Compound Interest Calculator: The Math Behind 'Start Investing Early'

Everyone tells you to start investing early. Very few people show you the actual math.

Here it is.

$5,000/year invested from age 25 to 65 (40 years) at 7% average return: $1,068,000

$5,000/year invested from age 35 to 65 (30 years) at 7% average return: $472,000

Same annual investment. Same return rate. The 10-year head start is worth $596,000.

Why the Numbers Look Like That

The "magic" of compound interest is that your returns earn returns. In year one, you earn 7% on your $5,000. In year two, you earn 7% on your contributions plus 7% on last year's earnings. The longer this runs, the more of your final balance comes from earnings-on-earnings rather than your own contributions.

After 40 years of $5,000/year contributions, your total contribution is $200,000. Your final balance is $1,068,000. You contributed 19% of it. The other 81% is compound growth.

The Variables That Change Everything

Rate of return matters less than you think — in the short run. The difference between 6% and 8% over 10 years is modest. Over 40 years, it's enormous. Time is the most powerful variable; chasing higher returns is secondary.

Contribution frequency matters. Monthly contributions outperform the same annual amount because money gets invested earlier in the year and compounds longer. The difference is small but real.

Inflation adjustment matters for planning. A million dollars in 40 years is not the same as a million dollars today. Our calculator shows both nominal and inflation-adjusted final values.

[Try the compound interest calculator →](https://doesitaddup.com)

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