Compound interest is often called one of the most powerful forces in personal finance, and the math behind why is actually simple.
Simple interest vs. compound interest
With simple interest, you only earn interest on your original amount. With compound interest, you earn interest on your original amount plus all the interest you've already accumulated. Over short periods the difference looks small; over many years it becomes large.
A worked example
Say you invest $1,000 at a 7% annual return, and never add another dollar:
- Year 1: $1,000 grows to $1,070
- Year 10: roughly $1,967
- Year 20: roughly $3,870
- Year 30: roughly $7,612
Notice the growth from year 20 to 30 ($3,742) is larger than the entire first 20 years combined. That's compounding — growth building on growth.
Why starting early matters more than the amount
Because compounding needs time to snowball, a smaller amount invested early can end up ahead of a larger amount invested later. Someone who invests $200/month starting at 25 can end up with more at retirement than someone investing $400/month starting at 35, purely because of the extra years of compounding — even though the second person put in more money overall.
The other side: compounding debt
The same math works against you with debt that charges compound interest, like many credit cards. Unpaid interest gets added to your balance, and then you're charged interest on that interest too — which is why credit card debt can grow faster than expected.
This is a general explanation, not financial advice
The example above uses a fixed 7% return for simplicity; real investments fluctuate and can lose value. This page explains how the math works — it isn't a projection or a recommendation for any specific return or investment.