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 on the original amount — compound interest makes your money grow exponentially over time.
Albert Einstein allegedly called it the "eighth wonder of the world." Whether or not the quote is real, the math behind it is genuinely powerful.
The Compound Interest Formula
The standard formula is:
A = P × (1 + r/n)^(n × t)
Where:
- A = Final amount (principal + interest)
- P = Principal (initial investment)
- r = Annual interest rate (as a decimal)
- n = Number of times interest compounds per year
- t = Number of years
A Quick Example
Invest ₹1,00,000 at 8% annual interest, compounded monthly, for 10 years:
A = 1,00,000 × (1 + 0.08/12)^(12 × 10) A = 1,00,000 × (1.00667)^120 A = ₹2,21,964
You earn ₹1,21,964 in interest — more than doubling your money — without adding a single rupee after the initial deposit.
Compare that with simple interest: 1,00,000 × 0.08 × 10 = ₹80,000. Compound interest earns you ₹41,964 more. That gap only grows with time.
How Compounding Frequency Changes Your Returns
The same principal, rate, and time period produce different results depending on how often interest compounds:
| Frequency | Times/Year (n) | Final Amount (₹1L, 8%, 10y) | Interest Earned |
|---|---|---|---|
| Annually | 1 | ₹2,15,892 | ₹1,15,892 |
| Semi-annually | 2 | ₹2,19,112 | ₹1,19,112 |
| Quarterly | 4 | ₹2,20,804 | ₹1,20,804 |
| Monthly | 12 | ₹2,21,964 | ₹1,21,964 |
| Daily | 365 | ₹2,22,535 | ₹1,22,535 |
The difference between annual and daily compounding on ₹1 lakh over 10 years is about ₹6,643. Not life-changing on a small sum, but at larger amounts and longer periods, it adds up significantly.
The Real Power: Time, Not Amount
Here's the counterintuitive truth about compound interest — when you start matters more than how much you invest.
Consider two people:
Person A starts investing ₹5,000/month at age 25 and stops at 35 (invests for 10 years, then lets it grow). Person B starts investing ₹5,000/month at age 35 and continues until 60 (invests for 25 years).
At 8% annual return:
- Person A invests ₹6,00,000 total → has ₹1,04,22,747 at age 60
- Person B invests ₹15,00,000 total → has ₹95,10,267 at age 60
Person A invested less than half the amount but ends up with more money. That's the time advantage of compound interest.
Simple Interest vs. Compound Interest
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Formula | A = P(1 + rt) | A = P(1 + r/n)^(nt) |
| Interest basis | Original principal only | Principal + accumulated interest |
| Growth pattern | Linear | Exponential |
| Best for | Short-term loans | Long-term savings |
| Typical use | Car loans, short-term deposits | Savings accounts, mutual funds, bonds |
Simple interest is predictable and easier to calculate. Compound interest rewards patience and time.
Where Compound Interest Works in Real Life
- Savings accounts: Most banks compound interest daily or monthly on savings deposits.
- Fixed deposits: Indian FDs typically compound quarterly. A 7% FD for 5 years yields more than simple 7% × 5 = 35%.
- Mutual funds and SIPs: Returns compound as gains generate their own gains over years.
- Credit card debt: This is where compound interest works against you — unpaid balances compound, often at 36–42% annual rates.
- Home loans: EMIs include both principal and interest, and early payments are interest-heavy due to compounding.
The Rule of 72
Want a quick estimate of how long it takes to double your money? Divide 72 by the annual interest rate:
Doubling time ≈ 72 ÷ interest rate
- At 6%: ~12 years to double
- At 8%: ~9 years to double
- At 12%: ~6 years to double
- At 15%: ~4.8 years to double
This is an approximation, but it's surprisingly accurate for rates between 6% and 15%.
Try It Yourself
Numbers are convincing, but playing with your own figures makes it real. Use our free Interest Calculator to compare simple vs. compound interest side by side, or check our EMI Calculator to see how compounding affects loan repayments.