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Investment growth.

See what consistent investing actually does over time. The eighth wonder of the world, plotted clearly.

Growth over time
Contributed Total balance
Balance in 30 years
$854,537
Assuming 8% annual return
Total Contributed
$190,000
Interest Earned
350% gain on contributions
$664,537
The math, plainly

You'll put in $190,000. Compounding adds $664,537 on top. That's the eighth wonder doing its thing.

Read the math

Every formula on this page is explained in plain English, with the worked examples that produced it.

8 min read

Compound Interest: Why Starting at 25 vs 35 Matters More Than You Think

The math of compounding doesn't care about your intentions, only your start date. Here's what a 10-year head start actually costs (or saves) you in real dollars, and why it's almost impossible to catch up later.

Common questions

What return should I assume?

Long-term S&P 500 returns average around 10% per year before inflation, or about 7% after inflation. We default to 8% as a reasonable middle-ground assumption, adjust based on your portfolio's risk profile.

Why is compound interest such a big deal?

Because your earnings start earning their own earnings. Investing $500/month at 8% for 30 years gives you about $745,200, but you only put in $180,000. Compounding did the other $565,200 of work.

Roth IRA vs. 401(k), which first?

If your employer offers a 401(k) match, contribute enough to capture it first (free money). After that, a Roth IRA gives you more investment options and tax-free withdrawals in retirement. Most planners suggest: match → max Roth IRA → back to 401(k).

How does inflation affect this calculator?

We don't subtract inflation, the numbers shown are nominal dollars. To estimate real purchasing power, mentally subtract 2–3% from your assumed annual return.