# Simple Interest Calculator Calculate simple interest, the final amount owed, and total repayment amount based on your principal, rate, and loan term. --- - **Canonical URL:** https://dothecalculation.com/calculators/simple-interest-calculator - **Category:** Financial calculators - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## Simple Interest Calculator — Find Interest, Payoff, and Final Balance Calculate simple interest, total repayment, and final balance on loans, notes, and short-term investments that grow in a straight line instead of compounding. - Interest, total amount, and principal breakdown - Works for loans, notes, and short-term deposits - Growth chart across the full term ## What Is Simple Interest? Simple interest is the most basic interest model in finance: it is calculated once, on the original principal, and never recalculated on top of interest that has already accrued. Contrast this with compound interest, where each period's interest gets added to the balance and starts earning interest itself. Because simple interest always uses the same starting principal, the dollar amount it adds is identical every single period — the balance grows in a straight line rather than curving upward. That predictability is exactly why simple interest shows up where clarity matters more than yield: short-term promissory notes between individuals or businesses, certain auto loans and personal loans, add-on interest consumer credit, some certificates of deposit, and — very commonly — introductory finance and accounting courses, because it isolates the relationship between rate, time, and return before compounding is introduced. ## The Simple Interest Formula The full calculation runs on one equation: $$I = P \times r \times t$$ Where: * \(I\) — total interest earned or owed. * \(P\) — principal, the original amount borrowed or deposited. * \(r\) — the annual interest rate, expressed as a decimal (6% becomes 0.06). * \(t\) — time, expressed in years (18 months becomes 1.5). Once you know the interest, the total amount owed or received at the end of the term follows directly: $$A = P + I = P(1 + rt)$$ This second formula, \(A = P(1 + rt)\), is the one worth memorizing — it gets you straight from principal to payoff amount in a single step. ## Worked Example: $10,000 at 6% for 3 Years Say you borrow $10,000 at a 6% annual simple interest rate for 3 years. Step 1 — convert the rate to a decimal: 6% → 0.06. Step 2 — apply the formula: \(I = 10{,}000 \times 0.06 \times 3 = 1{,}800\). Step 3 — add it to the principal: \(A = 10{,}000 + 1{,}800 = 11{,}800\). So over the 3-year term you would pay exactly $600 in interest per year — the same amount every year, because each year's interest is still calculated on the original $10,000, never on $10,600 or $11,200. That flat, linear pattern is the entire point of simple interest, and it is what the growth chart above shows visually as you change the principal, rate, or term. A second example with a shorter, more common note: you lend a friend or family member $2,500 at 4% simple interest for 90 days. Converting the term to years first (90/365 ≈ 0.2466), the interest owed is \(I = 2{,}500 \times 0.04 \times 0.2466 \approx 24.66\), so the total repayment is about $2,524.66 — a useful sanity check for informal personal loans where both sides just want a fair, easy-to-verify number. ## Simple Interest vs. Compound Interest The dividing line between the two models is whether interest already earned gets folded back into the balance that future interest is calculated on. Simple interest never does this — every period starts from the same principal. Compound interest does, so the base grows each period and interest accelerates on top of interest. The gap is small over short terms and large over long ones. $10,000 at 6% for 3 years earns $1,800 in simple interest ($11,800 total) versus roughly $1,910 compounded annually ($11,910 total) — a modest $110 difference. Stretch the same rate out to 20 years and simple interest totals $22,000 while annual compounding grows to roughly $32,071 — a difference of over $10,000 on the exact same principal and rate. This is why lenders overwhelmingly prefer compounding on long-term debt, and why simple interest tends to survive mainly in short-term, transparent agreements where both sides want the math to stay easy to verify by hand. ## Where Simple Interest Shows Up in Real Agreements Personal and business promissory notes between individuals, family members, or small businesses commonly use simple interest specifically because it is easy for both parties to verify without a spreadsheet. Add-on interest auto loans and some short-term personal or payday-style loans calculate the finance charge up front using simple interest on the original amount, then divide the total (principal plus interest) into equal payments — which is one reason these loans can carry a higher effective rate than the stated rate once fees are factored in. Certain short-duration certificates of deposit, T-bills quoted on a discount basis, and some corporate trade-credit terms ("2/10 net 30" style early-payment discounts) are also modeled using simple interest math over the relevant period rather than a compounding schedule. ## Handling Months, Days, and Day-Count Conventions Because \(t\) must be in years, partial periods need converting first: months are divided by 12 (7 months = 7/12 ≈ 0.5833 years), and days are divided by either 365 (exact interest, common for consumer loans) or 360 (ordinary/banker's interest, still used in some commercial and international lending). The two conventions produce slightly different results on the same stated rate — a 360-day count effectively charges a marginally higher rate than a 365-day count for the same number of calendar days, so it is worth confirming which one a lender is using before comparing offers. ## Solving for Principal, Rate, or Time Instead of Interest The same formula rearranges cleanly whenever you already know the interest and need a different variable: * To find the principal: \(P = I / (r \times t)\) — useful when you know how much interest a note charged and want to back into the original amount. * To find the rate: \(r = I / (P \times t)\) — useful for checking whether a lender's stated rate matches the interest they actually charged. * To find the time: \(t = I / (P \times r)\) — useful for figuring out how long a fixed-rate note has been (or will be) outstanding based on the interest amount. For example, if a note charged $450 in interest on a $5,000 principal over 18 months, the effective rate is \(r = 450 / (5{,}000 \times 1.5) = 0.06\), or 6% — a quick way to sanity-check a quoted rate against the actual dollar figures on a contract. ## Common Mistakes When Calculating Simple Interest The most frequent error is leaving the rate as a whole percentage instead of converting it to a decimal — using 6 instead of 0.06 inflates the result by a factor of 100. The second most common mistake is entering time in months or days without converting to years first, which silently produces a number far larger or smaller than intended. A subtler mistake is assuming a loan is simple interest when it is actually a compounding or amortizing loan with a fixed monthly payment (like a standard auto loan or mortgage) — those use a very different formula, and running simple interest math on them will understate the true cost. Always confirm with the lender or loan documents which interest method actually applies before using this calculator to make a financial decision. ## Related Calculators If the loan or account in question actually compounds, switch to the [compound interest calculator](/calculators/compound-interest-calculator), or run both methods side by side with the [interest calculator](/calculators/interest-calculator) to see the exact dollar gap. ## Frequently asked questions ### What is simple interest? Simple interest is interest calculated only on the original principal for the entire term — prior interest is never added back into the base, so the amount charged or earned each period stays the same. ### What is the formula for simple interest? I = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years. Total payoff is A = P(1 + rt). ### How do I calculate simple interest for months instead of years? Divide the number of months by 12 to convert to years before plugging it into the formula. For example, 9 months becomes 9/12 = 0.75 years. ### How is simple interest different from compound interest? Simple interest is always calculated on the original principal, so growth is linear. Compound interest recalculates on the growing balance (principal plus prior interest), so growth accelerates over time. ### Is simple interest better for borrowers or lenders? Borrowers generally come out ahead with simple interest over long terms since the cost stays flat instead of accelerating. Lenders and long-term investors generally prefer compound interest because it grows returns faster. ### Do banks still use simple interest? Most modern savings accounts, mortgages, and credit cards compound. Simple interest mainly survives in short-term notes, some add-on-interest auto and personal loans, certain CDs, and educational examples. ### What is the difference between the 360-day and 365-day method? Both convert days into a fraction of a year, but dividing by 360 (the "banker's rule") produces a slightly higher effective rate than dividing by 365 for the same stated annual rate and number of days. ### How do I find the interest rate if I already know the interest paid? Rearrange the formula to r = I / (P × t). Divide the interest paid by the principal multiplied by the time in years. ### Can simple interest apply to investments, not just loans? Yes. Some short-term deposits, T-bills, and note-style investments credit simple interest instead of compounding, especially over terms of a year or less. ### Why does the interest stay the same every year with simple interest? Because the formula always multiplies the rate and time by the original principal — not the growing balance — so each period produces an identical dollar amount of interest. ### Is an add-on interest car loan the same as simple interest? Add-on loans start with a simple interest calculation on the original amount, then spread principal plus that interest evenly across fixed payments — which can make the effective rate higher than a standard simple-interest note, so it is worth comparing the annual percentage rate (APR), not just the stated rate. ### Does simple interest ever get used on savings accounts? It is uncommon today — most banks compound savings interest daily or monthly — but some short-term certificates of deposit and note-style products still credit interest using the simple interest method. ## Related concepts - **Principal** — The original sum borrowed or invested before any interest is applied. - **Linear growth** — Growth that adds the same amount per period rather than accelerating, as with compounding. - **Compound interest** — Interest earned on both the principal and previously accumulated interest — the opposite growth pattern from simple interest. - **Day-count convention** — The method (360-day or 365-day year) used to convert a number of days into a fraction of a year for interest calculations. ## Related guides - [Simple vs Compound Interest: Linear Growth vs Interest on Interest](https://dothecalculation.com/blog/finance/simple-vs-compound-interest) — Compare simple interest and monthly compound interest, learn when each model fits, and use worked examples aligned to the calculators. - [Compound Interest: Formula, Examples, and Calculator](https://dothecalculation.com/blog/finance/compound-interest-deep-dive) — Learn how compound interest works, calculate growth with monthly deposits, compare assumptions, and avoid common projection mistakes. ## Related calculators - [Interest Calculator](https://dothecalculation.com/calculators/interest-calculator) — Calculate simple and compound interest scenarios instantly to see how principal, rate, and time affect your total earnings. - [Compound Interest Calculator](https://dothecalculation.com/calculators/compound-interest-calculator) — Model compound growth of savings or investments with regular deposits, interest rate, and time horizon to project future value. - [CAGR Calculator](https://dothecalculation.com/calculators/cagr-calculator) — Calculate the compound annual growth rate between a starting and ending value to measure investment performance over time. - [Payment Calculator](https://dothecalculation.com/calculators/payment-calculator) — Find your exact monthly loan payment instantly from the loan amount, repayment term, and interest rate you enter into the tool. - [Student Loan Calculator](https://dothecalculation.com/calculators/student-loan-calculator) — Estimate student loan monthly payments, total interest paid, and payoff timeline based on your balance, rate, and repayment term. - [Auto Loan Calculator](https://dothecalculation.com/calculators/auto-loan-calculator) — Estimate monthly car payments, total financing cost, and interest owed based on vehicle price, down payment, and loan term. --- _This calculator is for planning and education, not financial advice. Verify rates, day-count conventions, and contract terms with your lender, institution, or a licensed financial advisor before relying on these figures for a real loan or investment decision._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/simple-interest-calculator). Quote freely with attribution and a link to this page._