Compound Interest Calculator

How compound interest works

Compound interest is interest earned on both your original principal and the interest already accumulated. At each compounding interval — yearly, half-yearly, quarterly or monthly — the interest is added to the balance, and the next interval earns interest on that larger amount. The calculator uses the formula A = P × (1 + r/n)^(n·t), where n is the number of compounding periods per year, to project the final value.

This snowball effect is the single most important idea in personal finance. Simple interest grows in a straight line; compound interest curves upward, accelerating over time.

Why compounding rewards time

Because growth is exponential, the later years add far more than the early ones. A ₹1 lakh investment at 10% compounded quarterly reaches about ₹2.69 lakh in 10 years, but roughly ₹7.2 lakh in 20 years — the second decade adds more than four times what the first did. This is why starting early beats investing larger amounts later, and why the Rule of 72 (72 divided by the rate gives the doubling time) is such a handy mental shortcut.

What affects the final amount

• Principal — the initial sum you invest.
• Interest rate — the higher the rate, the steeper the curve.
• Time period — the most powerful lever, thanks to exponential growth.
• Compounding frequency — monthly beats quarterly beats annual, lifting the effective yield.

The gap between nominal rate and effective annual rate (EAR) comes entirely from frequency: 12% compounded monthly is really about 12.68% a year.

Common mistakes to avoid

The biggest is waiting to start, which sacrifices the most valuable compounding years. Another is spending interest or dividends instead of reinvesting them, which breaks the chain. On the borrowing side, people underestimate how compounding punishes them — credit card debt at 36–42% a year balloons fast, so clearing it should come before most investing. Finally, remember inflation compounds too, quietly eroding purchasing power, so aim for returns comfortably above the inflation rate to build real wealth.