How Compound Interest Works (With Simple Examples)
Understand compound interest, the formula behind it, and why starting early matters so much — with worked examples you can try yourself.
Compound interest is often called the most powerful force in finance, and once you see how it works, it's easy to understand why. In this guide we'll break it down in plain English, show the formula, and walk through real examples.
What is compound interest?
Simple interest only ever pays you on your original deposit. Compound interest pays you on your deposit and on all the interest you've already earned. That "interest on interest" is what makes balances snowball over time.
The compound interest formula
The classic formula is:
A = P × (1 + r/n)^(n·t)
- P — the principal (your starting amount)
- r — the annual interest rate (as a decimal)
- n — how many times interest compounds per year
- t — the number of years
You don't need to do this by hand — our Compound Interest Calculator does it instantly, including monthly contributions.
A worked example
Say you invest $10,000 at 7% compounded monthly for 20 years, adding $100 every month:
- Total you put in: about $34,000
- Final balance: roughly $97,000
- Interest earned: around $63,000
You more than doubled your contributions — purely from compounding.
Why starting early beats investing more
Because compounding accelerates over time, the years matter more than the amounts. Someone who invests for 30 years usually ends up far ahead of someone who invests twice as much but only for 15 years.
Try it yourself
Plug your own numbers into the Compound Interest Calculator. If you're comparing this against paying down debt, the Loan Calculator shows what interest is costing you, and the ROI Calculator helps you compare returns across different investments.