Albert Einstein may or may not have actually called compound interest the “eighth wonder of the world” — but whoever said it was onto something. Compound interest is the reason patient investors get rich and why credit card debt feels impossible to escape. Understanding it changes how you think about every financial decision.
What Is Compound Interest?
Simple interest earns interest only on the original principal. Compound interest earns interest on the principal AND on the interest you’ve already earned — “interest on interest.” Over time, this creates exponential rather than linear growth.
Formula: A = P(1 + r/n)^(nt)
- A = final amount
- P = principal (starting amount)
- r = annual interest rate (decimal)
- n = number of times interest compounds per year
- t = time in years
Use our free compound interest calculator to skip the formula entirely — just enter the numbers.
The Power of Compound Interest Over Time
| Initial Investment | Rate | After 10 Years | After 20 Years | After 30 Years |
|---|---|---|---|---|
| $5,000 | 7% | $9,836 | $19,348 | $38,061 |
| $5,000 | 10% | $12,969 | $33,637 | $87,247 |
| $10,000 | 7% | $19,672 | $38,697 | $76,123 |
Compound Interest Working Against You — Debt
The same force that makes savings grow also makes debt grow. Credit card interest compounds daily on most cards. A $5,000 credit card balance at 22% APR, paying only minimum payments ($100/month):
- Takes approximately 7 years and 5 months to pay off
- Costs approximately $3,800 in interest alone
- Total paid: approximately $8,800 for a $5,000 debt
Use the loan calculator to see your own credit card payoff timeline and find what monthly payment gets you out of debt in 1, 2, or 3 years.
The Rule of 72 — Quick Mental Math for Doubling Time
Divide 72 by the interest rate to find approximately how long your money takes to double:
- 72 ÷ 6% = 12 years to double
- 72 ÷ 8% = 9 years to double
- 72 ÷ 10% = 7.2 years to double
- 72 ÷ 22% (credit card) = 3.3 years for debt to double
Where to Find Compound Interest Working for You
- High-yield savings accounts (HYSA) — 4-5% APY as of 2025, compounding daily
- 401(k) and Roth IRA — tax-advantaged compound growth; Roth IRA grows completely tax-free
- Index funds (S&P 500) — historical average ~10% annually, compounded
- I-Bonds — inflation-linked government bonds, compound interest
Daily compounding gives the best results, but the difference between daily and monthly compounding is small at typical rates. The frequency of compounding matters far less than the rate and the time invested.
Yes — compound interest is the standard in banking and investing. All savings accounts, investments, and most loans use compound interest. The key is understanding whether it’s working for you (savings) or against you (debt).
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