What is the difference between simple and compound interest?

Simple interest calculates on original principal only. Compound interest calculates on principal plus accumulated interest ('interest on interest'). For ₹1 lakh at 10% for 10 years: SI = ₹1 lakh extra; compound = ₹1.59 lakh extra. Most modern financial products use compound interest.

How do I convert monthly interest rate to annual rate?

For simple interest: multiply by 12 (1.5%/month = 18%/year). For compound interest: use EAR = (1 + monthly rate)^12 - 1 (1.5%/month compound = 19.56%/year). The difference matters when comparing loan offers — compound monthly rates cost more than their simple equivalent.

Simple Interest Calculator

Calculate simple interest on loans and investments — find total interest earned or owed, final amount, and interest rate using the SI formula. Free online interest calculator.

Why Use a Simple Interest Calculator?

Quick loan cost estimation: Calculate total interest cost on short-term personal loans, car loans, and credit before committing — compare the true cost of borrowing across different lenders.
Fixed deposit returns: Many bank FDs and short-term deposits use simple interest for periods under 1 year — calculate exact interest earned before maturity without compounding complexity.
Educational clarity: Simple interest is the foundational concept for understanding all interest calculations — mastering SI makes compound interest, APR, and amortization easier to understand.
Short-term investment analysis: Treasury bills, commercial paper, and money market instruments typically use simple interest — compare returns across different instruments at different rates.
Trade credit calculations: Suppliers offering 2/10 net 30 terms (2% discount if paid in 10 days) can be compared to borrowing at a rate — simple interest calculation reveals the implied annual rate of the discount.

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How to Use the Simple Interest Calculator

Enter the principal: Input the initial amount borrowed or invested (P) — this is the starting balance before any interest accrues.
Enter the interest rate: Input the annual interest rate as a percentage (R) — for monthly rates, multiply by 12 to get the annual equivalent first.
Enter the time period: Input duration in years (T) — for months, divide by 12; for days, divide by 365.
Calculate: The formula SI = P × R × T / 100 runs instantly — results show total interest, final amount (P + SI), and verification of inputs.
Solve backwards: Use any three known variables to find the fourth — calculate required principal, interest rate, or time period needed to reach a target interest amount.

Real-World Use Case

A small business owner takes a 90-day working capital loan of ₹5,00,000 at 12% per annum simple interest to fund seasonal inventory. Using the simple interest calculator: SI = ₹5,00,000 × 12% × (90/365) = ₹14,794. Total repayment = ₹5,14,794 after 90 days. The same owner is offered an alternative loan at 1.1% per month — converting to annual: 1.1% × 12 = 13.2% per annum. The second loan would cost ₹16,274 in interest for the same period, making the 12% annual loan significantly cheaper despite the monthly rate sounding smaller. The calculator reveals that the "lower" monthly rate is actually more expensive over the loan term.

Best Practices

Confirm time period conventions: Some lenders calculate interest on 365-day years while others use 360-day years (banker's rule) — ask which convention applies as it affects total interest by ~1.4%.
Compare APR not rate: Annual Percentage Rate (APR) includes fees in addition to interest — a loan at 10% interest rate with 2% processing fee has a higher effective APR; use the all-in cost for comparison.
Distinguish simple from flat rate: "Flat rate" loans (common for vehicle loans in some markets) charge interest on the original principal throughout the loan, even as you repay — a 10% flat rate is equivalent to approximately 18-19% reducing balance rate.
Account for day-count conventions: For precise calculations involving treasury instruments and bonds, verify whether actual/365, actual/360, or 30/360 day-count basis applies to your specific instrument.
Use reducing balance for multi-year loans: Simple interest is only appropriate for single-period calculations — multi-year loans with periodic payments should use compound or amortized interest calculations.

Performance & Limits

Formula: SI = (P × R × T) / 100; Amount = P + SI — exact formula with no rounding errors.
Principal range: Supports amounts from pennies to billions — suitable for personal loans and institutional instruments alike.
Rate precision: Handles rates up to 4 decimal places (e.g., 8.375% p.a.) for precise financial modeling.
Time period flexibility: Input years, months, or days — automatic conversion ensures correct SI calculation regardless of unit.
Reverse calculation: Solve for P, R, or T given the other three variables — finds required principal, target rate, or needed time period.

Common Mistakes to Avoid

Confusing annual rate with monthly rate: A lender quoting "1% per month" is charging 12% per year — always convert rates to the same time basis before comparing lenders.
Using simple interest for long-term savings: Bank savings accounts, mutual funds, and FDs over 1 year use compound interest — using simple interest underestimates returns and debt costs significantly for multi-year periods.
Ignoring processing fees in total cost: Interest is not the only cost of borrowing — processing fees, GST, insurance premiums, and pre-payment charges are all part of the true cost of a loan.
Applying the formula to reducing balance loans: EMI-based loans reduce the principal with each payment — applying simple interest to the original principal overestimates the total interest cost for amortizing loans.

Privacy & Security

Client-side calculation: All interest computations run in your browser — financial figures are never transmitted to servers.
No data stored: Principal, rate, and time inputs are not logged or retained between sessions.
No account required: Calculate without registration or personal information.
Session-only: All inputs clear when you navigate away from the page.

Frequently Asked Questions

What is the simple interest formula?

The simple interest formula is: SI = (P × R × T) / 100, where P is the principal amount (initial deposit or loan amount), R is the annual interest rate as a percentage, and T is the time period in years. The total amount at maturity is A = P + SI. For example: ₹10,000 at 8% for 3 years gives SI = (10,000 × 8 × 3) / 100 = ₹2,400, so the total amount is ₹12,400. Unlike compound interest, simple interest always calculates on the original principal — interest does not earn further interest, making it predictable and straightforward to calculate manually.

What is the difference between simple interest and compound interest?

Simple interest calculates interest only on the original principal throughout the entire period. Compound interest calculates interest on both the principal and accumulated interest — "interest on interest." For the same rate and period, compound interest always yields more than simple interest for an investor, and costs more for a borrower. For short periods (under 1 year) and low rates, the difference is small. For longer periods, the gap is significant: ₹1 lakh at 10% for 10 years earns ₹1 lakh in simple interest but approximately ₹1.59 lakh in compound interest. Most modern financial products use compound interest — savings accounts, mutual funds, mortgages, and credit cards all compound.

When is simple interest used in practice?

Simple interest is commonly used for: short-term loans under 1 year (personal loans, working capital, trade credit); some fixed deposits for periods less than 12 months; treasury bills, commercial paper, and certain money market instruments; post-maturity interest calculation on FDs after the maturity date; and accrued interest on bonds between coupon payment dates. In India, the RBI mandates that banks calculate interest on savings accounts using daily balance methods (effectively compound), while many NBFC personal loans still use flat/simple rate calculations. Always confirm whether a financial product uses simple or compound interest before comparing returns or costs.

How do I convert a monthly interest rate to an annual rate?

For simple interest, multiply the monthly rate by 12: 1.5% per month × 12 = 18% per annum. This direct multiplication works because simple interest adds linearly, not exponentially. However, for compound interest, use the effective annual rate (EAR) formula: EAR = (1 + monthly rate)^12 - 1. At 1.5% monthly compound rate, EAR = (1.015)^12 - 1 = 19.56% per annum — not just 18%. This distinction matters when comparing loan offers: a lender quoting compound interest at 1.5% per month costs more than 18% per annum simple interest, even though both sound like the same "1.5% monthly rate."