Simple Interest Calculator
Our free Simple Interest Calculator computes interest the straightforward way - principal times rate times time. Enter your amount, annual rate, and period to see the interest earned and the total.
Simple interest
Calculate simple interest: principal x rate x time, with total amount and breakdown.
FAQs
When is simple interest used instead of compound interest?
Simple interest appears in short-term personal loans, some auto loans, certain bonds that pay periodic coupons, and many textbook problems. Most savings accounts, credit cards, and mortgages use compound interest, so always check which method applies before comparing rates.
How do I handle periods shorter than a year?
Convert the time to years as a fraction: 6 months is 0.5 years, 90 days is roughly 90/365 = 0.247 years. Multiply principal x annual rate x that fraction. The calculator handles the conversion if you enter the period in months or days.
How it works
Simple interest is calculated only on the original principal, never on accumulated interest. The formula is: interest = principal x annual rate x years. It's the most transparent way interest can work, and it's still used for many short-term loans, some bonds, and basic savings products.
For example, $5,000 at 6% simple interest for 3 years earns $5,000 x 0.06 x 3 = $900 in interest, for a total of $5,900. Each year's $300 interest payment is identical because it's always computed on the original $5,000.
Compare that with compound interest at the same rate: after 3 years you'd have about $5,955 because each year's interest starts earning its own interest. The gap looks small over 3 years but grows dramatically over decades - which is why knowing which type you're dealing with matters.
Use this calculator whenever a loan or deposit quotes simple interest, or as a teaching baseline before moving to compound calculations. If your numbers don't match a bank's quote, check whether they're compounding - that's the most common source of confusion.
Simple vs. Compound: A Tale of Two Formulas
The difference between simple and compound interest is small at first and enormous eventually. On $10,000 at 8%, simple interest earns a flat $800 every year - $8,000 over a decade. Compounded annually, the same rate yields about $11,589 in interest over 10 years, because each year's gains join the principal.
This is why lenders and savers should both care: borrowers generally prefer simple interest (less total paid), while investors prefer compounding (more total earned). When comparing two products, make sure you're comparing the same interest type - or convert both to an effective annual figure.
Reading the Fine Print on Loans
A loan advertised with simple interest is refreshingly honest: you can verify every number with one multiplication. If a lender quotes a rate but the payments don't match the simple-interest math, compounding or extra fees are hiding somewhere in the terms.
Ask two questions before signing: is interest calculated on the original balance or the declining balance, and how often does it compound? The answers determine the true cost far more than the headline rate does.
Why Simple Interest Still Matters
In a world of compounding everything, simple interest remains the clearest mental model for interest itself. It isolates the pure cost of borrowing or reward for lending, free from the exponential curve that makes compound math harder to intuit.
It's also the right tool for short horizons. Over a few months, the difference between simple and compound is negligible, so the simpler formula gives quick, accurate-enough answers for deposits, bridge loans, and informal lending between friends and family.