What Is Personal FinancePeople often call compound interest one of the strongest forces in personal finance. The math behind why is simple.
Simple interest vs. compound interest
With simple interest, you earn interest only on your original amount. With compound interest, you earn interest on your original amount plus all the interest you already accumulated. Over short periods the difference looks small. Over many years it grows 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 build, a smaller amount invested early often ends up ahead of a larger amount invested later. Someone who invests $200/month starting at 25 ends 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 pay interest on that interest too. That's why credit card debt grows faster than expected.
Real investments fluctuate and lose value during down periods, unlike the fixed 7% return used above for simplicity. This page explains how the math works. It isn't a projection or a recommendation for any specific return or investment.