Einstein supposedly called it the most powerful force in the universe. The math, the Rule of 72, and why 10 years of patience beats 20 years of cleverness.
Simple interest pays you on your original deposit. Compound interest pays you on your deposit AND on all the interest you've already earned.
Quick way to see how fast money doubles: divide 72 by your interest rate.
Two friends. Both invest $200/month at 8% return.
Key takeaway: Compound interest is what makes early starters unstoppable. The interest you earn starts earning interest of its own — and over decades, the snowball gets enormous. Time is more powerful than amount.
Questions people ask
Does compounding work in savings accounts too?
Yes, just at lower rates. A 4-5% high-yield savings account compounds the same way — just less dramatically than the long-term stock market.
What's a realistic long-term return to assume?
The S&P 500 has averaged about 10% annually over the past 100 years. After inflation, ~7%. Use 7-8% in your projections to stay realistic.
Can compounding work against me?
Yes — credit card interest compounds against you. That's why $1,000 at 24% APR can balloon if you only pay the minimum. Same math, opposite direction.
Should I wait to invest until I have more money?
No. The math strongly favors starting smaller, sooner. $20/month from 18 beats $200/month from 30 at retirement, even with 10x smaller contributions.
How often is compounding calculated?
Varies by product. Savings: usually daily. Investments: continuously as values fluctuate. Loans: monthly (or daily for credit cards).
What's the biggest threat to compounding?
Interrupting it. Withdrawing early, panic-selling, or stopping contributions during downturns all break the snowball. Boring consistency wins.