Simple Interest Calculator

Simple interest is interest on the original amount only, never on interest that piles up. The formula is I = P·r·t: principal, times the annual rate, times the time in years. Enter a principal, a rate, and a length of time and it returns the interest and final total instantly. It's free, runs entirely in your browser, and recalculates the moment you change a field.

Most simple interest calculators only go one direction. You give them principal, rate, and time, they hand back the interest, done. This one runs the formula backward too. Say you paid $120 in interest on a loan and want to know the rate the lender actually charged. Switch the mode and it solves for the rate. Same deal for principal or time. Simple interest is the rare finance formula that reverses cleanly, so you can check a lender's arithmetic yourself rather than trust the number they quoted. That's why I'd pick this over the calculators that only ever go forward.

1. Pick what you're solving for. The "Solve for" dropdown starts on "Interest & total", the everyday case where you know the principal, rate, and time. Switch it to Principal, Rate, or Time to work a number backward.
2. Enter the principal in dollars, the original sum borrowed or deposited.
3. Set the annual rate as a percentage, like 5 or 6.5. Zero is allowed and just returns zero interest.
4. Enter the time and choose its unit: years, months, or days. The tool converts internally, so 6 months and 0.5 years give the same answer.
5. Read the result. Forward mode shows the interest and the final amount side by side. In the reverse modes you also type in the interest you already know, and the calculator fills in the missing piece.

Every field is live. Change the rate from 5 to 6 and the interest updates as you type, no button to press.

This is the feature most simple-interest pages leave out, and it's the one I built this around. The four "Solve for" modes all lean on the same identity, I = P·r·t, just rearranged, so they can never disagree with each other and each answer is exact rather than an estimate.

Solving for rate is the one you'll want most. A lender quotes a dollar figure of interest but stays vague on the actual rate. You borrowed $2,000 and paid $120 in interest over a year. Switch to Rate mode, enter those plus the time, and you get 6%. Now you can hold that against a competing offer, a real rate instead of a marketing headline.

Solving for time answers "how long until this pays what I want." You've got $5,000 earning 4% simple and want $1,000 of interest out of it. Time mode returns 5 years, and breaks it out into months and days too, so you can see it's 60 months without a second calculation.

Solving for principal reverse-engineers a quote. A lender says you'll owe $300 in interest on a two-year loan at 5%. Principal mode tells you the loan behind that figure is $3,000, handy for checking whether the numbers on a term sheet hang together before you sign.

So you never take a lender's arithmetic on faith. Feed it any three numbers and it hands back the fourth, off the same reversible equation every time.

The units are the one real trap here. The rate is annual, so the time has to be in years, or the calculation is off. A 5% rate over 6 months only earns half of that 5%, because six months is half a year, so 2.5% of the principal.

The unit dropdown handles the conversion, but it's worth seeing the arithmetic. Months divide by 12, so 6 months is 0.5 years. Days divide by 365, so 90 days is about 0.2466 years. Enter $5,000 at 6% for 6 months by picking "Months" and typing 6, and you get $150 in interest, the identical number you'd get typing 0.5 years. If switching the unit ever changes the answer in a way you didn't expect, that's your signal the rate and the time weren't in the same terms.

One wrinkle on the day count. Some lenders figure daily interest on a 360-day year instead of 365, the old "banker's year," which nudges the daily rate up a touch. Tiny on one small loan, real money across a large commercial balance. This tool uses 365, so read a 360-based contract as running slightly higher than what you see here.

Before you trust any of these numbers, make sure the thing you're calculating really is simple interest. Most places money grows use compound instead: savings accounts, credit cards you carry a balance on, mortgages, long-term investments. Compound charges interest on the interest already added, so the base keeps growing and each period earns a bit more than the last. Over a few months the two barely differ, over decades compound pulls far ahead, which is why credit cards use it and why it's rarely on your side when you borrow. If you are unsure which one an account uses, assume compound, and switch to the compound interest calculator.

Simple interest tends to live in a handful of specific spots. A lot of auto loans figure interest on the balance you still owe with no compounding, which is why throwing extra at the principal directly shrinks your future interest. Short-term personal and installment loans often use it too, since over a few months compounding barely moves the needle. Bond coupons behave the same way: a $10,000 bond at a 4% coupon pays $400 a year, and won't grow unless you reinvest it yourself. The thread through all of these is a short term or a fixed payout nobody's reinvesting.

When you know you've genuinely got simple interest, it's the rare finance formula you can run in your head. 10% of $2,000 for 3 years is $600, no tool required. For a loan you're paying down month by month, though, this gives the total interest cost, not the payment breakdown, so pair it with the loan calculator for the full schedule.