Interest is the price of money — what you earn when you save and what you pay when you borrow. But not all interest is calculated the same way. The difference between simple and compound interest seems small at first, yet over years it decides whether your money grows steadily or explosively. Understanding it is one of the most valuable pieces of financial knowledge you can have.
Simple interest: interest on the principal only
Simple interest is calculated only on the original amount (the principal), never on the interest already earned. The formula is:
SI = P × R × T ÷ 100
where P is the principal, R is the annual rate, and T is the time in years. For example, ₹1,00,000 at 10% simple interest earns a flat ₹10,000 every year — ₹50,000 over five years. The interest never changes, because it is always based on the same ₹1,00,000.
Compound interest: interest on interest
Compound interest is calculated on the principal plus all the interest earned so far. Each period, your interest itself starts earning interest. The formula is:
A = P × (1 + R/100)T
Take the same ₹1,00,000 at 10%, compounded annually. In year one you earn ₹10,000. In year two you earn 10% on ₹1,10,000 — that is ₹11,000. In year three, 10% on ₹1,21,000, and so on. After five years you have about ₹1,61,051, versus ₹1,50,000 with simple interest.
The gap widens dramatically over time
| Years | Simple (₹1L @ 10%) | Compound (₹1L @ 10%) |
|---|---|---|
| 5 years | ₹1,50,000 | ₹1,61,051 |
| 10 years | ₹2,00,000 | ₹2,59,374 |
| 20 years | ₹3,00,000 | ₹6,72,750 |
| 30 years | ₹4,00,000 | ₹17,44,940 |
Over 30 years, compound interest turns ₹1 lakh into more than ₹17 lakh — over four times what simple interest produces. This is why Albert Einstein reportedly called compound interest "the eighth wonder of the world".
Why this matters for your decisions
- When saving or investing, you want compounding on your side — and you want it working for as many years as possible. Starting early beats investing more later.
- When borrowing, compounding works against you. Credit card debt compounds monthly, which is why an unpaid balance grows alarmingly fast.
- Frequency matters: the more often interest compounds (monthly vs yearly), the faster your money grows.
The one lesson to remember
Time is the most powerful ingredient in compounding. A person who invests a modest amount for 30 years usually ends up with far more than someone who invests a larger amount for 15 years. The best day to start was years ago; the second best day is today.
Frequently asked questions
Compound interest, without question — it earns interest on your past interest, so your money grows faster over time.
Mostly in short-term loans, some vehicle loans, and certain fixed-income products where interest does not roll over.
Yes. At the same annual rate, monthly compounding produces slightly more than annual compounding because interest is added and reinvested more often.
This article is for general education only and is not financial advice. Figures are illustrative; actual returns depend on the rate, frequency and product.