Divya (an illustrative profile) is 25, works in Pune and has just received her first increment. She can spare ₹5,000 a month, but her friends say there is no hurry: "Enjoy now, invest seriously at 35 when you earn more." Her cousin Dinesh (an illustrative profile) did exactly that. At 35 he started the same ₹5,000 a month and assumed ten years would not make much difference.
This chapter answers the question behind that choice: how much is time actually worth in money? The answer surprises most people, and it comes down to one idea, compounding.
In this chapter
- What the time value of money is and why a rupee today beats a rupee later
- How compounding differs from simple interest, step by step
- The rule of 72 and how to use it for both returns and costs
- Why starting early beats investing more later, with real arithmetic
Why a rupee today is worth more than a rupee later
Offer someone ₹1 lakh today or ₹1 lakh in five years, and almost everyone picks today. That instinct is sound for two reasons. Money you hold today can be invested and earn a return, and money promised later will buy less because prices rise.
This is the time value of money: the idea that the same rupee amount is worth more the sooner you have it. It explains why banks pay interest, why loans cost money, and why delaying an investment has a real cost even when no rupee is lost.
Simple interest versus compounding
Simple interest pays you only on the amount you originally invested, called the principal. Compounding pays you on the principal plus everything it has already earned. Think of a snowball rolling downhill: it picks up snow on its existing size, so the bigger it gets, the faster it grows.
Worked example: ₹1 lakh at an assumed 10% a year
- Year 1: ₹1,00,000 earns ₹10,000. You end with ₹1,10,000.
- Year 2: now ₹1,10,000 earns 10%, which is ₹11,000. You end with ₹1,21,000.
- Year 3: ₹1,21,000 earns ₹12,100. You end with ₹1,33,100.
- Under simple interest you would have earned a flat ₹10,000 each year and ended year 3 with ₹1,30,000. The extra ₹3,100 is interest earned on interest.
Three years in, the gap is small. Stretch it out and it becomes the main event:
| Years | Simple interest at 10% | Compounded at 10% | Extra from compounding |
|---|---|---|---|
| 10 | ₹2.00 lakh | about ₹2.59 lakh | about ₹0.59 lakh |
| 20 | ₹3.00 lakh | about ₹6.73 lakh | about ₹3.73 lakh |
| 30 | ₹4.00 lakh | about ₹17.45 lakh | about ₹13.45 lakh |
Notice that most of the growth arrives late. Between year 20 and year 30, the compounded amount grows by more than ₹10 lakh, more than in the first twenty years combined. That is why patience matters as much as the rate itself.
The rule of 72
You do not need a calculator to estimate compounding. Divide 72 by the annual rate, and the answer is roughly the number of years it takes money to double. It is accurate enough for everyday decisions.
| Annual rate | Rule of 72 estimate | Exact doubling time |
|---|---|---|
| 4% | 18 years | about 17.7 years |
| 6% | 12 years | about 11.9 years |
| 8% | 9 years | about 9.0 years |
| 10% | 7.2 years | about 7.3 years |
| 12% | 6 years | about 6.1 years |
The rule works for costs as well. At 6% inflation, prices double in about 12 years, so a household budget of ₹50,000 a month today would need about ₹1 lakh a month in 12 years to buy the same things. The next chapter looks at inflation in detail.
Starting early beats investing more later
Back to Divya and Dinesh. Each invests ₹5,000 a month through a SIP (systematic investment plan, a fixed monthly investment into a mutual fund) and each stops at 60. We assume a return of 1% a month, roughly 12% a year, which is an illustrative long-term assumption for a diversified equity portfolio and is not guaranteed.
Worked example: the cost of a ten-year delay
- Divya invests for 35 years: 35 × 12 = 420 instalments of ₹5,000, a total of ₹21 lakh.
- Dinesh invests for 25 years: 300 instalments, a total of ₹15 lakh.
- At 1% a month, Divya's SIP grows to about ₹3.25 crore by 60.
- Dinesh's grows to about ₹94.9 lakh.
- Divya put in only ₹6 lakh more, yet ends with about 3.4 times as much. The difference of roughly ₹2.3 crore is almost entirely the result of ten extra years of compounding.
- To reach Divya's ₹3.25 crore in 25 years, Dinesh would need to invest about ₹17,100 a month, more than three times her amount.
| Divya: starts at 25 | Dinesh: starts at 35 | |
|---|---|---|
| Monthly SIP | ₹5,000 | ₹5,000 |
| Years invested to 60 | 35 | 25 |
| Total invested | ₹21 lakh | ₹15 lakh |
| Approximate value at 60 | about ₹3.25 crore | about ₹94.9 lakh |
| Monthly SIP needed to match Divya | n/a | about ₹17,100 |
There is a caveat. Returns in real life arrive unevenly, with bad years and good years, and nobody can promise 12%. But the logic holds at any positive rate: the earlier money goes in, the more doubling periods it gets. You cannot control markets, and you can only partly control how much you earn, but you fully control when you start.
The key idea: The first rupees you invest work the longest. A small SIP started now is usually worth more than a large one started 'once things settle down', because things rarely settle down on their own.
Compounding in reverse
Compounding is neutral; it works on debts too. Credit card interest is commonly charged at around 3 to 3.75% a month on unpaid balances. At 3.5% a month, an unpaid ₹50,000 grows to about ₹75,500 in a year, an effective rate of about 51%, before any late fees or GST on the interest.
Inflation compounds against idle money in the same way. The habits that build wealth are therefore two-sided: start investing early so compounding works for you, and avoid high-cost debt so it does not work against you.
Common mistakes to avoid
- Waiting for a "big enough" amount. Each year of delay removes years of growth from the end of your plan, which is where compounding is strongest.
- Withdrawing early for small wants. Breaking an investment resets the snowball; the lost growth is far larger than the amount withdrawn.
- Assuming high returns are guaranteed. Plan with sensible assumptions and treat anything better as a bonus, not a promise.
- Ignoring the cost of debt. A 51% effective card rate will outrun any reasonable investment return.
- Judging progress in the first few years. Early growth looks slow by design; most of the gain comes in the later decades.
Try this yourself: Take the amount you could invest every month and your current age. Use the SIP calculator on our tools page to see the value at 60 if you start today, then again if you start five years later. Write down the difference: that is the price of waiting, in rupees.
Frequently asked questions
What is the time value of money in simple words?
It means a rupee today is worth more than a rupee in the future, because today's rupee can be invested to earn a return and because prices rise over time. It is the reason delaying an investment has a real cost even if you never lose money.
How accurate is the rule of 72?
For rates between about 4% and 15% it is very close, usually within a few months of the exact answer. It is a mental shortcut, not a planning tool, so use a calculator for actual decisions.
Does compounding work in a savings account or FD?
Yes. Bank deposits compound when interest is added to the balance, often quarterly for FDs. The catch is that the rate after tax and inflation may be low, so the snowball grows slowly; chapter 2 shows how to check this.
Is it too late to benefit from compounding if I start at 40?
No. A 40-year-old still has 20 years to retirement and more beyond it, which is enough for several doubling periods. You may need a higher monthly amount than an early starter, but starting now is still far better than starting at 45.
To see compounding on your own numbers, try the calculators on our planning tools page, or read how a monthly SIP can build a ₹1 crore corpus.