$200 a month invested for 20 years

Want more scenarios like this? Explore the weekly hub and monthly hub, then run your own numbers in the calculator.

Looking specifically for the S&P 500 example? Read the S&P 500 version here →

A steady monthly habit plus time can create big results. Use the calculator to test conservative and optimistic assumptions.

What drives the outcome?

Your monthly contribution builds the base. Over time, compounding begins to dominate — especially in the later years.

Then change the timeline to 25 years and see how much time adds.

Keep exploring — these pages connect directly to calculators so you can run your own numbers.

Should I include an initial amount?

If you’re starting from zero, leave it at $0. If you already have savings/investments, enter your current balance.

Why do results vary so much by return rate?

Compounding magnifies differences. Small rate changes can create large long-term differences.

Does the calculator assume smooth returns?

Yes. Real markets fluctuate; the rate here is an average assumption.

Should I model inflation?

This shows nominal growth. If you want “today’s dollars,” you can reduce the return rate to approximate inflation.

Where can I learn the maths?

See /how-compound-interest-works.html.