Compound Interest

Compound interest is interest earned on previously earned interest. Unlike simple interest — which is always calculated only on the original principal — compound interest lets each period's earnings join the balance and start producing their own interest, so growth accelerates over time rather than staying flat.

How it works

The key idea is that interest keeps earning its own interest. The general formula is:

A = P × (1 + r/m)^(m × t)

• P — the starting principal
• r — the annual interest rate (as a decimal)
• m — how many times interest compounds per year (e.g. 12 for monthly, 4 for quarterly)
• t — the number of years
• A — the amount you end with

Why frequency and time matter

The more often interest compounds, the faster your money grows for the same nominal rate. Compounding monthly yields slightly more than compounding annually because each month's interest starts earning immediately.

The most powerful ingredient is time. Consider ₹100,000 at 8% compounded annually:

• After 10 years: ≈ ₹215,892
• After 20 years: ≈ ₹466,096
• After 30 years: ≈ ₹1,006,266

The growth is not linear — it bends upward as the accumulated interest becomes a bigger chunk of the balance. Starting even a few years earlier can make a dramatic difference, which is why advisors stress beginning to invest early and reinvesting dividends or interest rather than spending them.

A worked example of compounding frequency

Suppose you invest ₹1,00,000 for 5 years at 8%. If it compounds annually, the maturity value is about ₹1,46,933. Compounded monthly, it reaches roughly ₹1,48,985. The gap looks small over five years, but widens considerably over two or three decades as each compounding interval compounds on more.

The rule of 72

A handy shortcut is the Rule of 72: divide 72 by the annual interest rate to estimate how many years it takes your money to double. At 8%, 72 ÷ 8 = 9 years to double. It is an approximation, but it makes compounding's power instantly graspable for planning without a calculator.

Common misconceptions

• "Compounding only matters for the rich." False — small, regular contributions add up precisely because they compound over decades.
• "Higher frequency always means better." Only up to a point; the gains from daily versus monthly compounding are small.
• "It applies only to savings." It also works against you — credit-card and loan interest compounds on what you owe.

See the effect of different rates and timeframes with the Compound Interest Calculator.