How Compound Interest Works: The Power
A clear explanation of compound interest, how it differs from simple interest, the Rule of 72, and how to make it work in your favour.
Albert Einstein allegedly called compound interest the "eighth wonder of the world." Whether or not he said it, the maths is undeniable: compound interest is the most powerful force in personal finance — working either for you (when you invest) or against you (when you borrow).
Simple Interest vs Compound Interest
Simple interest is calculated only on the original principal:
Simple Interest = P × r × tCompound interest is calculated on the principal plus all previously earned interest:
A = P × (1 + r/n)^(n×t)Where A = final amount, P = principal, r = annual rate, n = compounding frequency, t = time in years.
Example: ₹1,00,000 at 10% for 20 Years
- Simple interest: ₹1,00,000 + ₹2,00,000 interest = ₹3,00,000
- Compound interest (annually): ₹6,72,750
- Compound interest (monthly): ₹7,32,817
Monthly compounding grows your money 2.4× more than simple interest — same rate, same period.
The Rule of 72
Divide 72 by the annual interest rate to find how many years it takes to double your money:
- At 6%: 72 ÷ 6 = 12 years to double
- At 9%: 72 ÷ 9 = 8 years to double
- At 12%: 72 ÷ 12 = 6 years to double
Compounding Frequency Matters
The more frequently interest compounds, the more you earn. On ₹1,00,000 at 10% for 10 years:
- Annually: ₹2,59,374
- Quarterly: ₹2,68,506
- Monthly: ₹2,70,704
- Daily: ₹2,71,791
How to Make Compound Interest Work for You
- Start early. 10 years of compounding at 30 vs 40 can double your final corpus.
- Reinvest dividends. Never withdraw interest — let it compound.
- Increase contributions over time. Even small annual increases dramatically boost outcomes.
- Minimise fees. A 1% fund management fee eats 15–20% of your wealth over 25 years.
Compound Interest Working Against You: Debt
Credit card debt at 36–42% per annum is compound interest working in reverse. A ₹50,000 outstanding balance left unpaid for 3 years at 40% becomes over ₹1,37,000. Always clear high-interest debt before investing.
See how your money grows using our Compound Interest Calculator.