The Power of Compound Interest: How It Affects Your Mortgage and Wealth

Compound Interest: Your Greatest Ally or Enemy

Albert Einstein reportedly called compound interest the "eighth wonder of the world." Whether or not he actually said it, the concept is genuinely powerful. Compound interest is interest that earns interest — and it works in both directions.

How It Works Against You: Your Mortgage

With a mortgage, compound interest means you pay interest on your outstanding balance, and if you only make minimum payments, the interest portion keeps the principal from shrinking quickly.

On a $400,000, 30-year mortgage at 6%:

• Total payments over 30 years: approximately $863,000
• Total interest paid: approximately $463,000
• You pay more than double the original loan amount

This is compound interest working against you.

How It Works For You: Investments

When you invest, compound interest (or compound returns) works in your favor. Your returns generate their own returns, creating exponential growth over time.

Investing $500/month at 8% average annual return:

• After 10 years: approximately $91,000
• After 20 years: approximately $274,000
• After 30 years: approximately $680,000

Notice how the growth accelerates in later years — that is compounding at work.

The Key Insight

When you make extra mortgage payments, you are essentially stopping compound interest from working against you. When you invest, you are making it work for you. The question is: which effect is stronger for your specific situation?

Use our prepay vs. invest calculator to see which strategy builds more wealth in your specific case.

Time Is the Critical Factor

The longer your time horizon, the more powerful compounding becomes. This is why starting early — whether paying extra on your mortgage or investing — makes such a dramatic difference in outcomes. Even a few years head start can translate to tens of thousands of dollars.