How compounding actually works

Everyone is told that money compounds. Almost nobody is told what that feels like from the inside, which is where the damage gets done.

The arithmetic is simple enough: returns earn returns, so growth accelerates over time. The consequence is not simple at all. It means the outcome is wildly back-loaded. It means the first years produce almost nothing visible, and then a decade later the same money starts moving in amounts that look like errors.

Take a ₹10,000 monthly SIP at 12% a year, run for twenty years. After ten years you have ₹22.4 L. After twenty you have ₹91.99 L. The second decade added ₹69.58 L — around 3.1x what the first decade produced, for exactly the same monthly effort.

Your app shows one number, and that number is misleading. A portfolio is not a pot that grows. It is a stack of individual purchases, each bought at a different price on a different day, each ageing independently.

This matters because it changes what an SIP instalment is. It is not a contribution towards a goal. It is its own small investment, with its own thirty-year life ahead of it, and its own eventual multiple. The instalment you pay this month is the one that will still be compounding in 2056.

This is the single most useful rule on this site, and it is phrased deliberately. Not “invest for ten years” —make the promise instalment by instalment, as you pay each one. This month’s ₹10,000 is not to be touched until 2036. Next month’s until 2036 as well, and so on.

The reason is statistical rather than moral. Over one year, equity markets do more or less anything. Over ten years, the range of outcomes narrows sharply, and a 10-12% result stops being a hope and starts being the base case. The promise is how you convert an uncertain asset into a fairly reliable one, and the only cost is that you must not break it.

The practical test: if you cannot make that promise about a particular amount, the amount is too large or the category is too aggressive. Move it to the reserve bucket and stop pretending.

An SIP is not mathematically optimal. Over the last forty-five years the Indian market closed higher in thirty-six of them — roughly four years up for every one down. If you have money now and a twenty-year horizon, spreading it over the next twelve months means deliberately sitting out of a market that goes up four times more often than it goes down.

So why does everyone recommend SIPs anyway? Because the SIP is not optimising returns. It is optimising for you continuing to do it. A salaried person does not have a lump sum; they have a monthly surplus, and matching the investment to the income is what makes it survive twenty years of moods.

The two are also less different than they look. A business owner putting in ₹1 lakh this month and ₹3 lakh in four months is running an SIP of irregular amounts at irregular intervals. Over twenty years, that is the same machine.