What is Compound Interest?

Simple explanation

With simple interest, you earn (or pay) interest only on the original principal amount, every single period. With compound interest, once interest is added to the principal at the end of a period, the next period's interest is calculated on this new, larger amount — meaning you start earning "interest on interest."

This effect is small at first but becomes more noticeable the longer money is left to compound, and the more frequently interest is added (for example, compounding monthly grows money faster than compounding yearly, at the same annual rate).

Compound interest works both ways: it helps savings and investments grow faster over time, but it can also make debts grow faster if interest isn't paid off and keeps compounding.

A quick way to estimate how long compounding takes to double your money is the Rule of 72: divide 72 by the annual interest rate. At 8%, that works out to roughly 72 ÷ 8 = 9 years to double. It's only an estimate, but it's a fast mental shortcut for comparing opportunities.

In India, bank fixed deposits typically compound quarterly, so the effective annual yield is a little higher than the annual rate quoted on the FD receipt. This is why two FDs quoting the same annual percentage can pay slightly different final amounts — the compounding frequency decides the difference.

Every time interest is added, the base for the next round of interest becomes larger. That is the whole engine of compounding: the gap between compound and simple interest keeps widening with every passing year, which is why long horizons matter more than the rate alone.

Why does it matter?

Understanding compounding helps explain why starting to save early — even in small amounts — can make a big difference over many years, and why letting certain debts (like unpaid credit card balances) sit unpaid for a long time can become expensive quickly.

Example

Important things to know

• The compound interest formula is A = P × (1 + r/n)^(n×t), where P is principal, r is annual interest rate, n is how many times per year it compounds, and t is time in years.
• The more frequently interest compounds (yearly, half-yearly, monthly), the faster the amount grows, even at the same stated annual rate.
• Compounding rewards time — money compounding for a longer period grows disproportionately more than the same amount compounding for a short period.
• Compound interest applies to many everyday products: fixed deposits, recurring deposits, and some loans and credit card balances all use compounding in some form.
• A small difference in interest rate can lead to a surprisingly large difference in final amount over long periods, due to compounding.
• The Rule of 72 is a quick mental shortcut: divide 72 by your annual rate to estimate the years needed to double the amount at that rate.
• In India, bank fixed deposits usually compound quarterly, so the effective annual yield is a little higher than the quoted annual percentage rate.

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