Does this account for inflation or taxes?

No — this is a simplified illustration of compounding math, not a financial projection. It doesn't account for taxes, fees, inflation, or the sequence of real market returns, which are never a smooth straight line.

Why does the monthly contribution make such a big difference?

Each new contribution gets its own compounding runway. A dollar added in year one compounds for the full period, while a dollar added in year 20 has much less time to grow — so starting early and contributing consistently both matter more than the exact dollar amount.

Investing

Compound Interest Calculator

Drag the chart or adjust the sliders to see how a one-time $1,000 investment, plus an optional monthly contribution, compounds over time at a 7.5% average annual return — roughly the long-run average of a broad U.S. stock index.

Interactive · drag to explore

What your $1,000 can become

Added each month$100

Years invested30

Cash, not invested

$1,000

$1,000 invested once

$9,422

$1,000 + monthly

$144,166

Assumes a 7.5% average annual return, compounded monthly — roughly the long-run average of a broad U.S. stock index. Markets don't move in straight lines; this shows the math of staying invested, not a guarantee.

Frequently Asked Questions

A 7.5% average annual return, compounded monthly. This approximates the long-run historical average of a broad U.S. stock index fund like an S&P 500 tracker, after accounting for some inflation drag. Actual returns vary significantly year to year.

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