How Compound Interest Works

Q: What is compound interest?

Compound interest is interest earned on both your original money and on the interest it has already earned. Because each period's growth builds on the last, balances grow faster and faster over time; the curve gets steeper the longer you stay invested.

Q: What is the Rule of 72?

The Rule of 72 is a quick mental shortcut: divide 72 by your annual return to estimate how many years it takes money to double. At 8%, money doubles in about 9 years; at 6%, about 12 years.

Q: How much difference does starting early make?

An enormous difference. At 7%, $500 a month starting at age 25 grows to roughly $1.2 million by 65, while the same contributions starting at 35 reach only about $570,000. The extra decade of compounding roughly doubles the outcome.

Einstein supposedly called compound interest the eighth wonder of the world. Whether he said it or not, the math earns the hype: it's how ordinary savings become serious money. This guide explains how it works with real numbers. To run your own, use the free compound interest calculator.

In short: compound interest means you earn interest on your interest. Growth accelerates over time: slowly at first, then dramatically. Time in the market is the single biggest ingredient.

Simple vs. compound interest

Simple interest pays only on your original amount. Compound interest pays on the whole balance: original money plus every dollar of interest earned so far. The gap is small at first and enormous later:

$10,000 at 7%	Simple	Compound
After 10 years	$17,000	~$19,672
After 20 years	$24,000	~$38,697
After 30 years	$31,000	~$76,123

By year 30, compounding hasn't just beaten simple interest; it's produced nearly two and a half times as much.

The Rule of 72

Want a fast estimate of doubling time? Divide 72 by your annual return. At 8%, money doubles in about 9 years; at 6%, about 12; at 10%, about 7. Stack a few doublings and you see why long horizons matter: two doublings is 4× your money, three is 8×.

Why starting early wins

At 7% growth, $500 a month from age 25 reaches roughly $1.2 million by 65. Start at 35 instead and the same $500 a month reaches about $570,000. Ten fewer years of contributions ($60,000 less invested), yet roughly half the final result. The first dollars you invest are the most powerful ones you'll ever save, because they compound the longest.

Contributions supercharge it

Compounding rewards consistency. A one-time $10,000 at 7% becomes ~$76,000 in 30 years, but add just $200 a month and it becomes ~$319,000. Regular contributions keep feeding the curve, and each contribution starts its own compounding clock. Try different monthly amounts in the calculator and watch the chart change shape.

Compounding frequency: does it matter?

Less than you'd think. $10,000 at 7% for 20 years grows to $38,697 compounded annually, $40,387 monthly, and $40,546 daily. Frequency helps, but it's a rounding error next to the two things that dominate: your rate of return and your time horizon.

Frequently asked questions

What is compound interest?

Interest earned on both your original money and its accumulated interest. Each period builds on the last, so growth accelerates over time.

What is the Rule of 72?

Divide 72 by your annual return to estimate doubling time. At 8%, money doubles in ~9 years.

How much difference does starting early make?

At 7%, $500/month from 25 → ~$1.2M by 65; from 35 → ~$570k. The extra decade roughly doubles the outcome.

Run your own numbers: the free compound interest calculator shows your final balance, interest earned, and a year-by-year growth chart.

More guides: The Rule of 72: how fast money doubles · Daily vs. monthly compounding

Examples assume constant returns for illustration. Real markets vary and can lose money. Educational only, not investment, tax, or financial advice.

← Back to the calculator