How Compound Interest Works
Compound interest is the closest thing to magic in personal finance: growth earning growth on itself. Here is how it works, why starting early beats investing more, and the numbers behind it.
The snowball effect
Simple interest pays you only on your original deposit. Compound interest pays you on your deposit plus everything it has already earned. Each cycle, the base gets bigger, so the growth accelerates — slowly at first, then dramatically.
Put $10,000 in an account earning 7% a year. After year one you have $10,700. In year two you earn 7% on $10,700, not $10,000. After 30 years of compounding, that single deposit is worth about $76,123 — with no further contributions.
Why time beats amount
Someone who invests $200 a month from age 25 to 35 ($24,000 total) and then stops will usually end up with more at 65 than someone who invests $200 a month from 35 to 65 ($72,000 total), assuming the same 7% return. The early starter's money compounds for an extra decade.
The lesson is not that small amounts do not matter — it is that starting now matters more than waiting until you can invest big. Try it yourself with the SIP investment calculator or the lump sum calculator.
The Rule of 72
A quick mental shortcut: divide 72 by your annual return to estimate doubling time. At 7%, money doubles roughly every 10 years. At 10%, every 7 years. At 3% (a typical savings account), every 24 years.
Flip it around to sanity-check promises: anyone guaranteeing to double your money in 2 years is implying a 36% annual return — which should tell you everything about the risk involved. Use the CAGR calculator to check what growth rate a claim really implies.
Compounding works against you too
The same math applies to debt. A credit card balance at 24% APR compounds monthly against you, doubling roughly every 3 years if you only pay minimums. Paying down high-interest debt is usually the highest "return" available to you — risk-free and tax-free.