Simple Interest Calculator — SI = (P × R × T) / 100
Simple interest is the most straightforward form of interest: you pay or earn interest only on the original principal, not on any accumulated interest. Enter your principal, annual rate, and time period to instantly calculate the total interest and final amount — useful for personal loans, treasury bills, pawn shop loans, car financing, and short-term savings.
P = Principal amount, R = Annual interest rate (as a percentage), T = Time in years. For months: T = months ÷ 12. For days: T = days ÷ 365.
Simple Interest vs Compound Interest — The Core Difference
With simple interest, interest is always calculated on the original principal. If you borrow ₹50,000 at 10% for 5 years, you pay ₹5,000 interest per year — every year — for a total of ₹25,000. With compound interest, the interest from Year 1 is added to the principal, and Year 2 interest is calculated on ₹55,000 — so each year you pay more than the last. Over 5 years at 10% compounded annually, the same ₹50,000 generates ₹30,526 in interest. The longer the time period, the larger the gap between SI and CI.
When Simple Interest is Used in the Real World
Simple interest is the basis for several real financial products. Car loans in India and the US often use a daily simple interest method on the outstanding balance, so paying early reduces the total interest cost. Treasury bills (91-day, 182-day, 364-day T-bills) issued by RBI are discount instruments that effectively use simple interest. Short-term personal loans from NBFCs and co-operative banks commonly quote simple interest rates. Pawn shop loans and gold loans also use simple monthly interest rates, typically 1–2% per month.
Calculating Monthly Simple Interest
When interest is expressed as an annual rate but you need a monthly figure: Monthly SI = (P × Annual Rate) / (100 × 12). For example, ₹1,00,000 at 12% per annum = ₹1,000 per month in simple interest. Alternatively, express time as a fraction of a year: 6 months = 0.5 years, 3 months = 0.25 years. For daily interest, divide the annual rate by 365: ₹1,00,000 at 12% p.a. = ₹32.88 per day in simple interest.
Practical Examples — What Simple Interest Looks Like
Example 1: Fixed deposit at a co-operative bank — ₹2,00,000 at 7.5% simple interest for 2 years: SI = (2,00,000 × 7.5 × 2) / 100 = ₹30,000. Total maturity = ₹2,30,000. Example 2: 91-day T-bill — ₹95,000 invested, matures at ₹1,00,000 in 91 days. Effective simple interest rate = (5,000 / 95,000) × (365 / 91) × 100 ≈ 21.15% annualized. Example 3: Gold loan — ₹50,000 at 1.5% monthly simple interest for 6 months: Total interest = 50,000 × 1.5% × 6 = ₹4,500.
Quick Reference
- Car loans (daily simple interest on declining balance)
- Treasury bills (91-day, 182-day, 364-day T-bills)
- Gold loans and pawn shop borrowing
- Short-term personal and NBFC loans
- Some fixed deposits at co-operative banks
- Inter-company loans and invoice discounting
Frequently Asked Questions
What is simple interest?
Simple interest is interest calculated only on the original principal, not on accumulated interest. If you deposit ₹10,000 at 5% for 3 years, you earn ₹500 per year — always on the original ₹10,000, never on a growing balance. The formula is SI = (P × R × T) / 100, where P is principal, R is the annual rate as a percentage, and T is time in years.
How is simple interest different from compound interest?
The key difference: simple interest is always calculated on the original principal, while compound interest is calculated on the principal plus all previously accumulated interest. Over short periods the difference is small. Over long periods, compound interest grows exponentially while simple interest grows linearly. On a ₹1,00,000 loan at 8% for 10 years: SI = ₹80,000; compound interest (annual) ≈ ₹1,15,892.
When is simple interest used in real life?
Simple interest is used in car loans (many use a simple daily interest calculation), personal loans with fixed EMIs, treasury bills and short-term government securities, pawn shop loans, and some savings bonds. It is also the basis for most short-term borrowing where interest is paid in full each period rather than added to the balance.
Can simple interest be calculated monthly?
Yes. For monthly calculations, convert the annual rate: Monthly rate = Annual rate / 12. For example, 12% annual = 1% monthly. Then SI per month = P × 1% = P × 0.01. Multiply by number of months for the total interest. Alternatively keep the formula in years and use a fractional time value: 3 months = 0.25 years.
Does the bank always use compound interest?
Most banks use compound interest for savings accounts, fixed deposits, and credit cards. However, some loan products — particularly certain vehicle loans, personal loans, and subsidized education loans — use simple interest on the outstanding principal. Always check the product terms: look for phrases like 'reducing balance' (effectively simple interest on declining principal) versus 'compound interest'.
What is the difference between interest rate and APR?
Interest rate is the base rate charged on the principal. APR (Annual Percentage Rate) includes the interest rate plus all additional fees, processing charges, and costs, expressed as an annual rate. For simple interest products, APR is typically close to the stated rate. For compound interest products with monthly compounding, APR is the nominal rate while APY (Annual Percentage Yield) reflects the true effective annual cost including compounding.
Is simple interest better for borrowers or lenders?
For short-term borrowing, simple interest is generally better for borrowers because there is no compounding penalty — you pay only on what you originally borrowed. For long-term investments, lenders (investors) prefer compound interest because returns grow exponentially. Simple interest is actually better for lenders in scenarios where the borrower repays early, because the interest due is proportional to the exact time used.