Compound interest is often called the eighth wonder of the world because of how powerfully it grows money over time. This guide explains how it works, shows the formula with examples, and lets you run your own numbers.
Compound interest, explained simply
Simple interest is calculated only on your original amount (the principal). Compound interest is calculated on the principal plus all the interest already earned. Because you earn “interest on interest”, the total grows faster and faster over time.
The compound interest formula
The standard formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is how many times interest compounds per year, and t is the number of years. You do not need to do this by hand — our Compound Interest Calculator does it instantly.
A worked example
Invest $5,000 at 5% interest compounded monthly for 10 years and you end up with about $8,235 — roughly $3,235 in interest, far more than the $2,500 you would get with simple interest. The longer you leave it, the bigger the gap becomes.
Why starting early matters
Because growth compounds, money invested in your twenties can outgrow much larger amounts invested later. Time is the most important ingredient. If you are weighing a loan instead, our Loan Calculator shows how interest works against you on borrowing.
Frequently asked questions
What does compounding frequency mean? It is how often interest is added — annually, quarterly, monthly or daily. More frequent compounding produces slightly higher returns.
Is compound interest good or bad? It is excellent for savings and investments, but works against you on debt like credit cards. Understanding both sides helps you make better money decisions.