Simple Interest Calculator
Interest on the original amount only, the maths behind most short loans and deposits, with the formula shown.
- Total at the end
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- Compound would give
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Free forever, no sign-up and no limits, and this tool installs on its own so you can keep it on your home screen.
Principal, rate and time in, interest and total out, with the formula printed and, alongside it, what compound interest would have produced. The gap between those two numbers is the entire story of interest.
How to use it
- Enter the three inputsThe amount, the yearly rate, and the time, in days, months or years, whichever the loan is quoted in.
- Read the interestThe headline is the interest alone; the total adds it to the principal.
- Compare with compoundThe side figure shows the same money compounding yearly, the difference grows with time.
The straight line of interest
Simple interest is interest that never meets itself: it is always calculated on the original principal, so every year adds the same amount. Borrow 1,000 at 5% and the cost is 50 a year, flatly, forever. That linearity makes it the easiest interest to reason about, and the reason it survives in short-term lending, where the simplicity is worth more than the precision.
Short-term is also why the time field takes days and months as readily as years. Late fees run in days and bridging loans in months, so the tool converts whatever unit the paperwork uses, a year counted as 365 days, and prints the conversion inside the worked formula so nothing happens out of sight.
Why the compound figure sits beside it
The most useful thing a simple interest calculator can do is show what it is not. The side-by-side compound figure uses the same principal, rate and time with yearly compounding, and the gap between the two answers is the compounding effect isolated, negligible at one year, decisive at thirty. If you are deciding where the distinction matters for you: money you hold long-term compounds; money you borrow short-term mostly does not. When the compound figure startles you, the compound interest calculator next door breaks it down year by year.
Questions people ask
What is the simple interest formula?
Interest = principal × rate × time, with the rate as a fraction. 1,000 at 5% for 3 years: 1000 × 0.05 × 3 = 150. The tool prints this line with your numbers in it.
Where is simple interest actually used?
Short-term lending mostly: many personal and car loans accrue interest daily on the outstanding balance in a simple-interest way, plus bonds' coupon payments, some deposit accounts, and late-payment penalties. Long-term savings and mortgages are compound territory.
How different is it from compound interest?
Over short periods, barely, one year at 5% is identical. Over long ones, dramatically: 1,000 at 5% simple for 30 years earns 1,500; compounded yearly it earns 3,322. Simple interest grows in a straight line, compound in a curve, and the side-by-side figure on this page shows the gap for your exact numbers.
How do I enter months or days instead of years?
Pick the unit on the pill beside the time field and type the number as quoted: 18 months, or 90 days. The tool converts to years for the formula, counting a month as a twelfth of a year and a year as 365 days, and the worked line shows the conversion it used.
Is this the same as APR?
No. APR includes fees and reflects compounding where it happens, which is why it exists, headline simple rates understate true cost. Use this tool to understand the mechanics; compare real loan offers by APR.
How do I work out interest for a number of days?
Take the yearly rate, divide it by 365, then multiply by the balance and the number of days. A 2,000 balance at 6% accrues about 33 cents a day. Switch the time unit beside the field to days and the tool prints that line with your own numbers in it.