Compounding

Interest earns interest. That is the whole mechanism, and it is far less interesting than what it does.

Two people

Both invest ₹5,000 a month. Both earn 10% a year. The only difference is when.

Priya starts at 25 and stops at 35. Ten years of instalments — ₹6,00,000 in total — and then she never adds another rupee. She leaves it alone until she is 60.

Arjun starts at 35, the year Priya stops, and keeps going until he is 60. Twenty-five years of instalments: ₹15,00,000.

Arjun invests two and a half times as much money, for two and a half times as long. At 60:

	Priya	Arjun
Invested	₹6,00,000	₹15,00,000
Value at 60	₹1,11,89,652	₹66,89,452

Priya ends with ₹45,00,200 more, having put in ₹9,00,000 less.

Move the return slider and watch what happens: the gap widens as the return rises and narrows as it falls. Compounding is not a fixed law that early always wins — it is that early money gets multiplied more times, and the value of "more times" depends on the rate.

Why the last decade does the heavy lifting

Priya's ₹6,00,000 was worth about ₹10.3 lakh when she stopped at 35. Over the next twenty-five years, untouched, it became ₹1.12 crore. Nearly ₹1 crore of that arrived without her doing anything at all.

That is the part people find hard to believe, so it is worth stating the mechanism plainly: each year's growth is a percentage of a larger number than the year before. Growth of 10% on ₹10 lakh is ₹1 lakh. Ten per cent on ₹1 crore is ₹10 lakh. Same rate, same effort, ten times the effect — and the only thing that produced the larger base was time.

The practical consequence: the final years of any long plan produce most of the money, which means the early years are the ones that buy them. A decade of investing in your twenties is not worth a decade in your fifties. It is worth vastly more, and the difference is not recoverable later by trying harder.

What actually drives it

Three inputs, in order of how much they matter over a long horizon.

Time. The exponent. Doubling the years does far more than doubling the amount, which is what the table above demonstrates.

Rate. Multiplies, and the effect is larger than it looks — but it is also the input you control least and the one people spend the most attention on.

Amount. Linear. Twice the instalment is exactly twice the outcome, no more.

Most beginners rank these in reverse: they hunt for a better rate, worry about the amount, and treat time as something they will get round to. Chapter 3 made the same point from the savings-rate side — the two chapters are the same idea seen from opposite ends.

Where it goes wrong

Interruptions cost more than they look. Stopping for two years in your thirties does not cost two years of instalments; it costs those instalments and every year of growth they would have had. This is why chapter 4's emergency fund exists: it is what stops a bad year from becoming a hole in the middle of the compounding.

The rate is not a promise. Every figure above assumes exactly 10% every year, which no real investment delivers. Chapter 9 is about the difference between an average and a sequence, and it matters more than this chapter's arithmetic suggests.

It works against you too. Chapter 5 priced a credit card at 51% a year. That is this same machinery, pointed the other way, running much faster than any investment.

The point

Time is the input with the exponent on it. Starting now with a small amount beats starting later with a large one, and no rate you can realistically find makes up for the years.