Quick answer
Simple interest is calculated only on the original principal: interest = principal × rate × time. Borrow or deposit $8,000 at 5% for 3 years and the interest is exactly $1,200, for a total of $9,200 — the same $400 every year, with no interest on interest at any point in the term.
How the Simple Interest Calculator works, step by step
Enter the principal, the annual interest rate and the term in years and months. The calculator converts the term to years — 18 months is 1.5 years — then multiplies the three together. There is no compounding step, so interest never earns interest, and the result shows the interest per year and per month alongside the total.
Because growth is linear, each year adds the same amount of interest: in the example, $400 a year, every year. Doubling the term exactly doubles the interest, which makes simple interest easy to check by hand and easy to spot in a loan quote — if the interest charge is identical in year one and year five, the loan is simple.
Switch on the compound comparison to see what monthly compounding at the same rate would give instead. On the worked example the difference is only $92 over 3 years, which is why the two methods look similar over short terms. Over 20 years the same $8,000 at 5% earns $8,000 of simple interest but about $13,701 compounded monthly — the gap is a rounding error at first, then most of the return.
Simple interest shows up in short-term lending, many auto loans and some personal loans, where interest is charged on the outstanding balance without compounding. US Treasury bills and some short-term notes are also priced on a simple basis. Most savings products compound instead, so this calculator is the wrong tool for projecting a savings account.
Mortgages work on a related idea: each month's interest is charged on the current balance only, and the payment is set so the balance reaches zero at the end of the term. That schedule is called amortization, and interest is never charged on past interest as long as you pay on time. If a product does compound, this calculator understates the true growth or cost, so confirm the method in the product terms first.
The formula
Interest = P × r × t; Amount = P × (1 + r × t), where P is principal, r the annual rate as a decimal and t the time in years (years plus months ÷ 12).
Worked example — try it on the calculator above
Worked example: $8,000 at 5% for 3 years
Interest = $8,000 × 0.05 × 3 = $1,200. The final amount is $8,000 + $1,200 = $9,200, with $400 of interest landing each year and $33.33 each month. If the same money compounded monthly, the final amount would be $9,292 — only $92 more over 3 years. Stretch the term to 20 years and the picture changes: simple interest pays $8,000 in total, while monthly compounding pays about $13,701. Time, not the rate, is what turns the compounding gap from a footnote into the main event.
Frequently Asked Questions
Where is simple interest actually used?
Short-term loans, many auto loans, and some personal loans calculate interest on the outstanding balance without compounding. US Treasury bills and some short-term notes are also priced on a simple basis. Most savings products compound instead, which is why a savings projection needs a compound calculator.
How do I calculate simple interest for months instead of years?
Convert the term to years: 18 months is 1.5 years. Interest = principal × rate × 1.5. The same works for days — divide the number of days by 365 (some lenders use 360; check the loan terms). This calculator takes years and months separately and does the conversion for you.
Is simple interest better for borrowers or lenders?
For borrowers, when the alternative is compounding at the same rate, because unpaid interest does not itself accrue interest. For savers it is worse. In practice, the rate and fees usually matter more than the method — a 2-point rate difference swamps the simple-versus-compound gap on most short loans.
Does a mortgage use simple interest?
Effectively yes, in a specific way: each month's interest is charged on the current balance only, and the payment is set so the balance reaches zero at the end of the term. That schedule is called amortization, and interest is never charged on past interest as long as you pay on time.
Estimates, not advice. This calculator provides educational estimates based on the inputs you enter and the assumptions shown on this page. It is not financial, tax or investment advice, and it cannot see your full situation. Verify important figures against official sources, and consider a qualified professional for decisions about your own money. See our Disclaimer and Methodology.