What is compound interest?
Compound interest is interest earned on your original money and on the interest it has already earned. In other words, your interest starts earning its own interest. That small idea is the single most powerful force in personal finance.
Simple vs compound interest
With simple interest, you earn a fixed amount each year on the original sum only. With compound interest, each year’s interest is added to the balance, so next year you earn interest on a slightly bigger amount. Early on the difference is small; over many years it becomes enormous.
The formula (in plain language)
The compound interest formula is A = P (1 + r/n)^(nt), where P is the principal, r is the annual interest rate (as a decimal), n is how many times a year interest is compounded, and t is the number of years. A is the final amount; the interest earned is A − P. You don’t need to memorise it to use the idea — just remember that more time and more frequent compounding both help.
A rupee example
Suppose you invest ₹10,000 at 10% per year. With simple interest you’d earn ₹1,000 every year. With compound interest you earn ₹1,000 in year one, but about ₹1,100 in year two (10% of ₹11,000), and more each year after. Over 30 years, ₹10,000 at 10% compounded annually grows to roughly ₹1.7 lakh — without you adding a single rupee.
The rule of 72
A handy shortcut: divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8% a year, money doubles in about 9 years (72 ÷ 8). At 12%, in about 6 years. It’s a quick way to feel how powerful a higher rate — or more time — really is.
Why time matters most for students
The biggest lever in compounding is not the amount or even the rate — it’s time. A student who starts investing a small amount at 18 can end up far ahead of someone who starts a much larger amount at 30. Understanding this early is one of the most valuable lessons financial literacy offers.
Frequently asked questions
What is the compound interest formula?
A = P (1 + r/n)^(nt): P is the principal, r the annual rate as a decimal, n the number of times interest compounds per year, and t the number of years. Interest earned is A − P.
What is the rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes money to double. For example, at 9% it takes roughly 8 years.
Where do students see compound interest in real life?
In savings accounts, fixed and recurring deposits, mutual fund SIPs — and, less happily, on credit-card balances and loans, where it works against you.
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