Compound Growth Calculator

Growth on growth — that's the whole trick

Simple interest pays you on your deposit and nothing else. Compound growth pays you on your deposit plus everything it has already earned — growth earning its own growth. Early on the difference is pocket change. Given decades, the growth can quietly outgrow everything you deposited.

Here is the shape of it. Put in $1,000, add $200 a month, and assume an 8% average yearly return for 10 years. You deposit $25,000 of your own money — but the account ends near $38,800. Roughly $13,800 of that is growth: money you never earned at a job or saved from a paycheck. Time did that.

That 8% is an assumption, not a promise. Real markets do not pay any rate like clockwork — they lurch up and down, and the average only shows up across long stretches. Fees and inflation also take a quiet cut every year, which this estimate leaves out. A lower, honest return assumption beats a rosy one.

The Rule of 72 below is the shortcut: 72 divided by your return is roughly the years money takes to double. It is most accurate near 8%. The exact-math divisor is 69.3 — 72 just divides cleanly by more rates, which is why the shortcut stuck. Either way, the biggest lever is starting early.

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