Compound Interest: The Eighth Wonder of the World Explained

There's a quote often attributed to Einstein: "Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." Einstein may not have actually said it — but the principle is mathematically irrefutable.

Simple vs Compound Interest

Simple interest earns returns only on your original principal. If you invest ₹1,00,000 at 10% simple interest for 10 years, you earn ₹10,000 per year — ₹1,00,000 total. Your investment grows to ₹2,00,000.

Compound interest earns returns on your principal AND on the returns you've already earned. The same ₹1,00,000 at 10% compounded annually for 10 years grows to ₹2,59,374. That extra ₹59,374 is compounding at work — you earned returns on returns.

The frequency effect

How often interest compounds matters significantly. ₹1,00,000 at 12% for 10 years:

• Compounded annually: ₹3,10,585
• Compounded quarterly: ₹3,26,204
• Compounded monthly: ₹3,30,039

More frequent compounding = faster growth. This is why mutual funds (which effectively compound daily as NAV changes) tend to outperform FDs compounded quarterly.

The Rule of 72

A quick mental shortcut: divide 72 by the annual interest rate to find how many years it takes to double your money.

• At 6% (PPF): 72 ÷ 6 = 12 years to double
• At 8% (debt funds): 72 ÷ 8 = 9 years to double
• At 12% (equity): 72 ÷ 12 = 6 years to double

Why time matters more than amount

Ravi starts a ₹5,000/month SIP at age 25 and stops at 35 (10 years invested). Priya starts the same SIP at 35 and invests until 55 (20 years invested). At age 55, assuming 12% annual returns:

• Ravi invested ₹6 lakh total → corpus: ~₹1.76 crore
• Priya invested ₹12 lakh total → corpus: ~₹49.9 lakh

Ravi invested half the money and still comes out with more than 3x Priya's corpus. This is the power of starting early.

Run your own numbers with our SIP Calculator →