Everything you need to know about compound interest — the formula, how it works, real examples, and strategies to maximize your compound growth.
Compound interest is interest earned on both your original principal and on interest that has already been added to your balance. This creates a snowball effect — your money grows faster over time because the base keeps getting larger.
Albert Einstein allegedly called compound interest "the eighth wonder of the world." Whether or not he actually said it, the math supports the sentiment. A modest investment left to compound over decades can produce extraordinary results.
The standard formula is: A = P(1 + r/n)^(nt)
Where A = final amount, P = principal (starting amount), r = annual interest rate (as a decimal), n = number of times interest compounds per year, and t = number of years.
Example: $10,000 invested at 6% annual interest, compounded monthly, for 10 years:
A = 10,000(1 + 0.06/12)^(12×10) = 10,000(1.005)^120 = $18,193.97
That's $8,193.97 in interest earned — an 82% return on your investment, without adding another dollar.
Simple interest pays only on the original principal: I = P × r × t. On $10,000 at 6% for 10 years, simple interest earns $6,000. Compound interest (monthly) earns $8,194 — a $2,194 difference.
Over 30 years, the gap becomes dramatic. Simple interest: $18,000. Compound interest: $60,226. Same starting amount, same rate — compound interest earns 3.3× more.
More frequent compounding produces slightly higher returns. $10,000 at 6% for 10 years:
The difference between annual and daily is $311.96 over 10 years. It matters, but the rate and time period matter far more.
1. Start early. Every year you wait costs you. 10 extra years of compounding can double your final amount.
2. Contribute regularly. Even $100/month at 7% for 30 years grows to $121,997.
3. Reinvest all earnings. Every dollar pulled out stops compounding forever.
4. Use tax-advantaged accounts. 401(k)s and IRAs eliminate annual tax drag.
5. Stay invested. Market dips are temporary. Compound growth is permanent — if you stay in.