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)
This formula assumes the rate applies to the original principal for the stated time. Loan contracts can instead use a daily declining balance, payment schedule, fees, or another calculation method.
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 |
| Effect at a positive rate | Interest grows evenly with time | Interest can grow faster as prior interest is added |
| Before using | Verify the contract applies the rate to original principal | Verify compounding frequency, fees, and the applicable rate |
Continue Your Interest Planning
Compare a compounding scenario with the Compound Interest Calculator. For a monthly repayment schedule, use the EMI Calculator instead of applying this simple-interest result.
You can also explore an eligible lump-sum repayment with the Home Loan Prepayment Calculator or check how a future amount compares in today's money with the Inflation Calculator.
Simple-interest formula vs loan schedules
This page uses the classroom formula on a fixed original principal. Some lenders also use the phrase simple interest for loans where interest accrues daily or monthly on the outstanding balance. Those balances change after payments, so their results require an amortization schedule and will not match this fixed-principal illustration.
The Consumer Financial Protection Bureau explanation of auto-loan interest describes how outstanding-balance simple interest differs from precomputed interest. The external resource opens in a new tab.
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
Written Agreements
A simple-interest calculation can help parties describe a principal, annual rate, and fixed term. The agreement remains the source of truth.
Short-Term Examples
Simple interest is often used in finance education and illustrations for a fixed principal and period.
Quoted Instruments
Some instruments may quote returns using a simple-interest convention. Always check the issuer's current terms and day-count method.
Private Lending
Friends or family may use a simple formula for clarity, but should agree on payment dates, fees, and applicable local requirements.
Product Comparisons
Deposit and loan products can use different interest methods. Compare the total cost or return under the exact product terms.
Financial Education
Simple interest is a useful starting point for understanding how principal, rate, and time relate.
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.