How SIP and compounding actually work

Imagine Ramesh, a school teacher in Tezpur. At twenty-five, he decided to save five hundred rupees every month — about the price of two thalis at his canteen. He kept doing it, without break, for thirty years.

On his fifty-fifth birthday, his statement showed seventeen lakh, sixty-five thousand rupees.

Out of his own pocket he had put in ₹1,80,000 — six lakhs less than two lakhs. The market had given him the rest. About fifteen and a half lakh of pure, uninvited growth.

Most explanations of this stop here, with that magical-sounding number. They use words like "the eighth wonder of the world" and move on. That is not a lesson; that is a advertising poster.

In this lesson we are going to do the work that the marketing posters skip. We will build the intuition for why Ramesh's numbers look the way they do — with arithmetic you can do on paper — and identify the three mistakes that cost most Indians their version of his story.

Two ideas, not one

People often say "SIP" and "compounding" as if they are the same word. They are not. They are two separate ideas that happen to multiply each other.

• Compounding is what happens to any single amount when it earns return on its return. You can compound a one-time deposit. You can compound a fixed deposit. You can even compound a savings account, very slowly.
• SIP — Systematic Investment Plan is the act of adding fresh money on a schedule. Same date every month, same amount, no thinking required. SIP, on its own, is just regular saving.

Ramesh's ₹17.65 lakh comes from thirty years of SIP (the regular saving) feeding compound growth (the return on return). Take away either piece and the story collapses. Save regularly into a sock under the mattress: no compounding, no growth. Drop a one-time ₹500 into a mutual fund and forget about it: compounding works on a single tea-cup of money, gives you a cup-and-a-half thirty years later. Underwhelming.

The interesting things happen only when both ideas are doing their jobs at the same time.

Compounding, from scratch

Let us start without SIP, just compounding, so the idea is clean.

You have ₹100. A bank gives you 10% per year. After one year you have ₹110.

This is where two roads diverge.

Simple interest ignores last year's gain. It keeps paying you 10% on the original ₹100, every year, forever:

Compound interest remembers what it did last year and pays you 10% on the new total. It is the same 10% — but applied to a slightly larger amount each time:

Same 10%, same starting amount, same length of time. Compound made you more than five times the money simple interest did.

A mental shortcut: the Rule of 72

Compound calculations are not friendly to mental arithmetic. The Rule of 72 is a one-line approximation that takes you most of the way there:

Use it to sanity-check claims. If a salesman offers a scheme that "doubles your money in three years", the Rule of 72 says that's a 24% annual return. That number should make you lift an eyebrow — sustained 24% returns are very, very rare and almost never come without serious risk.

Now add the SIP

So far, we have compounded a single ₹100. Real investors don't do that. Real investors do what Ramesh did: add a bit more, every month, for years.

The math gets messier here, because each month's ₹500 gets a different amount of time to compound. The first month's deposit sits in the fund for the full thirty years. The last month's deposit sits there for one month. Each contribution lives a different lifetime.

Without doing the algebra, here is the rule of thumb that generations of advisers have memorised. At a 12% annual return, compounded monthly:

For Ramesh, scale this down: ₹500 a month for thirty years at 12% is roughly half of ₹35.30 lakh — call it ₹17.65 lakh. Hence the number we started with.

The three mistakes that ruin the math

Mistake 1 — Treating 12% as a guarantee

Twelve percent is not a guarantee. It is a long-term, hindsight-based assumption for diversified equity investments in India. Year-on-year, real returns swing wildly: −24% in 2008, +75% in 2009, +5% one year, +28% the next. The 12% emerges only over decades — and is not assured to repeat.

Always plan with humility:

• If you assume 12%, plan as if you might get 9%.
• Add 30% to your target SIP amount to absorb that gap.
• Treat any year above 15% as a windfall, not as the new normal.

Mistake 2 — "I'll start later, when I have more money"

This is the costliest mistake in personal finance. Look at this:

The lesson is brutal but simple. The amount you invest matters less than the years you give it. Two extra zeroes on the corpus come from time, not from contribution size.

Mistake 3 — Pausing the SIP when markets fall

When the market crashes, your monthly SIP buys more units for the same money. This is the part of SIP that most people fail to internalise: the bad years are when the SIP is doing its best work. You are loading up at a discount.

If Ramesh had stopped his SIP during the 2008 crash and the 2020 crash — both times when his existing investments looked terrible — he would have missed exactly the months that bought the cheapest units. His ₹17.65 lakh might have ended up as ₹12 lakh or less.

The instinct to pause is the instinct that destroys SIP returns. Build the discipline to do the opposite: when markets fall, if anything, contribute more.

Try it yourself

Pick up a piece of paper and answer two questions before you close this page. They take two minutes; the discipline of doing them is half the lesson.

1. At what age do you plan to retire? Subtract your current age. That is the number of years your SIP gets to run.
2. Look at the practitioner's table above (₹1,000 a month at 12% for various durations). Multiply the corresponding number by the SIP amount you can comfortably commit, divided by 1,000.

That is your back-of-the-envelope retirement corpus, assuming a 12% return that may or may not show up. If the number scares you (too small), the answer is almost never "earn 18% instead." The answer is "start sooner" or "save more." Those are the two levers you actually control.

What this lesson did not cover (yet)

We deliberately kept this lesson narrow. There are several real issues we left out, and we will address each in later lessons:

• Inflation. A ₹17.65 lakh corpus in 2055 is not equivalent to ₹17.65 lakh today. We'll cover real vs nominal returns in the next lesson.
• Taxation. The tax you pay on different fund categories changes the corpus you actually keep.
• Choosing the fund. All of this assumes the underlying investment delivered close to 12%. Picking the right fund category — and avoiding bad ones — is a separate skill.
• Why 12%? The number is anchored in long-term Indian equity-index history. Whether it repeats over the next 30 years is a real question, and we will not pretend otherwise.