Key Takeaways
- Compound interest earns interest on interest.
- Time is the biggest driver — growth accelerates later.
- More frequent compounding gives a higher final value.
- Simple interest, by contrast, only ever earns on the principal.
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The core idea
With simple interest, you always earn on the original principal alone. With compound interest, each period's interest is added to the balance, so future interest is calculated on a larger and larger amount. This "interest on interest" is what makes long-term growth accelerate.
The formula
A = P × (1 + r/n)n×t
where P is principal, r is the annual rate, n is compounding periods per year, and t is years. Adding regular contributions makes the balance grow faster still.
Illustration: ₹1 lakh at 8% over 10, 20 and 30 years
| Years | Simple interest total | Compound (annual) total | Extra from compounding |
|---|---|---|---|
| 10 | ₹1,80,000 | ₹2,15,892 | ₹35,892 |
| 20 | ₹2,60,000 | ₹4,66,096 | ₹2,06,096 |
| 30 | ₹3,40,000 | ₹10,06,266 | ₹6,66,266 |
The simple-interest line rises by a fixed ₹8,000 a year. The compound line roughly doubles every nine years (the "rule of 72": 72 ÷ 8 ≈ 9). In the third decade the compound deposit earns more interest than it did in the first two decades combined — which is why starting early matters more than finding a slightly higher rate.
Does compounding frequency matter?
Less than most people expect. At 8% nominal, annual compounding yields exactly 8.00%; quarterly yields 8.24%; monthly yields 8.30%. The difference between quarterly and monthly on ₹1 lakh over a year is about ₹60. Rate and time dominate; frequency is a second-order effect. Indian bank FDs usually compound quarterly, PPF annually, and most loan interest is computed monthly on the reducing balance.
Compounding works against borrowers too
Unpaid credit-card balances compound monthly at rates that can exceed 36% a year, so ₹50,000 left unpaid for a year can grow to over ₹70,000 before fees. The same mathematics that builds a corpus also builds a debt — the direction depends only on whether you are the lender or the borrower.
Try your own figures in the Compound Interest calculator, and compare with the Simple Interest calculator.
What matters most
- Time: the longer money compounds, the more dramatic the effect.
- Rate: higher rates compound to much larger sums over long periods.
- Frequency: more frequent compounding adds a little extra.
Experiment with all three in the compound interest calculator and watch how small changes in time or rate produce large changes in the final value.
Sources
We reference primary and official sources. Rate- and rule-dependent details must be verified against the latest official information.
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