Simple Interest Calculator

Simple interest is interest earned only on the original principal using I = P x r x t, never on previously accumulated interest.

Simple interest stays flat each period; compound interest earns interest on prior interest, so it grows faster over time.

Yes. Toggle between Years and Months — the calculator converts months to years internally (months / 12).

Yes, the Simple Interest Calculator is 100% free with no registration, no hidden fees, and no usage limits. All processing happens locally in your browser, ensuring complete privacy.

Absolutely. The Simple Interest Calculator processes everything client-side in your browser. No data is uploaded to or stored on any server. Your content remains private on your device at all times.

Yes, the Simple Interest Calculator is fully responsive and works on smartphones and tablets. You can use it on any device with a modern web browser -- no app download required.

No account or registration is needed. Simply open the Simple Interest Calculator in your browser and start using it immediately. There are no sign-up walls or usage restrictions.

The Simple Interest Calculator uses industry-standard formulas and algorithms to ensure accurate results. However, for critical financial or medical decisions, always consult a qualified professional.

Simply enter your input in the provided field, adjust any settings to your preference, and the tool will process it instantly. You can then copy the result to your clipboard or download it.

The Simple Interest Calculator works in all modern browsers including Chrome, Firefox, Safari, Edge, and Opera. For the best experience, use the latest version of your preferred browser.

Multiply three numbers: the principal you borrow, the annual rate as a decimal, and the time in years — that is I = P x r x t. Convert the rate by dividing the percentage by 100, so 5% becomes 0.05. For example, a $10,000 loan at 5% for 3 years gives 10,000 x 0.05 x 3 = $1,500 in interest, and the total you repay is principal plus interest, A = P + I, or $11,500. Because the base never changes, the interest amount stays flat for every year of the term, which makes flat-rate loans easy to verify by hand. Enter your principal, rate, and time above and the calculator returns the interest, the total owed, and a month-by-month schedule you can download as CSV to check against a lender's figures.

On a simple-interest basis, $10,000 at 5% per year for 3 years earns $1,500 in interest, for a total of $11,500 at maturity. The math is 10,000 x 0.05 x 3 = 1,500, and each year adds an identical $500 because simple interest is paid only on the original principal, never on interest already accrued. Stretch the same deposit to 5 years and the interest rises to $2,500; shorten it to 1 year and it drops to $500. Note that most real savings accounts compound, so they would pay slightly more than these flat figures over the same period. To test your own numbers, type the principal, rate, and term into the calculator — it shows the interest, total amount, effective return, and interest per month and per day instantly.

They are essentially the same idea: a flat-rate loan charges simple interest on the original principal for the whole term, so the interest cost does not shrink as you pay the balance down. That is why a flat 5% can feel deceptively cheap — you keep paying interest on the full amount borrowed even after months of repayments. By contrast, a reducing-balance loan recalculates interest on the outstanding balance each period, so the effective annual percentage rate ends up noticeably higher than the headline flat rate for the same number. Auto loans and many short-term personal loans are commonly quoted flat. This calculator models the flat, simple-interest case exactly, showing total interest in dollars so you can compare a flat quote against a reducing-balance offer side by side before signing.

The most common reason is compounding. This calculator applies simple interest, which charges or pays interest only on the original principal, while most savings accounts, credit cards, and long-term mortgages compound — adding interest on top of previously accrued interest. Over longer terms that gap widens, so a compounding product will show a higher total than the flat figure here. Other smaller causes include day-count conventions, fees, or rounding written into a real contract, plus rate changes the bank may apply mid-term. If your product compounds, the numbers here will understate the true cost or return, so reach for a compound interest calculator instead. Use this tool when the agreement is genuinely flat-rate, then treat the result as a reliable estimate and confirm critical figures with the lender or a qualified advisor.

Simple interest uses time measured in years, so a partial period is just a fraction. Flip the Years / Months toggle to Months and the calculator divides your entry by 12 before applying I = P x r x t — so 6 months becomes 0.5 years and 18 months becomes 1.5. For a $10,000 balance at 5%, six months yields 10,000 x 0.05 x 0.5 = $250 in interest. The tool also surfaces interest per month and interest per day, which is handy for prorating an early payoff or a stub period that does not land on a clean month boundary. The monthly breakdown table lists each month's interest, the running cumulative interest, and the balance, and you can export the full schedule as a CSV to attach to a loan record or reconcile against a statement.