Simple Interest

1. Enter the **Principal (P)** — the initial amount of money borrowed or invested (e.g., $5,000).
2. Enter the **Annual Interest Rate (R)** as a percentage (e.g., 7%). The calculator converts it to a decimal automatically.
3. Enter the **Time (T)** in years, or select months and let the calculator convert to years for you.
4. Click **Calculate** to instantly see the total **Simple Interest (I)** and the **Total Amount (A = P + I)**.
5. Review the results. Adjust any input field to explore different scenarios in real time.

I = P × R × T

• I — The simple interest earned or charged, expressed in currency (e.g., dollars). This is the output of the formula.
• P — The principal — the original sum of money borrowed, invested, or deposited before any interest is applied.
• R — The annual interest rate expressed as a decimal. Convert a percentage to a decimal by dividing by 100 (e.g., 6% becomes 0.06).
• T — The time the money is borrowed or invested, expressed in years. If the term is given in months, divide by 12 (e.g., 18 months = 1.5 years).

I = P × R × T = $8,000 × 0.05 × 3 = $1,200

Result: Simple Interest (I) = **$1,200** | Total Amount (A) = $8,000 + $1,200 = **$9,200**

In this example, a $8,000 principal at a 5% annual rate over 3 years generates $1,200 in simple interest. The borrower or account holder ends up with a total balance of $9,200. Notice that the interest is the same each year ($400/year), because simple interest does not compound — it is always calculated on the original principal, not on accumulated interest.

What Is Simple Interest?

Simple interest is one of the most fundamental concepts in personal finance. Unlike compound interest, which charges or earns interest on previously accumulated interest, simple interest is always calculated on the original principal only. This makes it predictable and easy to understand.

When Is Simple Interest Used?

• Short-term personal loans and payday loans often use simple interest.
• Auto loans are frequently structured on simple interest so that paying early reduces total interest.
• U.S. Treasury Bills (T-Bills) use a simple interest-based discount rate.
• Savings bonds and some certificates of deposit may accrue simple interest.
• Academic finance courses use simple interest as a foundational teaching concept before introducing compounding.

Simple Interest vs. Compound Interest

| Feature | Simple Interest | Compound Interest | |---|---|---| | Calculated on | Original principal only | Principal + accumulated interest | | Growth pattern | Linear | Exponential | | Common uses | Short-term loans, auto loans | Mortgages, savings accounts, investments | | Benefit for borrower | Lower total interest on long terms | — | | Benefit for saver | — | Faster wealth accumulation |

Practical Tips

• Paying off a simple interest loan early reduces total interest because interest accrues daily on the outstanding balance.
• The rate must be annual unless specified otherwise. Monthly rates must be converted: divide by 12 before applying the formula.
• Time must match the rate period. If you use an annual rate, express time in years.

⚠️ Financial Disclaimer: Results from this calculator are estimates intended for educational and planning purposes only. Actual loan or savings amounts may differ based on your lender's calculation method, fees, compounding schedule, or other terms. Always consult your lender or a qualified financial advisor for precise figures.

• **Using the percentage rate instead of the decimal:** Always divide the percentage by 100 before plugging into the formula (e.g., 5% → 0.05, not 5).
• **Mixing time units:** If the annual rate is used, time must be in years. Convert months to years by dividing by 12 (e.g., 9 months = 0.75 years).
• **Confusing Interest (I) with Total Amount (A):** The formula gives you the interest earned/charged, not the final balance. Add I to P to get the total amount.
• **Assuming all loans use simple interest:** Mortgages, credit cards, and most savings accounts use compound interest, which yields different (usually higher) totals.
• **Forgetting fees and other charges:** Simple interest calculations do not include origination fees, late fees, or other lender charges that affect the true cost of a loan.

• Compound Interest and Simple Interest – Investor.gov — U.S. Securities and Exchange Commission (SEC) – Investor.gov
• Simple Interest – Khan Academy — Khan Academy
• Consumer Credit and Interest Rates — Board of Governors of the Federal Reserve System

Only sources that have been reviewed are shown. Unverified citations are never published.