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. 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 |
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.
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.
- Federal Reserve. "Consumer Handbook on Adjustable-Rate Mortgages." federalreserve.gov.
- U.S. Securities and Exchange Commission. "Interest Rate Basics." investor.gov.
- Consumer Financial Protection Bureau. "What Is Simple Interest?" consumerfinance.gov.
- Financial Industry Regulatory Authority. "Simple vs Compound Interest." finra.org.
- Investopedia. "Simple Interest Definition." investopedia.com.
Last updated: May 12, 2026