Simple Interest Calculator

What does this Simple Interest Calculator do?

Simple interest is the most straightforward form of interest calculation: it's charged only on the original principal amount, for the entire duration of the loan or investment, with no compounding.

The formula

Simple Interest = (P × R × T) ÷ 100

Where P is the principal, R is the annual interest rate (as a percentage), and T is the time in years. Unlike compound interest, this amount never changes based on interest already accrued — it's a flat, linear calculation.

Step-by-step example

For ₹1,00,000 at 6% annual interest over 3 years: SI = (1,00,000 × 6 × 3) ÷ 100 = ₹18,000. Total amount payable/receivable = 1,00,000 + 18,000 = ₹1,18,000.

Tips & common mistakes

• Simple interest grows linearly (a straight line), while compound interest grows exponentially — over long periods, this difference becomes significant.
• Simple interest is common for short-term loans, some auto loans, and certain bonds — always check which method a lender or product actually uses before comparing rates.
• Don't forget to divide by 100 when the rate is expressed as a percentage — a common manual calculation error.

Where simple interest is still used today

While most modern loans and savings products use compound interest, simple interest remains common in specific contexts: certain short-term personal loans, some auto loans, treasury bills and certain bonds, and many basic textbook and classroom finance problems designed to teach the underlying concept before introducing compounding.

A quick comparison

On the same ₹1,00,000 principal at 6% for 3 years, simple interest gives exactly ₹18,000, while compound interest (annually) gives slightly more — about ₹19,102 — because each year's interest starts earning its own interest. The gap grows larger with higher rates and longer time periods.

When simple interest is actually the better deal for a borrower

Because simple interest doesn't compound, a borrower generally pays less total interest under simple interest than under compound interest at the same nominal rate and term — which is exactly why borrowers often prefer simple-interest loans, while savers and investors generally prefer compound-interest products.

Deriving the total repayment amount

Once you know the simple interest amount, total repayment is simply principal plus interest: Total = P + SI = P + (P×R×T)/100 = P × (1 + RT/100). This can be rearranged to solve for any single unknown (principal, rate, or time) if you know the other three values and the total.

A worked example: solving for the rate

Suppose you know a ₹50,000 loan accrued ₹9,000 in interest over 3 years, and you want to find the rate: rearranging the formula, R = (SI × 100) ÷ (P × T) = (9,000 × 100) ÷ (50,000 × 3) = 900,000 ÷ 150,000 = 6%. This kind of reverse calculation is common when comparing loan offers that don't clearly state their rate upfront.

Frequently asked questions