Simple Interest Calculator
Calculate Interest, Principal, Rate, or Time — any variable instantly
Find simple interest for any loan or deposit. Switch between calculating interest amount, the principal, the annual rate, or the time period. Works for years, months, and days — perfect for personal loans, short-term deposits, and financial studies.
What Is Simple Interest?
Simple interest is the most straightforward method of calculating interest on a loan or investment. Unlike compound interest, which adds earned interest back to the principal, simple interest is always calculated only on the original principal amount. This makes it easier to understand and predict.
The formula is: SI = P × R × T, where P is the principal, R is the annual interest rate (as a decimal), and T is the time in years. The total amount repaid or received is A = P + SI.
Simple interest is commonly used for car loans, personal loans, short-term fixed deposits, and educational financing. While it is less common for long-term investments (which use compound interest), understanding it is essential for comparing financial products accurately.
How to Use the Simple Interest Calculator
- Choose what to calculate: Use the "Calculate" dropdown to pick what you want to find — Interest Amount, Principal, Interest Rate, or Time Period.
- Fill in the known values: Enter the values you already know. For example, to find interest, enter the Principal, Rate, and Time.
- Select the time unit: Switch between Years, Months, or Days. The calculator automatically converts to years for the formula.
- Read the instant results: The result box shows your answer, plus a full breakdown of Principal, Total Interest, and Total Amount.
You can switch between all four modes at any time without losing your entered values. This makes it easy to cross-check calculations.
Simple Interest Formula and Derivations
Standard Formula
SI = P × R × T
Where: P = Principal, R = Annual Rate (decimal), T = Time (years). Total Amount = P + SI = P × (1 + R × T).
Finding Principal
P = SI / (R × T) — useful when you know the interest paid on a loan and want to find the original borrowed amount.
Finding Rate
R = SI / (P × T) × 100 — useful for comparing loans or deposits when the stated rate is not clear.
Finding Time
T = SI / (P × R) — useful when you want to know how long until a certain interest amount accumulates.
Simple vs Compound Interest
At 10% for 5 years on ₹1,00,000: Simple Interest = ₹50,000. Compound Interest (annual) = ₹61,051. The difference grows larger with higher rates and longer time periods.
Common Simple Interest Scenarios
| Principal | Rate | Time | Interest | Total Amount |
|---|---|---|---|---|
| ₹10,000 | 8% | 1 year | ₹800 | ₹10,800 |
| ₹50,000 | 10% | 2 years | ₹10,000 | ₹60,000 |
| ₹1,00,000 | 7% | 3 years | ₹21,000 | ₹1,21,000 |
| ₹2,00,000 | 9% | 5 years | ₹90,000 | ₹2,90,000 |
| ₹5,00,000 | 12% | 18 months | ₹90,000 | ₹5,90,000 |
Worked Examples
Example 1: Personal Loan Interest
You borrow ₹2,00,000 at 12% p.a. simple interest for 2 years.
SI = 2,00,000 × 0.12 × 2 = ₹48,000. Total repayment = ₹2,48,000.
Example 2: Find the Rate on a Loan
You borrowed ₹50,000 and paid ₹6,000 interest over 18 months.
Time in years = 18/12 = 1.5 years. Rate = (6,000 / (50,000 × 1.5)) × 100 = 8% p.a.
Example 3: How Long to Earn a Target Interest
You have ₹1,00,000 at 7.5% p.a. How long to earn ₹30,000 interest?
T = 30,000 / (1,00,000 × 0.075) = 4 years.
Tips for Using Simple Interest in Real Life
- Check whether your loan uses simple or compound interest: Most personal loans in India use flat rate (simple) interest, but the effective rate is much higher than the nominal rate stated.
- Compare flat rate to reducing balance: A 10% flat rate loan can effectively cost 18%+ when compared to a reducing balance loan. Always ask lenders which method they use.
- Use months for short loans: If your loan tenure is 6, 9, or 18 months, use the months option in the calculator to avoid manual conversion errors.
- Simple interest favors the borrower over time: Since interest doesn't compound, the total interest stays fixed regardless of when you pay. Early repayment reduces your actual effective cost.
- Verify your bank statement math: Use this calculator to cross-check interest charges on your loan statements to ensure accuracy.
Frequently Asked Questions
Simple Interest = Principal × Rate × Time (SI = P × R × T). Total amount = Principal + Interest. For example, ₹1,00,000 at 7% for 3 years: SI = 1,00,000 × 0.07 × 3 = ₹21,000. Total = ₹1,21,000.
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus accumulated interest each period — it grows faster over time. For the same rate and period, compound interest always yields a larger total than simple interest.
Simple interest is used for short-term personal loans, car loans (flat rate), some savings accounts, Post Office deposits, and educational financing. Most long-term investments like fixed deposits and mortgages use compound interest instead.
Principal = Interest / (Rate × Time). For example, if you paid ₹15,000 in interest at 10% over 2 years: P = 15,000 / (0.10 × 2) = ₹75,000. Use the "Calculate Principal" mode in our calculator above for instant results.
Convert to years first: months / 12 or days / 365. Then apply SI = P × R × T. Our calculator handles this automatically — just select "Months" or "Days" in the Time Unit dropdown and enter the number directly.
Total Amount (A) = Principal (P) + Simple Interest (SI) = P + (P × R × T) = P × (1 + R × T). This is linear — it grows by a fixed amount each period. Compound interest grows exponentially: A = P × (1 + R/n)^(n×t).
Yes. Select "Interest Rate" from the Calculate dropdown, then enter the Principal, known Interest amount, and Time period. The calculator instantly shows: Rate = Interest / (Principal × Time) × 100. This is useful for reverse-engineering rates on existing loans or deposits.