The Simple Interest Formula
Simple interest is calculated as: Simple Interest = (P × R × T) / 100, where P is the principal (original amount), R is the annual interest rate (as a percentage), and T is the time period in years. The total amount payable or receivable is the principal plus this interest.
A Worked Example
If you borrow ₹50,000 at a simple interest rate of 10% per year for 3 years: Interest = (50,000 × 10 × 3) / 100 = ₹15,000. The total repayable amount is ₹50,000 + ₹15,000 = ₹65,000 — and this ₹15,000 stays exactly the same each year (₹5,000/year), unlike compound interest where the yearly amount would grow.
Why Simple Interest Never Compounds
The defining feature of simple interest is that it is always calculated on the original principal only — never on interest that has already accrued. This means the interest amount for each period is identical, making the total interest a straightforward multiplication rather than a year-by-year calculation.
Where Simple Interest Is Still Used
Simple interest is commonly used for certain short-term loans, some fixed-term instruments, and situations where a lender or borrower wants a straightforward, predictable interest cost calculated upfront. It’s also the easiest way to sanity-check a quoted total repayment amount, since the calculation involves no compounding assumptions to verify.
Simple Interest vs Compound Interest at a Glance
For the same principal, rate, and time period, simple interest will always be less than or equal to compound interest (they are equal only when there is just one compounding period). The longer the time period, the bigger this gap becomes — which is why understanding which type applies to a loan or investment materially changes what you should expect to pay or earn.