Simple Interest Calculator
What this calculator does
This calculator computes simple interest on a principal amount over a specified time period at a given annual interest rate. Unlike compound interest, simple interest is calculated only on the original principal—it does not earn interest on accumulated interest. The calculator shows the interest earned, total amount, and allows comparison with compound interest to illustrate the difference.
Simple interest is used in a variety of real-world financial products: auto loans, some personal loans, short-term certificates of deposit, Treasury bills, and certain savings instruments. It is also the foundation for understanding more complex interest calculations, making it a key concept in financial literacy.
Understanding the difference between simple and compound interest is one of the most important financial distinctions. Over short periods, the difference is small. Over decades, it is enormous—which is why compound interest is called the "eighth wonder of the world" and why borrowers should understand which type applies to their loan.
How to use it
- Enter the principal amount (the initial sum, e.g. $5,000).
- Enter the annual interest rate (e.g. 6%).
- Enter the time period in years (or months, which the calculator converts).
- Review the simple interest earned, total amount (principal + interest), and optional comparison with compound interest.
Formulas used
Simple interest: I = P × r × t where P = principal, r = annual rate (as a decimal), t = time in years. Example: $5,000 × 0.06 × 3 = $900
Total amount: A = P + I = P × (1 + r × t) Example: $5,000 + $900 = $5,900
For time in months: t = months / 12 Example: 18 months = 1.5 years. I = $5,000 × 0.06 × 1.5 = $450.
Comparison with compound interest (annual): A_compound = P × (1 + r)^t Example: $5,000 × 1.06³ = $5,955.08. Compound interest = $955.08 vs. simple = $900. Difference = $55.08 over 3 years.
Examples
Example 1 — Short-term loan: P = $10,000, r = 8%, t = 2 years. Interest = $10,000 × 0.08 × 2 = $1,600. Total repayment = $11,600.
Example 2 — Car loan (simple interest): P = $22,000, r = 5.5%, t = 5 years. Interest = $22,000 × 0.055 × 5 = $6,050. Total = $28,050. (Note: actual car loans use amortization, which is similar but not identical to simple interest. This gives a close approximation.)
Example 3 — Savings for 6 months: P = $3,000, r = 4.5%, t = 0.5 years. Interest = $3,000 × 0.045 × 0.5 = $67.50. Total = $3,067.50.
Tips and common mistakes
- Simple interest grows linearly; compound interest grows exponentially. Over 1–2 years, the difference is minor. Over 20+ years, compound interest generates dramatically more wealth (or costs dramatically more on a loan).
- Most loans and savings accounts use compound interest. Simple interest is less common in practice. Always check whether your financial product uses simple or compound interest—the terms "APR" (often simple) and "APY" (always compound) signal the difference.
- APR vs. APY: Annual Percentage Rate (APR) is the stated rate before compounding. Annual Percentage Yield (APY) includes the effect of compounding. A 6% APR compounded monthly yields an APY of 6.17%.
- Time must be in years for the standard formula. Convert months to a fraction (e.g. 9 months = 0.75 years) or days to a fraction (e.g. 90 days = 90/365 = 0.247 years).
- Interest on Treasury bills and commercial paper is typically calculated using simple interest with a 360-day year (banker's convention) rather than 365. This slightly overstates the effective rate.
- Early payoff saves interest. With simple interest loans, paying off the principal early directly reduces the interest charged (since I = P × r × t, and you are reducing t). This is not always true for compound interest loans with prepayment penalties.