Simple Interest vs Compound Interest: Key Differences Explained
Why Your FD Earns More Than the "Stated" Rate
When a bank advertises 7% on a fixed deposit, you might expect ₹7,000/year on a ₹1 lakh FD. But with quarterly compounding, you actually earn ₹7,186 — that’s the power of compound interest. The difference seems small in year one, but over 10–20 years, compounding creates a massive gap between simple and compound interest.
The Fundamental Difference
| Simple Interest | Compound Interest | |
|---|---|---|
| Formula | P × R × T / 100 | P × (1 + R/n)^(n×T) – P |
| Interest earned on | Original principal only | Principal + accumulated interest |
| Growth pattern | Linear (same amount each year) | Exponential (accelerating) |
| Common use | Short-term loans, some car loans | FDs, savings accounts, SIPs, home loans |
The Gap Over Time: ₹1 Lakh at 10% for Different Periods
| Period | Simple Interest | Compound Interest (Annual) | Difference |
|---|---|---|---|
| 5 years | ₹50,000 | ₹61,051 | ₹11,051 |
| 10 years | ₹1,00,000 | ₹1,59,374 | ₹59,374 |
| 20 years | ₹2,00,000 | ₹5,72,750 | ₹3,72,750 |
| 30 years | ₹3,00,000 | ₹16,44,940 | ₹13,44,940 |
At 30 years, compound interest earns 5.5× more than simple interest on the same principal at the same rate. This is why long-term investing (via SIP or PPF) generates wealth — the compounding curve accelerates dramatically over time.
Compounding Frequency Matters
How often interest is compounded changes the effective return:
| Compounding | Effective Annual Rate (on 10% nominal) | ₹1L After 5 Years |
|---|---|---|
| Annual | 10.00% | ₹1,61,051 |
| Half-yearly | 10.25% | ₹1,62,889 |
| Quarterly | 10.38% | ₹1,63,862 |
| Monthly | 10.47% | ₹1,64,531 |
| Daily | 10.52% | ₹1,64,866 |
Most Indian FDs compound quarterly. Savings accounts compound daily. PPF compounds annually. The more frequent the compounding, the higher the effective return. Use the compound interest calculator to compare different frequencies.
The Rule of 72
A quick mental shortcut: divide 72 by the interest rate to find how many years it takes to double your money with compound interest. At 12%, your money doubles in 72/12 = 6 years. At 8%, it takes 72/8 = 9 years. This rule helps you quickly evaluate investment and loan propositions.
Where Each Type Is Used in Real Life
- Simple interest: Some personal loans, car loans, advance tax penalty calculations
- Compound interest: FDs, RDs, PPF, SIPs, home loan interest, credit card outstanding
- Reducing balance (a form of CI): Most EMI-based loans — interest is charged on remaining principal, not original amount
Try the simple interest calculator and compound interest calculator side by side to see the difference for your specific amounts and time periods.