Compound Interest Explained — How Your Money Grows

Albert Einstein supposedly called compound interest the “eighth wonder of the world.” Whether or not he actually said it, the math backs it up. Compound interest is the single most powerful force in personal finance.

Simple vs Compound Interest

Simple interest is calculated only on the original principal. If you invest $10,000 at 5% simple interest, you earn $500 per year — forever.

Compound interest is calculated on the principal plus accumulated interest. That $10,000 at 5% compound interest earns $500 the first year, then $525 the second year (5% of $10,500), then $551.25 the third year, and so on. The growth accelerates over time.

The Compound Interest Formula

A = P(1 + r/n)^(nt)

Where:
A = final amount
P = principal (initial investment)
r = annual interest rate (decimal)
n = number of times compounded per year
t = number of years

Why Time Matters More Than Amount

Consider two investors:

• Alice invests $200/month from age 25 to 35 (10 years, $24,000 total), then stops.
• Bob invests $200/month from age 35 to 65 (30 years, $72,000 total).

At 7% annual return, Alice ends up with more money at 65 than Bob — despite investing for only 10 years and contributing 3x less. That’s the power of starting early.

How to Maximize Compound Interest

1. Start as early as possible — time is the biggest multiplier
2. Contribute regularly — monthly contributions add up dramatically
3. Reinvest dividends — don’t withdraw your earnings
4. Minimize fees — a 1% fee difference can cost you hundreds of thousands over 30 years
5. Be patient — compound interest is boring for the first decade, then explosive

The Rule of 72

Want a quick estimate of how long it takes to double your money? Divide 72 by your interest rate. At 8% return, your money doubles in roughly 9 years (72 ÷ 8 = 9). At 6%, it takes 12 years.