The true power of compound interest

Albert Einstein reportedly called compound interest "the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." Whether he actually said it is disputed, but the sentiment is correct. Compound growth is one of the few things in finance that genuinely deserves the word "magical."

Simple vs compound: the dramatic difference

$10,000 at 8% simple interest for 40 years = $42,000. The same at 8% compound = $217,000. Same rate, same time, same starting amount. The only difference is letting the interest compound.

The rule of 72

Divide 72 by your rate to know how long it takes money to double. At 8%, money doubles every 9 years. At 12%, every 6 years. Small rate differences matter enormously over long periods.

Why starting early crushes starting late

Anna invests $5,000/year from age 25 to 35, then stops ($50,000 total). Ben invests $5,000/year from 35 to 65 ($150,000 total). At 8%, Anna ends up with $787,000 at 65. Ben ends up with $611,000. Anna invested 1/3 as much and came out ahead by $176,000. Time is the most valuable asset.

The dark side: compound debt

Compounding works against you too. A $10,000 credit card balance at 22% APR, paying only the minimum, takes 46 years to pay off and costs $25,000 in interest. Pay off high-interest debt before investing.

What 1% extra return costs

A portfolio growing at 7% instead of 8% over 40 years ends up with 30% less money. The investment industry quietly charges fees that look small (1-2%) but consume a huge chunk of final wealth. Always check expense ratios.