Simple Interest Calculator
Simple Interest Calculator
The Simple Interest Calculator computes interest for loans or investments using the simple interest formula. Unlike compound interest, simple interest accrues linearly over time based only on the original principal amount. Simple interest is commonly used for short-term loans, auto loans (in some cases), personal loans between individuals, Treasury bills, commercial paper, and certain types of bonds. Understanding simple interest is fundamental to financial literacy and provides the foundation for more complex interest calculations.
Simple interest is straightforward: you earn or pay interest only on the original principal, not on accumulated interest. [cfpb-simple] This makes it predictable and easy to calculate. For example, if you lend $10,000 at 5 percent simple interest for 3 years, you will earn $1,500 in total interest ($10,000 times 0.05 times 3 = $1,500). The borrower repays $11,500 total.
Real-world examples demonstrate the practical application of simple interest. A small business owner needs a $50,000 short-term loan to cover inventory for the holiday season. A private lender offers a 6-month loan at 8 percent simple interest. Using the formula, the interest would be $50,000 times 0.08 times 0.5, which equals $2,000, making the total repayment $52,000. If the loan used the Actual/360 convention for 180 days, the interest would be $50,000 times 0.08 times 180 divided by 360, which equals $2,000 as well since 180/360 equals 0.5. This precision matters for larger loan amounts and longer durations.
Simple interest appears in many everyday financial situations. Short-term personal loans between family members often use simple interest to keep calculations transparent and fair. Treasury bills, which are short-term government securities with maturities of 4, 13, 26, or 52 weeks, are sold at a discount and mature at face value, with the difference representing simple interest. A $25,000 auto loan at 6 percent simple interest for 5 years results in total interest of $7,500, and if paid off after 2 years, the borrower would only pay $3,000 in interest rather than the full $7,500.
Enter the principal amount (P) in dollars. Enter the annual interest rate (r) as a percentage. Enter the time period in years (t), which can include fractional years. Optionally select to use days with a day-count convention (Actual/365 or Actual/360) for partial-year calculations. Press Calculate to see the simple interest earned or owed and the total amount due.
For example, a $10,000 investment at 4.5 percent simple interest for 5 years yields $2,250 in interest and a total of $12,250. A $5,000 personal loan at 6 percent for 2 years results in $600 in interest and a total repayment of $5,600.
For a 90-day loan of $2,000 at 8 percent, using the Actual/365 convention, the interest is $2,000 times 0.08 times 90 divided by 365, equaling approximately $39.45, bringing the total repayment to $2,039.45.
For more information, see the Interest Rate Converter.
The simple interest formula:
Where I is the interest amount, P is the principal, r is the annual interest rate expressed as a decimal, and t is the time in years.
Total amount due (principal plus interest):
For partial years using days, using Actual/365:
Using Actual/360 (used in money markets):
Simple interest earned on $10,000 at various rates and terms:
| Rate | 1 Year | 3 Years | 5 Years | 10 Years |
|---|---|---|---|---|
| 2% | $200 | $600 | $1,000 | $2,000 |
| 3% | $300 | $900 | $1,500 | $3,000 |
| 4% | $400 | $1,200 | $2,000 | $4,000 |
| 5% | $500 | $1,500 | $2,500 | $5,000 |
| 6% | $600 | $1,800 | $3,000 | $6,000 |
| 8% | $800 | $2,400 | $4,000 | $8,000 |
| 10% | $1,000 | $3,000 | $5,000 | $10,000 |
Common day-count conventions:
| Convention | Description | Typical Use |
|---|---|---|
| Actual/365 | Actual days divided by 365 | Corporate bonds, personal loans |
| Actual/360 | Actual days divided by 360 | Money markets, commercial paper |
| 30/360 | 30-day months, 360-day year | Corporate bonds, mortgages |
| Actual/Actual | Actual days divided by actual days in year | Government bonds |
The fundamental difference between simple interest and compound interest is what earns interest over time. Simple interest applies only to the initial principal, producing a linear growth pattern where each period adds exactly the same amount. Compound interest applies to the principal plus any interest already accumulated, which means the base grows each period and produces exponential growth. This difference, while small in the first year, becomes dramatic over longer periods.
The simple interest formula:
The compound interest formula for annual compounding, where A is the total amount after time t:
The compound interest earned is A minus P:
Consider a $10,000 investment at 6 percent annually:
| Year | Simple Interest | Annual Compound Interest | Difference |
|---|---|---|---|
| 1 | $600 | $600 | $0 |
| 5 | $3,000 | $3,382 | $382 |
| 10 | $6,000 | $7,908 | $1,908 |
| 20 | $12,000 | $22,071 | $10,071 |
| 30 | $18,000 | $47,435 | $29,435 |
Year 1 produces identical interest because no compounding has occurred yet. By year 10, the compound investment has earned $7,908 in interest, which is $1,908 more than the simple interest amount of $6,000. By year 30, compound interest totals $47,435, more than 2.6 times the $18,000 earned with simple interest. The gap widens with each additional year as the compounding base expands.
Simple interest is the standard for short-term instruments. Auto loans in many states use simple interest, where prepaying principal saves future interest. Short-term personal loans, payday loans, and loans between individuals commonly specify simple interest for clarity. Treasury bills, commercial paper, and other money market instruments under one year use simple interest with specific day-count conventions. Accrued interest on bonds between coupon payment dates is also calculated using simple interest conventions.
Compound interest is universal for long-term financial products. Savings accounts, certificates of deposit, retirement accounts, mortgages amortized over 15 to 30 years, credit card balances, student loans, and most investment accounts all compound at regular intervals. A credit card balance at 18 percent APR compounded daily accumulates significantly more interest over a year than simple interest at the same rate. A retirement account earning 7 percent compound returns over 30 years produces nearly $76,123 on a $10,000 investment, more than double the $31,000 under simple interest. This exponential effect is why long-term investors benefit enormously from starting early and letting compounding work over decades.
For more information, see the APR Calculator.
Simple interest appears across many financial products, often in ways that materially affect total borrowing costs.
Auto loans are among the most common simple interest products. With a simple interest auto loan, interest accrues daily on the outstanding principal balance. Making extra payments or paying off the loan early directly reduces the total interest paid. A $30,000 auto loan at 5 percent simple interest over 60 months incurs $7,500 in total interest if held to full term. If the borrower pays off the loan after 24 months, the interest is only $3,000, saving $4,500. This contrasts with precomputed interest loans where total interest is fixed at origination regardless of early repayment.
Personal loans from banks and credit unions use simple interest on a declining balance. Each monthly payment reduces the principal, and the next month's interest is calculated only on the remaining balance. A $15,000 personal loan at 8 percent simple interest over 3 years has a nominal total interest of $3,600 if calculated on the full principal for the full term. In practice, the declining balance through monthly payments means the actual total interest paid is lower because principal is gradually repaid throughout the loan period. The APR captures this amortization structure.
Treasury bills are sold at a discount and redeemed at face value, with the difference representing simple interest calculated using the Actual/360 convention. A 26-week $10,000 T-bill at 4 percent discount rate costs $10,000 times 1 minus 0.04 times 182 divided by 360, which equals $9,797.78, and returns $10,000 at maturity for a gain of $202.22. The discount pricing structure means the effective yield is slightly higher than the stated discount rate.
Promissory notes between individuals specify simple interest for transparency. Unlike complex amortization schedules, a simple interest note can be calculated by either party with basic arithmetic. A $50,000 family loan at 3 percent simple interest over 5 years produces $7,500 in total interest and total repayment of $57,500. This simplicity makes private lending agreements enforceable and understandable without professional assistance.
The day-count convention used for partial-year calculations materially affects the interest amount. Different financial markets have standardized on different conventions, and using the wrong one can change the total cost.
Actual/360, also called the money market convention, divides actual days by 360 rather than 365. This produces interest approximately 1.39 percent higher than a 365-day year because the denominator is smaller. Actual/360 is used for US money markets, commercial paper, Treasury bills, short-term business loans, and bank certificates of deposit. Borrowers pay slightly more under this convention for the same nominal rate.
Actual/365 divides actual days by 365. This convention is standard in the United Kingdom, Australia, and many Commonwealth countries for consumer loans. It is also used for US Treasury coupon securities and many corporate bonds. This approach is more intuitive because it matches the calendar year.
Actual/Actual divides actual days in the interest period by the actual number of days in the current year, which is 365 normally or 366 in a leap year. This precision is used for US Treasury bonds, TIPS, and most government bonds internationally. It ensures borrowers pay only for the exact fraction of the year the money is outstanding.
30/360 assumes each month has 30 days and each year has 360 days, simplifying manual calculations. This convention is common for corporate bonds, mortgages, and agency securities. It was developed when calculations were performed by hand and remains widely used in certain markets.
The difference is material. Consider $10,000 at 5 percent for 90 days:
| Convention | Calculation | Interest |
|---|---|---|
| Actual/360 | $10,000 times 0.05 times 90 divided by 360 | $125.00 |
| Actual/365 | $10,000 times 0.05 times 90 divided by 365 | $123.29 |
| 30/360 | $10,000 times 0.05 times 90 divided by 360 | $125.00 |
| Actual/Actual | $10,000 times 0.05 times 90 divided by 365 | $123.29 |
Actual/360 and 30/360 produce $125.00, while Actual/365 gives $123.29. On a $1 million commercial loan at 6 percent for 90 days, the difference between Actual/360 at $15,000 and Actual/365 at $14,795 is $205. For institutional-scale borrowing, this difference justifies careful attention to the convention specified in the loan agreement.
Simple interest is best suited for short-term loans and investments where the compounding effect is minimal. For longer terms, compound interest calculations provide a more accurate picture of true financial outcomes. When borrowing money, always check whether the interest is calculated using simple or compound interest, as this significantly affects the total cost. For investments, look for accounts that compound interest frequently (daily or monthly) to maximize returns.
When lending money to friends or family, using simple interest with a written agreement ensures both parties understand the terms clearly. A simple interest loan of $5,000 at 4 percent for 2 years results in $400 in interest, which is fair and transparent. Simple interest is also commonly used for car loans between private parties, bridge loans, and seller-financed real estate transactions.
For short-term business financing, simple interest loans offer transparency and predictability. A business taking a 90-day working capital loan of $25,000 at 7 percent simple interest using Actual/365 would pay about $431 in interest, making the total repayment $25,431. This straightforward calculation helps business owners accurately compare financing options and understand their true cost of capital for inventory purchases, equipment financing, or seasonal working capital needs. When evaluating multiple loan offers, comparing the total interest cost using identical assumptions about principal, rate, and term reveals which financing option is most cost-effective for your business needs. Understanding the impact of day-count conventions on short-term interest calculations helps you negotiate better terms and avoid unexpected costs when borrowing or lending money for periods of less than one year.
When comparing loan offers, always verify whether the interest is simple or compound and which day-count convention applies. Two loans with identical nominal rates can have meaningfully different total costs depending on these factors. Ask lenders directly about their calculation method and confirm it in writing.
Understand the difference between APR and the simple interest rate. APR includes fees and other borrowing costs beyond the interest rate itself. On a simple interest loan with no fees, the APR equals the stated interest rate, but loans with origination fees, closing costs, or annual fees have an APR that exceeds the simple interest rate. Comparing APRs across offers gives a more complete picture of total borrowing costs.
Negotiate the day-count convention on large loans. For significant commercial loans or private lending arrangements, specifying Actual/365 instead of Actual/360 can save meaningful money. A $500,000 loan at 7 percent for 180 days costs $17,500 under Actual/360 versus $17,260 under Actual/365, a savings of $240 from a simple contractual change.
Verify interest calculations on loan statements using the simple interest formula. For an auto loan with a $20,000 balance at 4.5 percent, monthly interest should be approximately $20,000 times 0.045 times 30 divided by 365, which equals about $73.97. If the statement shows a significantly different amount, ask the lender to explain their calculation method before making additional payments.
This calculator ignores compounding and is unsuitable for instruments that compound interest periodically, such as most savings accounts, credit cards, and long-term investments. The day-count convention significantly affects accuracy for short-term calculations, so choose the convention that matches your specific financial instrument. Simple interest also does not account for fees, prepayment penalties, or variable rates. For long-term financial planning, the compound interest calculator provides a more realistic projection.
- What is the difference between simple interest and compound interest?
- Simple interest is calculated only on the original principal amount, while compound interest is calculated on the principal plus any previously accumulated interest. Over time, compound interest produces higher returns for lenders and higher costs for borrowers. For short-term loans under one year, the difference is minimal.
- Which day-count convention should I use?
- Use Actual/365 for most personal loans and corporate bonds. Use Actual/360 for money market instruments and commercial paper. Use 30/360 for corporate bonds and mortgages. Choose the convention that matches your specific financial instrument.
- When is simple interest used in real estate?
- Simple interest is commonly used for seller-financed real estate transactions where the seller acts as the lender. Bridge loans and hard money loans, which are short-term real estate financing tools, also frequently use simple interest due to their short durations ranging from six months to three years.
- What types of loans use simple interest versus compound interest?
- Simple interest is used for auto loans, Treasury bills, commercial paper, short-term personal loans, payday loans, and private promissory notes. Compound interest is used for savings accounts, certificates of deposit, mortgages, credit cards, student loans, retirement accounts, and most long-term investments. The dividing line is generally loan duration: short-term products favor simple interest while long-term products compound.
- How does prepaying a simple interest loan reduce the total interest paid?
- With simple interest, interest accrues only on the outstanding principal. When you make an extra payment or pay off the loan early, the principal balance drops, and no future interest accrues on the repaid amount. This differs from precomputed interest loans, where the total interest is fixed at origination and early payoff does not reduce it. Simple interest loans always reward early repayment.
- What is the difference between add-on interest and simple interest?
- Add-on interest calculates total interest on the original principal for the full term and adds it to the principal, dividing the sum into equal monthly payments. This makes the borrower pay interest on principal that has already been repaid, resulting in a much higher effective cost. Simple interest on a declining balance is significantly cheaper. Add-on interest is considered predatory in many jurisdictions and is banned for consumer loans in some states.
- How can I convert a simple interest rate to an estimate of compound interest?
- For annual compounding at the same nominal rate, use A equals P times (1 plus r) to the power of t, then subtract P to find the interest. As a rule of thumb, for moderate rates and terms under 5 years, the difference is modest. For longer terms or higher rates, compound interest substantially exceeds simple interest. Use this calculator for the simple portion and the Compound Interest Calculator for the compound portion to compare directly.
- What does a 360-day year convention mean for borrowers?
- A 360-day year convention means the daily interest rate is calculated by dividing the annual rate by 360 instead of 365. This results in a slightly higher daily rate, so borrowers pay about 1.39 percent more interest over a full year compared to a 365-day convention at the same nominal rate. The difference is most significant for large commercial loans and short-term money market instruments.
- Why do credit cards use daily compounding instead of simple interest?
- Credit card issuers use daily compounding because it maximizes the interest earned on outstanding balances. With daily compounding, each day the balance grows, the next day interest accrues on a slightly larger base. Over a full year, a 20 percent APR compounded daily produces an effective annual rate of approximately 22.13 percent, compared to exactly 20 percent under simple interest. This compounding is disclosed in the APR terms on credit card agreements.
- How can I verify interest calculations on a loan statement?
- Multiply the outstanding principal balance by the annual interest rate, divide by 365 or the applicable day-count denominator, and multiply by the number of days in the billing period. For a $20,000 auto loan at 4.5 percent over a 30-day period using Actual/365, the interest is $20,000 times 0.045 times 30 divided by 365, which is approximately $73.97. If your statement shows a significantly different amount, ask the lender to explain their calculation method.
- [1]Federal Reserve. (n.d.). Consumer Handbook on Adjustable-Rate Mortgages.
- [2]U.S. Securities and Exchange Commission. (n.d.). Interest Rate Basics.
- [3]Consumer Financial Protection Bureau. (n.d.). What Is Simple Interest?
- [4]Financial Industry Regulatory Authority. (n.d.). Simple vs Compound Interest.
- [5]Investopedia. (n.d.). Simple Interest Definition.
Last updated: July 10, 2026
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