Compound Interest Explained: Formula, Examples, the Rule of 72 and How to Use It for Savings and Loans

Compound interest means earning interest on your interest. It is the reason small, steady savings can grow into large amounts over decades, and the reason debts can balloon when left unpaid. Understanding it changes how you save, invest and borrow.

Simple vs compound interest

With simple interest, you earn only on the original amount. ₹1,00,000 at 8% simple interest earns ₹8,000 every year, so after 10 years you have ₹1,80,000. With compound interest, each year's interest is added to the balance, and the next year's interest is calculated on the larger sum. After 10 years at 8%, ₹1,00,000 becomes ₹215,892.

The formula

A = P × (1 + r/n)^nt, where P is the principal, r is the annual rate as a decimal, n is the number of times interest is compounded per year and t is the number of years. For annual compounding, this simplifies to A = P × (1 + r)^t. Use the compound interest calculator to test any combination.

Time matters more than rate

Here is ₹1,00,000 growing at different rates, compounded annually. These are illustrations, not promises.

At 10%, the money multiplies nearly 17 times in 30 years, but only about 2.6 times in 10 years. Most of the growth happens late, which is why starting early is more powerful than saving more later.

The Rule of 72

Divide 72 by the annual rate to estimate how many years it takes to double your money. At 8%, it takes about 9 years. At 12%, about 6 years. At 6%, about 12 years. It is an approximation, but a handy one for quick decisions. Inflation works the same way against you, so check what your money buys using the inflation calculator.

Compounding frequency

The more frequently interest compounds, the higher the final amount, though the difference is small. ₹1,00,000 at 8% for 10 years gives ₹215,892 with annual compounding and ₹220,804 with quarterly compounding. Bank fixed deposits in India commonly compound quarterly. See our FD vs RD comparison.

Regular investing and compounding

Most people add money regularly rather than a lump sum. Investing ₹10,000 every month at an assumed 12% yearly return grows to about ₹2,323,391 in 10 years, ₹9,991,479 in 20 years and ₹35,299,138 in 30 years, of which your own contribution is ₹12 lakh, ₹24 lakh and ₹36 lakh. Market returns are not guaranteed and vary year to year. Model it in the SIP calculator and read our SIP guide.

Compounding works against borrowers

• Credit card balances carry interest of roughly 3% a month or more. Unpaid, they grow quickly.
• Long loans accumulate large interest, as shown in our EMI guide.
• Paying the full credit card bill each month avoids interest altogether.

Taxes and real returns

Interest and gains are often taxed, and inflation reduces the buying power of the final amount. A 7% deposit with 6% inflation and tax gives a very small real return. Always think in post-tax, post-inflation terms.

Practical rules

1. Start early, even with small amounts.
2. Stay invested and do not withdraw interest if you want growth.
3. Increase contributions each year, for example with a step-up SIP.
4. Reduce high-interest debt first.
5. Be sceptical of any scheme promising high, guaranteed compounding returns.

All figures are illustrations at assumed rates and do not guarantee future returns.

A small example of starting early

Two friends invest for retirement at 60. Asha invests ₹5,000 a month from age 25 to 35 and then stops, investing for ten years in total. Ravi waits until 35 and invests ₹5,000 a month for 25 years until 60. At an assumed 10% a year, Asha's ten years of contributions can grow to more than Ravi's twenty-five years, because her money compounds for much longer. The figures are only illustrations, but the lesson holds: time in the market is worth more than a larger amount invested late. Run both cases in the SIP calculator.

Using compounding in daily decisions

Before buying something on a no-cost EMI or paying only the minimum on a card, calculate what the money could have earned or cost you over five or ten years. Direct your savings to goals in order: clear any debt costing more than 12% first, then invest the surplus. Review long-term deposits and funds once a year, and reinvest maturity proceeds quickly so that the compounding chain is not broken. Small leaks matter too: a fee of 1% a year on a long-term investment can cost a fifth or more of the final value over thirty years.

Frequently asked questions

What is compound interest?

Interest earned on both the original amount and the interest already added.

What is the Rule of 72?

Divide 72 by the annual rate to estimate the years needed to double money.

Does compounding frequency matter?

More frequent compounding gives slightly higher returns, but the rate and time matter much more.

How does compound interest affect loans?

Interest on unpaid balances grows quickly, especially on credit cards.

Are SIP returns compound?

Yes, in the sense that gains are reinvested, though returns are not fixed or guaranteed.

Is compound interest taxable?

Interest and gains are generally taxable depending on the product and your slab.

Last reviewed 8 November 2026. Rates, limits and rules change, so confirm figures with the official source before you act.