How Compound Interest Works: The Most Powerful Force in Finance

Albert Einstein reportedly called compound interest the “eighth wonder of the world.” Whether or not he actually said it, the sentiment is correct — compound interest is the most powerful force in personal finance. Here’s exactly how it works and how you can use it to build wealth.

What Is Compound Interest?

Compound interest is interest calculated not just on your original principal, but also on the interest you’ve already earned. In other words: your interest earns interest. This creates an exponential growth curve over time.

Simple vs. Compound Interest

Simple interest: $1,000 at 7% for 10 years = $700 interest. Total = $1,700.
Compound interest: $1,000 at 7% compounded annually for 10 years = $967 interest. Total = $1,967.

That’s a 38% difference — just from the compounding effect. Try it yourself with our Compound Interest Calculator.

The Formula

A = P × (1 + r)ⁿ
Where: A = final amount, P = principal, r = annual interest rate (decimal), n = number of years.

The Rule of 72

A quick way to estimate how long it takes to double your money: divide 72 by your interest rate. At 7%: 72 ÷ 7 = ~10.3 years to double. At 10%: 72 ÷ 10 = 7.2 years.

How to Use Compound Interest to Build Wealth

Start early. Time is the most critical variable in compound interest. A 25-year-old investing $5,000/year at 8% will have more than a 35-year-old investing the same amount — simply due to 10 extra years of compounding. The earlier you start, the less you need to invest.

Frequently Asked Questions

How often does interest compound?

It depends on the account or investment. Common compounding frequencies include annually, quarterly, monthly, and daily. More frequent compounding means slightly more growth.

Does compound interest work against me too?

Yes — on debts like credit cards or loans, compound interest works against you. This is why paying off high-interest debt quickly is so important.

What’s the difference between APR and APY?

APR (Annual Percentage Rate) doesn’t account for compounding. APY (Annual Percentage Yield) does. APY is the more accurate measure of what you’ll actually earn or owe.