Compound Interest Explained — The Eighth Wonder of the World

Compound interest is often called the “eighth wonder of the world” — and for good reason. It’s the engine behind most long-term wealth.

What compound interest is

Simple interest is always calculated on your original principal. Compound interest is calculated on the principal plus the interest you’ve already earned. Each period, your balance grows, so the next period earns even more.

The formula

A = P × (1 + r/m)^(m × t)

• P = principal
• r = annual interest rate
• m = compounding periods per year (1 yearly, 4 quarterly, 12 monthly)
• t = years

Worked example

Invest ₹1,00,000 at 7% for 10 years, compounded quarterly:

A = 1,00,000 × (1 + 0.07/4)^(4 × 10)
A ≈ ₹2,00,160

You earn about ₹1,00,160 in interest — more than your original investment. Simple interest over the same period would earn just ₹70,000. Compounding earned you ₹30,000 extra for doing nothing.

Why starting early beats starting big

This is the counterintuitive part. Consider two investors:

• Anil invests ₹10,000/month from age 25 to 35 (10 years, ₹12 lakh total), then stops.
• Bina invests ₹10,000/month from age 35 to 60 (25 years, ₹30 lakh total).

At 12% returns, Anil ends up richer at 60 — because his money compounds for 25 extra years. The same math applies to SIPs and provident funds: time in the market beats timing the market.

The practical takeaways

1. Start as early as you can — even small amounts.
2. Don’t interrupt compounding; keep contributions regular.
3. Reinvest your returns (don’t spend the interest).
4. Longer horizons let you take more equity risk, where compounding is strongest.

See your own growth curve with the Compound Interest Calculator and compare with a monthly SIP.