Compound Interest: How Small Amounts Become Big Money

Written by Cripto Adicto Last updated: 15 Jul 2026

Compound interest is often called the eighth wonder of the world, and once you see the maths you will understand why. It is the engine behind almost every long-term wealth story. In this lesson you will learn how compounding works, why time is your most powerful ally, and then test yourself before projecting your own numbers.

Interest on your interest

Simple interest pays you only on the money you put in. Compound interest pays you on your original amount and on all the interest you have already earned. Each period, your base gets a little bigger, so the next batch of growth is a little larger — a snowball that speeds up over time. Invest $1,000 at 8% and after one year you have $1,080; the second year you earn 8% on $1,080, not $1,000. Over decades that difference becomes enormous.

Why starting early wins

Because growth builds on growth, time matters more than the amount you invest. An investor who puts away $200 a month from age 25 will usually end up with more than someone who starts at 35 and invests twice as much — simply because the early money had ten extra years to compound. This is the single most important lesson in personal finance: the best time to start was years ago; the second best time is today.

The Rule of 72

A handy shortcut lets you estimate compounding in your head. The Rule of 72 says that dividing 72 by your annual return gives the number of years for your money to double. At 8%, money doubles in about 9 years (72 ÷ 8). At 6%, about 12 years. It shows why even a small improvement in your return — from high fees to low-cost index funds, for example — can dramatically change where you end up.