What is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, where you earn interest only on the original amount, compound interest creates a snowball effect — your money earns interest on interest, and the growth accelerates over time.
Where: P = principal, r = annual interest rate (decimal), n = compounding frequency per year, t = number of years, A = final amount.
Simple Interest vs Compound Interest — A Real Example
Invest ₹1,00,000 at 10% for 20 years:
| Year | Simple Interest | Compound Interest | Difference |
|---|---|---|---|
| 5 | ₹1,50,000 | ₹1,61,051 | ₹11,051 |
| 10 | ₹2,00,000 | ₹2,59,374 | ₹59,374 |
| 15 | ₹2,50,000 | ₹4,17,725 | ₹1,67,725 |
| 20 | ₹3,00,000 | ₹6,72,750 | ₹3,72,750 |
After 20 years, compound interest gives you more than double what simple interest delivers on the same principal and rate. The gap widens dramatically with time — this is why compounding is often called the "eighth wonder of the world."
The Rule of 72 — Quick Mental Math
The Rule of 72 gives you a quick estimate of how many years it takes to double your money at a given annual return:
- At 6% (bank FD): 72 ÷ 6 = 12 years to double
- At 8% (PPF/debt fund): 72 ÷ 8 = 9 years to double
- At 12% (equity SIP long-term): 72 ÷ 12 = 6 years to double
- At 15% (aggressive equity): 72 ÷ 15 = 4.8 years to double
Compounding Frequency Matters
The more frequently interest compounds, the faster your money grows. Indian bank FDs typically compound quarterly, while PPF compounds annually and savings accounts compound daily.
| Frequency | ₹1L at 10% for 10 Years |
|---|---|
| Annually | ₹2,59,374 |
| Quarterly | ₹2,68,506 |
| Monthly | ₹2,70,704 |
| Daily | ₹2,71,791 |
Why Starting Early Beats Investing More
Consider two people investing for retirement at age 60:
- Person A starts at age 25, invests ₹5,000/month for 35 years = ₹21 lakh invested. At 12% annual return, final corpus: approximately ₹3.25 crore.
- Person B starts at age 35, invests ₹10,000/month for 25 years = ₹30 lakh invested. At 12% annual return, final corpus: approximately ₹1.90 crore.
Person A invests ₹9 lakh less than Person B but ends up with ₹1.35 crore more. Those extra 10 years of compounding are worth more than doubling the monthly contribution. This is the single most powerful argument for starting to invest as early as possible.
Compounding Works Against You Too — Debt
The same compounding that grows investments also grows debt. Credit card debt in India typically carries 36–42% annual interest, compounded monthly. A ₹1,00,000 credit card balance left unpaid for 3 years grows to approximately ₹2,80,000 — nearly tripling. This is why paying only the minimum due on credit cards is so dangerous.
Key Takeaway
Compounding rewards patience and consistency. Start early, stay invested, reinvest returns, and avoid high-interest debt. Even modest amounts grow into substantial wealth given enough time. Use the Compound Interest Calculator to model your own scenarios and see the difference that time, rate, and frequency make.