Simple Interest Calculator

What is simple interest and when does it apply?

Simple interest is interest calculated on the original principal amount only — the base amount never changes for interest purposes, regardless of how long the money is held. The simple interest formula is SI = P × R × T ÷ 100, where P is the principal, R is the annual interest rate in percent, and T is the time in years. Total amount at maturity = P + SI.

Simple interest applies to many short-term loans (personal loans, car loans), some bonds, and flat-rate financing products. It contrasts with compound interest, which adds earned interest back to the principal each compounding period — making future interest charges larger. For students studying for exams, simple interest is the most frequently tested interest formula in mathematics curricula worldwide.

Step-by-step: how to calculate simple interest

Example: You invest $8,000 at 7.5% annual simple interest for 4 years. Step 1: Identify P = 8000, R = 7.5, T = 4. Step 2: Calculate SI = 8000 × 7.5 × 4 ÷ 100 = 240,000 ÷ 100 = $2,400. Step 3: Calculate total amount = 8000 + 2400 = $10,400. Step 4: Effective annual rate = SI ÷ P ÷ T = 2400 ÷ 8000 ÷ 4 = 7.5% (same as stated rate — always the case for simple interest).

For months or days: convert T to years first. For 9 months, T = 9 ÷ 12 = 0.75. For 200 days, T = 200 ÷ 365. The calculator handles this conversion automatically when you select the time unit.

Simple interest vs. compound interest — which costs more?

For the same principal, rate, and term, compound interest always produces more interest than simple interest (after the first period). The gap widens with time and compounding frequency. On a $10,000 loan at 10% for 5 years: simple interest total = $15,000 (fixed $1,000/year). Compound interest (annual) total = $16,105.10. Compound interest (monthly) total = $16,453.09.

The simple interest model is borrower-friendly for long terms: you always know exactly how much you owe. With compound interest, missed payments or extended terms can cause the outstanding balance to balloon well beyond the original principal. US federal student loans use simple interest precisely because it's more predictable for borrowers on income-driven repayment plans.

Real-world applications: car loans, student loans, and savings bonds

Auto loans: Most car loans accrue simple interest daily on the remaining balance. This means paying even one extra payment per year, or paying bi-weekly instead of monthly, reduces total interest paid because each early payment reduces the outstanding balance before the next interest calculation. On a $25,000 car at 7% over 60 months, making one extra annual payment saves roughly $400 in interest and shortens the loan by about 5 months.

US federal student loans: These use simple daily interest. If you're on income-driven repayment (IDR) and your payment doesn't cover all the monthly interest, the unpaid interest is tracked separately — it doesn't immediately capitalize onto the principal (it may capitalize annually or at specific events, depending on the plan). This is distinct from the compound interest structure of most private student loans, where unpaid interest capitalizes immediately and starts accruing its own interest.

Free, private, no signup — comparing to Bankrate and NerdWallet

Bankrate's simple interest calculator and NerdWallet's interest calculators both work well, but they're surrounded by loan comparison ads, rate tables, and affiliate links. Typing your numbers into Bankrate may trigger retargeting campaigns. NerdWallet's business model is loan and credit card referrals — entering your principal and rate into their tools is read by their ad systems.

This calculator is the opposite: no trackers, no accounts, no loan offers. The formula breakdown, the SI result, and the total amount appear immediately in your browser. Nothing is sent anywhere. Works on desktop and mobile, online and offline once loaded. If you're a student checking homework or a borrower comparing two loan quotes, you get the answer without any commercial baggage.

The simple interest formula fully unpacked: SI = P × R × T / 100

Each variable in SI = P × R × T / 100 carries a precise meaning. P (Principal) is the original sum of money lent or invested — the base on which interest is always calculated, never changing throughout the term. R (Rate) is the annual interest rate expressed as a percentage; if the lender quotes 8% per annum, R = 8. T (Time) is the duration in years; six months is T = 0.5, ninety days is T = 90/365. The division by 100 converts R from a percentage to a proportion. When R is already expressed as a decimal (0.08 instead of 8), the formula simplifies to SI = P × R × T — both forms are mathematically identical.

Worked example: You deposit ₹50,000 at 8% per annum for 3 years. SI = 50,000 × 8 × 3 / 100 = 12,00,000 / 100 = ₹12,000. Total amount at maturity A = P + SI = 50,000 + 12,000 = ₹62,000. Equivalently, A = P(1 + RT) = 50,000 × (1 + 0.08 × 3) = 50,000 × 1.24 = ₹62,000.

The formula is fully reversible. To find Principal when interest, rate, and time are known: P = (SI × 100) / (R × T). To find Rate: R = (SI × 100) / (P × T). To find Time: T = (SI × 100) / (P × R). Example: an investment earned ₹9,000 at 6% p.a. over 3 years — P = (9,000 × 100) / (6 × 3) = 9,00,000 / 18 = ₹50,000. Mastering these four rearrangements covers the entire simple interest question set in competitive exams (SSC, IBPS, CAT, GMAT) and school curricula alike.

Simple interest vs. compound interest: a precise comparison

The single defining difference is the base on which interest is calculated. Simple interest always uses the original principal P — the base never grows. Compound interest uses the accumulated amount at the end of each compounding period as the new base, so interest earns interest. After the very first period they are identical; every period after that, compound interest charges more.

Concrete comparison at scale: invest ₹1,00,000 at 10% per annum for 5 years. Under simple interest: SI = 1,00,000 × 10 × 5 / 100 = ₹50,000 interest, total = ₹1,50,000. Under compound interest (compounded annually): A = 1,00,000 × (1.10)⁵ = 1,00,000 × 1.61051 = ₹1,61,051, so interest = ₹61,051. The gap — ₹11,051 — represents the cost of interest-on-interest over five years at 10%. Extend the same comparison to 20 years: SI total = ₹3,00,000; CI (annual) total = ₹6,72,750. The gap has grown from ₹11,051 to ₹3,72,750, illustrating how compounding accelerates non-linearly over time.

For borrowers, simple interest loans are cheaper over long terms because no interest capitalises. For investors and savers, compound interest is better because the portfolio compounds its own gains. This asymmetry explains why financial regulation in most countries requires lenders to disclose the effective annual rate (EAR) or APR alongside the nominal rate — a flat-rate (simple interest) loan's true cost is substantially higher than it looks when compared naively to an APR-quoted compound-interest product.

Where simple interest is actually used today

US auto loans are the largest everyday category: virtually all car loans originated in the United States accrue interest daily on the outstanding principal balance — a pure simple interest model. Because interest is recalculated each day on what you still owe, every extra payment you make immediately reduces tomorrow's interest charge. A borrower who makes a single extra monthly payment per year on a $30,000 / 6% / 60-month auto loan can save over $700 in total interest and retire the loan several months early. US federal student loans similarly use simple daily interest; unpaid interest does not capitalise until specific trigger events (end of a grace period, change of repayment plan), so aggressive early payments carry outsized benefit.

Treasury Bills (T-Bills) are issued on a discount basis that is mathematically equivalent to simple interest: the investor pays a discounted price today and receives the face value at maturity. The quoted discount rate and the equivalent investment yield differ precisely because of the day-count convention used (Actual/360 for US T-Bills). Flat-rate consumer loans — common in several Asian and African markets, and in the hire-purchase agreements used historically for appliance and vehicle financing in the UK — apply simple interest to the original loan balance regardless of repayment, which means the effective APR is approximately double the quoted flat rate. Microfinance products in India and Southeast Asia also frequently quote flat rates, making borrower education around the true cost essential.

Short-term money market instruments — commercial paper, banker's acceptances, and interbank deposits with maturities under one year — almost universally use simple interest rather than compounding, because the term is too short for compounding to add meaningful precision. A 90-day interbank deposit at 5.5% p.a. carries interest of Principal × 0.055 × 90/360 (using the Actual/360 convention standard in US dollar markets), with no mid-term compounding event. Understanding this keeps traders from over- or under-estimating the interest accrued on short positions.

Day count conventions and interest rate literacy

The variable T in the simple interest formula seems straightforward — until you ask how lenders count days. Financial markets have standardised several day count conventions that produce measurably different interest amounts even for the same nominal rate and the same calendar period. Actual/360 (also written Act/360) is the US money market standard: the actual number of calendar days between two dates is divided by 360. This inflates the effective annual interest cost by a factor of 365/360 ≈ 1.0139 relative to a 365-day year, which is why Act/360 loans are more expensive than they look. Actual/365 (Act/365 Fixed) is used for UK gilts and some sterling instruments — the actual day count divided by 365 regardless of whether the year is a leap year. Actual/Actual (ICMA or ISDA) is used for government bonds and gives the most precise result because it counts actual days and actual days in the year. 30/360 treats every month as 30 days and every year as 360 days; it was historically standard for US mortgage calculations and corporate bonds, and it simplifies arithmetic at the cost of slight inaccuracy across month-end boundaries.

Why does the convention matter in practice? On a $10,000,000 interbank loan at 5% for 180 days: using Actual/360, interest = 10,000,000 × 0.05 × 180/360 = $25,000. Using Actual/365, interest = 10,000,000 × 0.05 × 180/365 = $24,657.53. A difference of $342.47 on a single transaction — multiply across thousands of trades and the convention choice becomes material. When comparing loan quotes, always confirm which day count the lender uses.

APR (Annual Percentage Rate) is a standardised disclosure created precisely to cut through these differences. In the US, the Truth in Lending Act (TILA) requires lenders to disclose APR on all consumer credit products; APR includes not just the interest rate but also origination fees, points, and other mandatory charges, amortised over the loan term. A car loan quoted at 6% interest with a $400 origination fee has an APR higher than 6%. The nominal rate is the stated rate before fees and before considering compounding frequency; the effective annual rate (EAR) is what you actually pay after accounting for intra-year compounding. Two mental shortcuts worth memorising: the Rule of 72 — divide 72 by the annual rate to estimate the years required to double your money (at 8%, 72/8 = 9 years); and the Rule of 114 — divide 114 by the rate to estimate the years to triple your money (at 8%, 114/8 ≈ 14.25 years). Both rules apply approximately to compound interest; simple interest gives an exact answer (double at 8% takes exactly 100/8 = 12.5 years), highlighting once again why the two methods diverge over time.