About the Interest Rate Calculator
Interest is the price of money — what a lender earns, or a borrower pays, for the use of a sum over time. This calculator works out simple interest: the interest charged on the original principal alone, along with the total amount due at the end. It is the right tool for short-term loans, certain fixed-period deposits and bonds, and any situation where interest does not reinvest.
The simple interest formula
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. The total you repay or receive is simply the principal plus that interest. For instance, ₹1,00,000 placed at 8% for 5 years earns ₹40,000 in interest, giving a total of ₹1,40,000. Because the interest is always a fixed slice of the unchanging principal, simple interest grows in a straight line — the same amount is added every year.
Simple versus compound interest
The crucial distinction in finance is whether interest itself earns interest. Simple interest never does — it is computed only on the original principal. Compound interest does — each period’s interest is added to the balance, so the next period earns interest on a larger sum. Over short periods the two are close, but over long periods they diverge dramatically. Take ₹1,00,000 at 8%:
| Period | Simple interest total | Compound interest total |
|---|---|---|
| 1 year | ₹1,08,000 | ₹1,08,000 |
| 5 years | ₹1,40,000 | ₹1,46,933 |
| 10 years | ₹1,80,000 | ₹2,15,892 |
After ten years the compound balance is over ₹35,000 ahead, and the gap keeps widening. This is why compound interest is described as working for you when you invest, and against you when you borrow.
Where simple interest applies
Despite compounding being more common, simple interest still appears in real life: many personal and short-term loans, some car loans, treasury bills, and bonds that pay a fixed coupon without reinvesting it. Knowing which one applies to a product changes the true cost or return significantly, so it is always worth asking a lender or checking the terms.
A handy shortcut: the Rule of 72
For compound interest, you can estimate how long money takes to double by dividing 72 by the annual rate: at 8%, roughly 9 years; at 12%, about 6 years. This rule is a quick sanity check on any growth claim. (Simple interest doubles only when the rate multiplied by the time reaches 100 — for example 10% for 10 years — so the Rule of 72 does not apply to it.)
Using the calculator
Enter the principal, the annual rate as a percentage, and the time in years to see the interest and the total. Treat the result as the contractual interest only; real products may add fees, taxes or compounding that change the effective figure. For monthly loan repayments use the EMI Calculator, and for regular investing the SIP Calculator. The maths runs entirely in your browser.
Frequently Asked Questions
What is simple interest?
Simple interest is calculated only on the original principal, never on previously earned interest. The formula is I = P × R × T ÷ 100, where P is the principal, R is the annual rate, and T is the time in years.
How is the total amount worked out?
Add the interest to the principal: Total = P + I = P × (1 + R × T ÷ 100). For example, ₹1,00,000 at 8% for 5 years earns ₹40,000 of simple interest, for a total of ₹1,40,000.
How does simple interest differ from compound interest?
Simple interest is charged only on the principal, so it grows in a straight line. Compound interest is charged on the principal plus accumulated interest, so it grows faster and faster. The longer the period, the bigger the gap.
When is simple interest actually used?
It is common on some short-term and personal loans, certain car loans, and many fixed-period instruments and bonds where interest does not reinvest. Most savings accounts, fixed deposits and credit cards, however, use compound interest.
What is the “Rule of 72”?
A quick mental shortcut for compound interest: divide 72 by the annual rate to estimate the years for money to double. At 8% that is about 9 years. It does not apply to simple interest, which doubles only when R × T reaches 100.
Should I enter the rate as a percentage or a decimal?
Enter it as a percentage, the way it is normally quoted — for example 8 for 8%. The formula divides by 100 internally.
Can I use this for any currency?
Yes. Interest maths is currency-neutral; the figures work the same whether you think in rupees, dollars or anything else.