A simple interest calculator computes interest on the original principal only — there is no compounding. It's used for short-term loans, personal loans, auto loans, bonds, and savings accounts that pay a flat rate. The formula is I = P × R × T.
How to Use This Calculator
- Enter the principal (P) — the initial amount borrowed or invested.
- Enter the annual interest rate (R) as a percentage.
- Enter the time period (T) in years (or months, converted to years).
- Click Calculate to see total interest earned/paid and the final total amount.
What This Calculator Measures
- Principal (P) — The original loan amount or initial deposit.
- Interest rate (R) — The annual rate, expressed as a decimal in the formula.
- Time (T) — The duration in years over which interest accrues.
- Total interest (I) — The flat interest charged or earned over the entire period.
Formula or Logic
Simple Interest: I = P × R × T
Total Amount: A = P + I = P × (1 + R × T)
Unlike compound interest, simple interest does not earn interest on previously accumulated interest. This is favorable for borrowers but less beneficial for long-term investors.
Example Calculations
Example 1: $5,000 principal, 6% annual rate, 3 years. I = $5,000 × 0.06 × 3 = $900. Total = $5,900.
Example 2: $20,000 car loan at 8% for 4 years. I = $20,000 × 0.08 × 4 = $6,400. Total repaid = $26,400. Monthly payment ≈ $550.
Understanding Your Results
Simple interest calculations are linear — interest does not accelerate over time. Compare this to compound interest on the same principal: a $5,000 deposit at 6% compounded annually for 3 years earns $955 vs. $900 under simple interest.
Common Mistakes to Avoid
- Confusing annual rate with monthly rate — divide the annual rate by 12 for monthly periods, and express T in months accordingly.
- Using simple interest to project long-term investment returns — compound interest applies to most investments.
- Not checking whether a "simple interest" loan actually uses the add-on method, which yields a higher effective rate than it appears.
- Forgetting to convert days to years (divide by 365) for short-term money market calculations.