Compound Interest & Why Time Beats Money

Compound interest is interest earned on interest. The longer it runs, the more aggressive it gets. Most people understand this in theory and underestimate it in practice.

Simple vs. Compound Interest

Simple interest: You invest $10,000 at 7%. You earn $700/year. After 10 years: $17,000.

Compound interest: You invest $10,000 at 7%, compounded annually. Year 1: $10,700. Year 2: interest on $10,700, not $10,000. After 10 years: $19,672. After 30 years: $76,123.

The Rule of 72 is the mental math shortcut for this: divide 72 by your annual rate and you get the approximate years to double. At 7%, that is about 10 years.

The Time Variable Is More Powerful Than the Money Variable

Two people. Same 7% annual return. Different start times:

• Alex invests $200/month starting at age 25, stops at 35, never invests another dollar
• Jordan invests $200/month starting at age 35, contributes until age 65

Alex invested for 10 years ($24,000 total). Jordan invested for 30 years ($72,000 total).

At age 65: Alex wins. By a lot. That 10-year head start compounded for 30 extra years beats 3x more contributions that started late.

What This Means Practically

The best investment decision you can make is to start now, even with a small amount, rather than waiting until you have "enough" to invest properly. The waiting cost is enormous and invisible - you don't feel it today, but the math is running either way.

Index Funds: The Practical Vehicle

You don't need to pick stocks to benefit from compound growth. A broad index fund (like one tracking the S&P 500) gives you exposure to the entire US stock market for minimal fees. Over any 20-year rolling period in the S&P 500's history, the index has been positive. That's not a guarantee - but it's the track record.

Low-cost index funds at Vanguard, Fidelity, or Schwab have expense ratios under 0.05%. That means fees cost you less than $5 per year on every $10,000 invested. Compare that to actively managed funds at 0.75-1.5%, which mathematically drag your returns down over decades.

Related

• The Rule of 72: the mental math shortcut for estimating doubling time at any rate
• How index funds actually work: the practical vehicle for capturing compound growth over time

Ten years of head start beats three times the contributions. That's the formula, not a motivational poster.