With simple interest you earn a return only on the money you originally put in. With compound interest, each period's interest is added to your balance, so the following period you earn interest on a larger sum. Interest earns interest, and then that earns interest too.
It is the single most important idea in personal finance, and it works against you just as reliably as it works for you.
Simple against compound, with numbers
Put Rs 1,00,000 away at 10% a year.
| Years | Simple interest | Compounded annually |
|---|---|---|
| 5 | Rs 1,50,000 | Rs 1,61,051 |
| 10 | Rs 2,00,000 | Rs 2,59,374 |
| 20 | Rs 3,00,000 | Rs 6,72,750 |
| 30 | Rs 4,00,000 | Rs 17,44,940 |
Look at what happens between year 20 and year 30. Simple interest adds another Rs 1 lakh, exactly as it did in every previous decade. Compounding adds nearly Rs 11 lakh. Same rate, same money, entirely different outcome, purely because of time.
Why compounding frequency matters
Interest can be added annually, half-yearly, quarterly, monthly or daily. The more often it is added, the sooner it starts earning on itself.
Rs 1,00,000 at 8% for 5 years:
- Compounded annually: about Rs 1,46,933
- Compounded quarterly: about Rs 1,48,595
- Compounded monthly: about Rs 1,48,985
The differences look small over five years and widen over twenty. In India, most bank fixed deposits compound quarterly, PPF and EPF compound annually, and savings accounts credit interest quarterly on a daily balance. This is also why the "effective annual rate" can be slightly higher than the advertised rate.
The rule of 72
A shortcut worth memorising: divide 72 by the annual return to estimate how many years your money takes to double.
- At 6%, money doubles in about 12 years
- At 8%, about 9 years
- At 12%, about 6 years
- At 15%, about 4.8 years
It also works in reverse for inflation. At 6% inflation, prices double in roughly 12 years, which is the same statement as your money losing half its purchasing power.
The cost of starting late
This is where compounding is most brutal, and most useful to understand at 25 rather than 45.
Two people both invest until 60 at an assumed 12%:
- Ananya invests Rs 5,000 a month from age 25. She contributes Rs 21 lakh over 35 years and ends near Rs 3.2 crore.
- Rohit starts at 35 with the same Rs 5,000 a month. He contributes Rs 15 lakh over 25 years and ends near Rs 95 lakh.
Rohit invested 71% of what Ananya did and ended with under a third of her corpus. The missing decade was the one where her early money had the longest to compound. Rohit would need roughly Rs 17,000 a month to catch up, which is why "start early" is advice about arithmetic rather than discipline.
Treat 12% as an assumption for illustration, not a promise. Equity returns are lumpy and no rate is guaranteed. Test different assumptions in our compound interest calculator.
It works against you too
Compounding is the reason credit card debt is so dangerous. A card charging around 3.5% a month is compounding at roughly 42% a year. An unpaid Rs 50,000 balance, left alone, becomes over Rs 71,000 in twelve months and more than doubles in two years.
The same mechanism that builds a retirement corpus builds a debt trap. Clearing high-interest debt before investing is usually the highest-return decision available to you, because you are guaranteed to avoid a rate that no investment reliably matches.
What actually determines your outcome
Three levers: how much you invest, what rate you earn, and how long you leave it. Most people obsess over the second, which they control least. Time is the lever with the steepest effect and the one that quietly expires.
This is the engine behind a SIP, behind PPF's 15 year term, and behind why interrupting a long-term investment to fund a short-term want is more expensive than it looks. See CAGR for how compounded returns are measured after the fact.