What is Compound Interest?
Compound interest is interest calculated on both the initial principal AND the accumulated interest from previous periods. Unlike simple interest (which only earns interest on the original principal), compound interest makes your money grow exponentially over time. It's often called "interest on interest."
Warren Buffett, one of the world's richest investors, attributes his wealth primarily to compound interest and time. He made 99% of his $100+ billion fortune after age 50, demonstrating how compounding accelerates with time.
The Compound Interest Formula
A = P × (1 + r/n)n×t
Where:
- A = Final amount (principal + interest)
- P = Initial principal (starting amount)
- r = Annual interest rate (in decimal form)
- n = Number of times interest is compounded per year
- t = Time in years
Simple Interest vs Compound Interest
Let's invest Rs. 1,00,000 at 10% for different periods:
| Years | Simple Interest | Compound Interest | Difference |
|---|---|---|---|
| 5 | Rs. 1,50,000 | Rs. 1,61,051 | Rs. 11,051 |
| 10 | Rs. 2,00,000 | Rs. 2,59,374 | Rs. 59,374 |
| 20 | Rs. 3,00,000 | Rs. 6,72,750 | Rs. 3,72,750 |
| 30 | Rs. 4,00,000 | Rs. 17,44,940 | Rs. 13,44,940 |
At 30 years, compound interest gives you 4.36x the amount of simple interest. The gap widens dramatically with time.
The Rule of 72
A quick way to estimate how long your money takes to double: divide 72 by the annual interest rate.
- At 6% (FD): 72 / 6 = 12 years to double
- At 8% (Debt fund): 72 / 8 = 9 years to double
- At 12% (Equity SIP): 72 / 12 = 6 years to double
- At 15% (High growth): 72 / 15 = 4.8 years to double
Compounding Frequency Matters
Rs. 1,00,000 at 10% for 5 years with different compounding frequencies:
- Annual compounding: Rs. 1,61,051
- Quarterly compounding: Rs. 1,63,862
- Monthly compounding: Rs. 1,64,531
- Daily compounding: Rs. 1,64,866
More frequent compounding yields slightly more, though the difference diminishes. FDs typically compound quarterly, while some savings accounts compound daily.
Real-World Compounding Examples
Starting Early: Age 25 vs Age 35
Both invest Rs. 10,000/month at 12% annual returns until age 60:
- Starting at 25 (35 years): Invests Rs. 42 lakh, accumulates Rs. 6.49 crore
- Starting at 35 (25 years): Invests Rs. 30 lakh, accumulates Rs. 1.90 crore
Starting just 10 years earlier (investing only Rs. 12 lakh more) results in Rs. 4.59 crore more wealth. Those first 10 years of compounding have an outsized impact.
See Compounding in Action
Use our Compound Interest Calculator to visualize how your money grows with different rates, tenures, and compounding frequencies.
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Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal amount. Compound interest is calculated on the principal plus all accumulated interest. Over long periods, compound interest significantly outperforms simple interest.
How often is compound interest calculated?
It depends on the financial product. Bank FDs compound quarterly. Savings accounts may compound daily or quarterly. Bonds may compound semi-annually. Mutual fund NAVs effectively compound daily. More frequent compounding results in slightly higher returns.
What is the Rule of 72?
The Rule of 72 is a quick mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes for an investment to double. At 12% returns, money doubles in approximately 6 years (72/12). It's reasonably accurate for rates between 4-20%.
Can compound interest work against me?
Yes - compound interest on debt (credit cards, loans) works against you. Credit card debt at 36% annual interest compounds monthly, causing your outstanding balance to snowball rapidly. Always pay credit card bills in full to avoid compounding interest working against you.
What investments give the best compound returns?
Historically: equity mutual funds (12-15% long-term), direct stocks (variable), real estate (8-12%), gold (8-10%), FDs (6-7%). Higher returns come with higher risk. A diversified portfolio across asset classes balances returns and risk.