How compounding actually works

Almost everyone can recite the definition of compounding: you earn returns on your returns. Far fewer people have looked at what that actually does to a number over twenty years, and the shape of it is genuinely surprising.

The arithmetic

Compounding is one formula:

The whole effect lives in that exponent.

Put ₹1,00,000 in at an assumed 12% a year:

Year	Value	Added that year
1	₹1,12,000	₹12,000
5	₹1,76,234	₹18,882
10	₹3,10,585	₹33,277
20	₹9,64,629	₹1,03,353
30	₹29,95,992	₹3,21,000

The rate never changed. The amount added in year 30 is roughly 27 times the amount added in year one, because each year's return is calculated on everything that came before it.

Where the money actually comes from

Here is the part that changes how people think about it. Over that 30-year period, the investment grows by about ₹28.96 lakh. Split by decade:

• Years 1–10 contribute about ₹2.11 lakh of growth
• Years 11–20 contribute about ₹6.54 lakh
• Years 21–30 contribute about ₹20.31 lakh

The final decade produces more than twice the first two combined. Nothing special happens in year 21. The rate is identical throughout. The base is simply much larger by then.

Why this makes starting early so powerful

The usual advice — "start early" — is normally offered as a platitude. The arithmetic makes it concrete.

Two people both invest ₹1,00,000 at 12%. One starts at 25 and stops. The other starts at 35. At age 60:

• The early starter has 35 years of compounding: about ₹52.8 lakh
• The later starter has 25 years: about ₹17 lakh

The early starter invested exactly the same amount of money and ends with more than three times as much. The ten years they gained were the ten years at the end, when the base was largest — even though those ten years happened at the start of their own timeline.

This is the single strongest argument for investing whatever you can now rather than waiting until you can afford more.

What this does not mean

Three honest caveats, because compounding is routinely oversold.

Markets do not deliver a steady rate. The table above assumes 12% every single year. Real equity returns arrive in an unpredictable order, and the sequence matters: a bad year early in a withdrawal phase does far more damage than the same bad year later. A steady-rate projection shows you the mechanism, not the outcome.

Inflation eats a large part of it. That ₹29.95 lakh in 30 years, at 6% inflation, buys roughly what ₹5.2 lakh buys today. Compounding works on prices too, and in the opposite direction from your point of view.

Costs and taxes compound against you. A 1% annual expense ratio does not cost you 1%. Over 30 years at 12%, it reduces the final figure by roughly 24%, because every rupee taken out early is also a rupee that never compounds.

Try it yourself

The Compound Interest calculator lets you change the rate, the duration and the compounding frequency. The most instructive experiment is to compare adding five years against adding two percentage points of return — time usually wins, and by more than people expect.

The Playground shows the same thing with an inflation line overlaid, which is the version worth internalising.