Compound interest is the engine behind every successful long-term investment — and the engine behind most debt disasters. Understanding it is non-negotiable for financial literacy.
Simple vs Compound Interest: The Core Difference
Simple interest earns only on original principal. Compound interest earns on principal and on accumulated interest. Example: ₹1,00,000 at 10% for 10 years:
- Simple interest: ₹2,00,000
- Compound (annual): ₹2,59,374
The ₹59,374 difference is pure compounding — 30% more wealth from the exact same investment.
The Compound Interest Formula
A = P × (1 + r/n)^(n×t)Where: P = Principal, r = Annual rate (decimal), n = Compounding frequency/year, t = Time in years.
Worked Example: ₹2 Lakh for 15 Years at 12%
A = 2,00,000 × (1 + 0.12/12)^(12×15)
= 2,00,000 × (1.01)^180
≈ ₹11,99,159Your ₹2 lakhs becomes almost ₹12 lakhs. Total interest earned: ₹9,99,159 — nearly 5× your original investment.
Why Compounding Frequency Matters
| Frequency | n | ₹1L after 10 years at 10% |
|---|---|---|
| Annual | 1 | ₹2,59,374 |
| Monthly | 12 | ₹2,70,704 |
| Daily | 365 | ₹2,71,791 |
The Rule of 72
Years to double = 72 ÷ Annual interest rate
At 8%: doubles in 9 years. At 12%: doubles in 6 years. At 15%: doubles in 4.8 years.
Starting Early vs Starting Late
Investor A invests ₹5,000/month from age 25–35 (10 years = ₹6L total) then stops. Investor B invests ₹5,000/month from age 35–60 (25 years = ₹15L total). At 12% compounded monthly: Investor A has ₹1.76 crore vs Investor B’s ₹94 lakh. Starting 10 years earlier, investing less than half the money, results in nearly double the wealth.
Use our free Compound Interest Calculator to model any scenario, or the SIP Calculator for regular monthly investment projections.