Albert Einstein reportedly called compound interest "the eighth wonder of the world." Whether he actually said it or not, the math behind compound interest is genuinely remarkable — and understanding it is essential for building wealth.
What Is Compound Interest?
Simple interest is calculated only on your original deposit. Compound interest is calculated on your deposit plus all previously earned interest. This creates exponential growth rather than linear growth.
Simple interest example: $10,000 at 7% for 30 years = $10,000 + ($700 × 30) = $31,000
Compound interest: $10,000 at 7% for 30 years = $76,123
Compound interest earns you $45,123 more — without any additional effort. That's the power of earning interest on interest.
The Rule of 72
Want to know how long it takes to double your money? Divide 72 by your annual return rate.
- At 6%: 72 ÷ 6 = 12 years to double
- At 7%: 72 ÷ 7 = 10.3 years
- At 10%: 72 ÷ 10 = 7.2 years
Why Starting Early Matters (Dramatically)
Consider two investors:
Investor A starts at age 25, invests $300/month for 10 years (ages 25-35), then stops. Total invested: $36,000.
Investor B starts at age 35, invests $300/month for 30 years (ages 35-65). Total invested: $108,000.
At 7% annual return, Investor A (who invested only $36,000) ends up with more money at age 65 than Investor B (who invested $108,000). That's because Investor A had 10 extra years of compounding, even though they contributed 3x less money.
Practical Applications
- 401(k) and IRA: Start contributing as early as possible. Even $100/month at age 22 becomes significant by retirement.
- Savings accounts: High-yield savings accounts compound daily, maximizing returns on your emergency fund.
- Debt: Compound interest works against you with credit card debt. A $5,000 balance at 20% APR costs $1,000/year in interest alone.
Use our compound interest calculator with its visual growth chart to see exactly how your investments will grow.