Interest: The Price of Time
Why starting ten years earlier can nearly triple what you end up with, and why the same maths makes credit cards so dangerous.
Two friends, the same ₹2,000 a month, very different endings
Meet and Krupa studied together in Surat and both decided to invest ₹2,000 a month until they turn 60. Meet started at 22, with his first stipend. Krupa started at 32, once she felt settled. Assume both earn an average of 10% a year, which is a reasonable long-run assumption for a diversified equity fund but is never guaranteed. At 60, Meet has about ₹87 lakh. Krupa has about ₹32 lakh.
Meet put in only ₹2.4 lakh more than Krupa over those extra ten years. Yet he ends up with ₹55 lakh more. The difference is not talent, luck or a better fund. It is time, working through a mechanism called compounding, which is the single most important idea in personal finance. This unit explains how it works, why it works so slowly at first and so fast later, and why the same mechanism quietly works against anyone who borrows at high interest.
The compounding calculator
Start ten years later, at 32, and the same ₹2,000 a month grows to only ₹32.2 lakh.
Change the monthly amount, the return and the starting age. Watch how much of the final number is growth rather than your own money.
Interest Is the Price of Time
Money today is worth more than the same money next year. If you lend ₹10,000 to someone for a year, you give up everything you could have done with it in that time, and you take a risk that it will not come back. Interest is the payment for that wait and that risk. When you deposit money, the bank pays you interest. When you borrow, you pay it.
An interest rate is quoted as a percentage per year. A 7% FD pays ₹700 a year on ₹10,000. A home loan at 8.5% costs ₹8,500 a year on every ₹1 lakh borrowed, at least at the start. The rate depends on how long the money is tied up, how risky the borrower is, and on the RBI’s repo rate, which acts as the base price of money in the whole economy.
Simple Interest and Compound Interest
Simple interest is paid only on the original amount, called the principal. Put ₹10,000 in at 10% simple interest and you earn ₹1,000 every year, forever. After 30 years you have ₹40,000.
Compound interest is paid on the principal plus all the interest already earned. In year one you earn ₹1,000. In year two you earn 10% of ₹11,000, which is ₹1,100. In year three, 10% of ₹12,100. The base keeps growing, so the interest keeps growing too. After 30 years, the same ₹10,000 becomes about ₹1.74 lakh.
Sit with that chart for a minute. For the first ten years the two lines are close, and it is tempting to think compounding does not matter much. After twenty years the gap is wide. After thirty, compound interest has produced more than four times as much. Compounding is slow, then sudden, and most of its reward arrives at the end. That is precisely why patience and an early start matter more than finding the perfect investment.
The Rule of 72
You do not need a calculator to estimate compounding. The rule of 72 says that if you divide 72 by an annual rate, you get roughly the number of years it takes for money to double.
At 6%, money doubles in about 12 years.
At 8%, in about 9 years.
At 12%, in about 6 years.
At 36%, a typical credit card rate, a debt doubles in about 2 years.
The rule works for anything that grows at a steady rate, including prices. At 6% inflation, prices double in about 12 years, which means today’s ₹100 will buy only about half as much in 2038. Unit 5 picks up that thread.
Why Starting Early Beats Investing More
Meet’s extra ten years of investing are the first ten years of his money’s life, and those are the years that have the longest time to compound. Every rupee he invested at 22 doubles roughly every seven years at 10%, so by 60 it has doubled more than five times. A rupee Krupa invests at 32 has time to double only about four times.
The practical lesson for a student is liberating. You do not need large amounts to begin. A ₹500 SIP started at 18 is worth more, in the end, than a ₹2,000 SIP started at 35. The hardest part of investing is not choosing a fund. It is starting, and then not stopping.
Nominal and Real Interest
The interest rate a bank quotes is the nominal rate. What you actually gain in buying power is the real interest rate: roughly the nominal rate minus inflation. An FD at 6.5% when inflation is 5.5% leaves you only about 1% better off in real terms. An FD at 6.5% when inflation is 0.25%, as it was in October 2025, leaves you more than 6% better off.
This is why the same FD can be a good deal one year and a poor one the next, and why long-term savings usually need some exposure to assets that can beat inflation over time, such as equity funds. Unit 5 explains inflation in detail, and Unit 8 explains how to hold equities sensibly.
Deep Dive: When Compounding Works Against You
Borrowing turns compounding upside down. When you owe money, interest is charged on what you owe, and unpaid interest is added to the debt, so it grows the same way savings do. The higher the rate, the faster it grows. Different kinds of borrowing carry very different prices.
A credit card is interest-free only if you pay the full bill by the due date. Pay just the “minimum due” and the rest attracts interest of 3% to 3.5% a month, which compounds to around 40% a year. At that rate a debt doubles in under two years. Unit 9 shows a ₹50,000 balance paid at the minimum taking more than fifteen years to clear. Compounding is the best friend of a patient saver and the worst enemy of a careless borrower.
Case Study: How the RBI Reaches Your Loan
On October 7, 2026, the RBI raised the repo rate by 0.25 percentage points to 5.50%, its first increase since 2023, to bring inflation back under control. Within weeks, banks began raising rates on new loans, and most floating-rate home loans, which are linked to an external benchmark such as the repo rate, were set to reset upwards at their next review.
On a ₹50 lakh home loan over 20 years, a rise from 8.25% to 8.5% adds about ₹790 to the monthly EMI. Deposit rates usually rise too, though more slowly. This chain, from a committee meeting in Mumbai to a family’s monthly budget in Vadodara, is the clearest example of how interest rates connect the whole economy. When you hear that the RBI has moved rates, you now know that it has changed the price of time for almost everyone in the country.
Check yourself
0/12 answered1. Interest is best described as:
2. ₹10,000 earns 10% simple interest for 30 years. What is it worth at the end?
3. In Fig 4.1, when does the gap between simple and compound interest become large?
4. Using the rule of 72, how long does money take to double at 8% a year?
5. At 6% inflation, roughly how long until prices double?
6. Meet invests from age 22 and Krupa from 32, both ₹2,000 a month at 10% until 60. Why does Meet end up with far more?
7. An FD pays 6.5% and inflation is 5.5%. The real return is roughly:
8. Which borrowing in Fig 4.3 is the most expensive?
9. At about 40% a year, roughly how long does an unpaid credit card debt take to double?
10. A credit card is interest-free only if you:
11. What did the RBI do on October 7, 2026?
12. Which loans usually feel a repo rate change most directly?
Ask an adult in your family about one loan or one investment they have: the amount, the interest rate and the period. Use the rule of 72 to estimate how long the money (or the debt) takes to double, then write 150 to 200 words on whether compounding is working for or against them, and what one change would improve the outcome.