A friend asks to borrow $8,000 and offers to pay 9 percent simple interest over 18 months. Good deal or bad? The math is one multiplication: $8,000 × 0.09 × 1.5 = $1,080 in interest, $9,080 total. Simple interest is the most honest interest there is — it is always calculated on the original principal, never on accumulated interest, so the amount earned each period is exactly the same. The formula is I = P × R × T divided by 100 (when the rate is written as a percent): principal times annual rate times time in years. This page computes the interest, the total amount, and the ROI for any loan or investment that works this way, and the guide below shows how to run the formula in every direction — including solving for the rate, the time, or the principal when those are the unknowns.

The formula, worked twice.

Take $5,000 at 6 percent for 2 years. Multiply principal by rate by time: 5,000 × 6 × 2 = 60,000, divide by 100 to handle the percent, and you get $600 in interest. Add it to the principal for the total amount: $5,600. The ROI is 600 ÷ 5,000 = 12 percent. Now a bigger one: $15,000 at 6 percent for 4 years gives 15,000 × 6 × 4 ÷ 100 = $3,600 in interest and an $18,600 total. Notice the signature property of simple interest — the second year earns exactly as much as the first ($900 each year here), because the principal never changes. That flatness is what separates it from compound interest, where each year's earnings join the principal and the growth accelerates.

What each letter means, precisely.

P is the principal: the starting amount, the full loan balance, the initial deposit. R is the annual interest rate as a percent — 6 means 6 percent per year, and the formula divides by 100 to convert it to the decimal 0.06. T is time in years, and this is where most errors happen: 18 months is 1.5 years, 6 months is 0.5, 90 days is 90/365 (about 0.2466), and a 26-week term is 0.5. A monthly-quoted rate must be annualized first — 1.5 percent a month is 18 percent a year. The quoted rate is always annual unless the contract says otherwise; forgetting that is the single most common simple-interest mistake, and it inflates a 6-month loan's interest by a factor of 12.

Running the formula backwards.

The calculator above solves forward, but the formula rearranges cleanly for any unknown. Finding the principal: what deposit earns $2,000 of simple interest at 5 percent over 8 years? P = I ÷ (R × T / 100) = 2,000 ÷ (5 × 8 ÷ 100) = 2,000 ÷ 0.4 = $5,000. Finding the rate: you borrow $12,000 and repay $14,160 after 3 years, so the interest is $2,160 and R = 2,160 ÷ (12,000 × 3) × 100 = 6 percent — a useful check when a lender quotes a total repayment instead of a rate. Finding the time: how long does $30,000 take to earn $4,500 at 3 percent? T = 4,500 ÷ (30,000 × 3 ÷ 100) = 4,500 ÷ 900 = 5 years. Each rearrangement is the same three numbers in a different order, and being able to run all four directions is what turns the formula from a classroom exercise into a negotiating tool.

Simple versus compound: the $7,348 lesson.

Put $50,000 to work at 5 percent for 10 years. Simple interest pays 50,000 × 5 × 10 ÷ 100 = $25,000 — a flat $2,500 every year, $75,000 total. Compounded monthly, the same money grows to about $82,350, earning $32,350 in interest — $7,350 more, from nothing but interest earning its own interest. On a smaller scale: $10,000 at 5 percent for 10 years earns $5,000 simple versus about $6,289 compounded annually. The gap starts invisible and ends enormous, which is why the direction of the comparison matters so much: as a borrower you want simple interest (you never pay interest on interest), and as a saver you want compound interest (your earnings earn earnings). Anyone quoting you a "5 percent" anything should be asked which kind — over a decade, the difference on $50,000 is a used car.

Where simple interest actually lives in American finance.

Most US auto loans use simple interest that accrues daily on the outstanding balance: each day's interest is balance × rate ÷ 365, so on a $25,000 loan at 8 percent the first month accrues about $164 in interest, and every payment attacks the balance that tomorrow's interest is computed on. Pay early or pay extra and you directly cut total interest — there is no prepayment penalty on most auto loans, and the daily-accrual math rewards it immediately. Short-term personal loans, promissory notes between individuals, and 90-day business notes ($10,000 at 6 percent for 90 days costs $147.95 in interest) all run on simple interest. Treasury bills are the purest example: a 26-week T-bill on $5,000 at a 4 percent discount rate earns about $99.73 over 182 days. Some CDs and savings products quote simple interest for short terms. Mortgages, credit cards, and most savings accounts use compound interest instead — which is exactly why a mortgage balance falls so slowly in the early years.

The flat-rate trap: when "simple" is not simple.

Car dealers sometimes quote a "flat rate," which sounds like simple interest but is a harsher cousin: interest is charged on the full original principal for the entire term, even as you pay the balance down. A $20,000 car loan at a 7 percent flat rate for 5 years costs 20,000 × 7 × 5 ÷ 100 = $7,000 in interest, period. The same loan as a normal reducing-balance simple-interest loan costs about $3,761 in total interest — nearly half. The flat-rate quote hides an effective rate close to 13 percent. Whenever a rate sounds too good on a dealer worksheet, ask whether it is flat or reducing-balance, then run both numbers. The $3,239 difference on this example is the most expensive sentence in car buying.

Days, dates, and the 360-day year.

Short-term interest lives and dies on day counts. A 90-day loan uses T = 90/365 (about 0.2466 years), not 90 — typing 90 into a years field inflates the interest 365 times, the classic spreadsheet catastrophe. Some banks use a 360-day "banker's year" instead: the same $5,000 T-bill example earns $99.73 on a 365-day basis but $101.11 on a 360-day basis, about 1.4 percent more. The difference is small on one note and real across a portfolio. Leap years add one more wrinkle: a loan spanning February 29 accrues an extra day of interest under daily accrual. None of this matters for a 5-year personal loan, and all of it matters for commercial paper.

Simple interest and the loans to avoid.

Because the formula is so transparent, it is also the lens that exposes predatory lending. A payday loan that charges $15 per $100 borrowed for two weeks is not "a $15 fee" — annualized, it is roughly 390 percent simple interest, which the formula makes undeniable: 15 ÷ 100 × (365 ÷ 14) × 100. Title loans and some "no credit check" installment products live in the same territory. The honest use of I = P × R × T is to annualize any fee, any term, any product, and compare it to a mainstream alternative before signing. If the annualized number has three digits, walk away.

Mistakes that cost real money.

First, time units: months and days must become years before they touch the formula — 18 months is 1.5, not 18. Second, the rate is annual by convention; a "2 percent monthly" quote is 24 percent a year, not 2. Third, solving backwards requires the rearranged formula, not the forward one with guessing — R = I ÷ (P × T) × 100 and T = I ÷ (P × R) × 100 are worth memorizing. Fourth, confusing flat-rate with reducing-balance interest, the $3,239 car-dealer trap above. Fifth, forgetting that most amortizing loans in the US accrue simple interest daily on the declining balance — which is good news, because it means extra principal payments cut interest immediately, with no compounding working against you.

Simple interest in savings products.

Banks sometimes quote simple interest on short-term products, and the quote can mislead by omission. A 6-month CD advertising "4 percent simple interest" pays 4 percent annualized for half a year — $10,000 earns $200, not $400, because T is 0.5. Promotional "bonus interest" offers work the same way: the bonus is simple, prorated to the days your money actually sits in the account. Money-market accounts and most savings accounts compound daily or monthly instead, which is strictly better for the saver — on $10,000 at 4 percent for a full year, daily compounding earns about $408 versus $400 simple. When comparing a simple-interest CD against a compounding savings account, annualize both with the formula and then ask the bank for the APY, the legally standardized figure that includes compounding. The higher APY wins, regardless of which formula produced it.

Amortization is simple interest with a payment plan.

A fixed-rate mortgage accrues simple interest on the declining balance each month, then a scheduled payment wipes out that month's interest plus a slice of principal. On a $300,000 loan at 6.5 percent, the first month's interest is 300,000 × 0.065 ÷ 12 = $1,625 — pure simple interest, no compounding involved. What makes amortization feel different is that unpaid interest never joins the balance (as long as payments cover it), so the "interest on interest" engine never starts. This is also why extra principal payments are so powerful on a mortgage: every extra dollar immediately reduces the balance that next month's simple-interest calculation runs on. Biweekly half-payments, one extra payment a year, or a lump sum at origination all exploit the same one-line formula.

Taxes treat interest as income, simply.

The IRS taxes interest income in the year it is paid or credited, whether it was computed simply or otherwise — the $600 from the $5,000 example is $600 of taxable interest income if it lands in a taxable account. Borrowers, symmetrically, can sometimes deduct interest: mortgage interest on a primary residence and student-loan interest (up to $2,500, with phaseouts) are the big two, while personal auto-loan interest is not deductible. The formula does not change for taxes; only the labels do. Keep the 1099-INT the bank sends, check it against P × R × T, and the numbers should agree to the penny — if they do not, the bank owes you an explanation, not the other way around.

What simple interest cannot do.

The formula assumes the principal sits untouched for the whole term — no deposits, no withdrawals, no missed payments. Real loans violate that constantly, which is why lenders pair the simple-interest accrual with an amortization schedule rather than the bare formula. It also ignores inflation: $600 of interest over 2 years is worth less in real terms if prices rose 6 percent in the meantime, a point the nominal formula never addresses. And it cannot price anything with changing rates — an adjustable-rate loan needs the rate re-applied each period. Treat I = P × R × T as the atom of interest math: everything complicated in finance is built from it, but nothing complicated is only it.

The one-paragraph history.

Interest on loans predates coinage — Sumerian tablets record barley loans with interest around 2000 BCE. Simple interest was the only kind for most of history because it can be computed by hand; compound interest tables were a specialist's tool until calculators arrived. Usury caps in American states, many still on the books, were written with simple interest in mind: a "maximum 8 percent" statute means 8 percent simple unless it says otherwise. The formula has not changed in four thousand years. Only the marketing around it has.

With the pre-filled defaults — Principal (P) $10,000, Annual Rate (R) 5%, Time (T) 3 — the Simple Interest Calculator returns Interest Earned $1,500.00, Total Amount $11,500.00, ROI 15.00%. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.