What Is Simple Interest?
Simple interest is the most basic method for calculating the cost of borrowing money or the return on a deposit. Unlike compound interest, simple interest is calculated only on the original principal amount — it never compounds on previously earned interest. This makes the interest amount the same in every period, growing linearly rather than exponentially over time.
Simple Interest Formula
SI = (P × R × T) ÷ 100
P
Principal — the original amount
R
Rate — annual interest rate (%)
T
Time — duration in years
Total Amount (Maturity Value) = P + SI = P(1 + RT/100)
For example: a $100,000 loan at 8% annual interest for 3 years generates SI = (100,000 × 8 × 3) ÷ 100 = $24,000 in interest, for a total repayment of $124,000.
Simple vs Compound Interest
The key difference between simple and compound interest is whether interest is calculated on just the principal, or on the principal plus all previously accumulated interest.
| Simple Interest | Compound Interest | |
|---|---|---|
| Calculated on | Original principal only | Principal + accumulated interest |
| Growth pattern | Linear (constant per period) | Exponential (grows each period) |
| Formula | SI = PRT / 100 | A = P(1 + R/100)ᵗ |
| Best for | Short-term loans, simple bonds | Long-term investments, savings |
| Favors | Borrowers (lower total cost) | Lenders/investors (higher returns) |
Real-World Simple Interest Examples
| Principal | Rate | Time | Interest | Total Amount |
|---|---|---|---|---|
| $50,000 | 6% | 1 yrs | $3,000 | $53,000 |
| $100,000 | 8% | 3 yrs | $24,000 | $124,000 |
| $250,000 | 10% | 2 yrs | $50,000 | $300,000 |
| $500,000 | 7.5% | 5 yrs | $187,500 | $687,500 |
| $1,000,000 | 9% | 4 yrs | $360,000 | $1,360,000 |
Where Simple Interest Is Used
Auto Loans
Many car loans use simple interest calculated on the outstanding principal.
Short-Term Loans
Personal loans and bridge loans under 1-2 years often use simple interest.
Treasury Bills
T-bills and certain government bonds calculate returns using simple interest.
Private Lending
Loans between friends, family, or informal lenders typically use simple interest for clarity.
Some Fixed Deposits
Certain short-term FDs and CDs pay simple interest rather than compounding.
Financial Education
Simple interest is the foundational concept taught before introducing compounding.
Frequently Asked Questions
Disclaimer: This calculator provides estimates based on the standard simple interest formula for educational and planning purposes. Actual loan or investment terms may include additional fees, different day-count conventions, or hybrid interest structures. Always consult your loan agreement or a financial advisor for exact figures.