference is bigger than it looks. On a $200,000 balance at 6%, daily interest under Actual/365 is about $32.88. Under Actual/360, it’s about $33.33. Over a full year of 365 days, the 360-day method produces roughly $167 more in interest. Your loan documents will name the convention — check the interest calculation or definitions section of the agreement.
Why Paying Early Cuts Your Cost
Because simple interest is calculated on the remaining principal, every extra dollar you put toward principal immediately shrinks the base for every future interest calculation. Halfway through a 5-year auto loan, an extra $1,000 payment toward principal means every remaining month’s interest is figured on a balance $1,000 lower.
One caveat: some loan agreements include prepayment penalties, fees the lender charges when you pay off the balance ahead of schedule. Federal law prohibits these entirely on high-cost mortgages; 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.1Office of the Law Revision Counsel. 15 USC 1639 – Requirements for Certain Mortgages For other loan types, state laws vary; some prohibit prepayment penalties on consumer loans, others allow them in certain conditions. Read the prepayment clause of your agreement before sending extra money.
The Short Version
Simple interest is the plainest way to charge for the use of money: a rate applied to the original principal, multiplied by time. On a lump-sum agreement, one calculation tells you the full cost. On an installment loan, the same rule runs monthly against a shrinking balance, which is why extra payments toward principal pay you back with lower interest every month that follows.