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Understand compound interest with real examples, the Rule of 72, daily vs monthly vs annual compounding, and how to use it to build wealth or avoid debt traps.
Everything you need to know
Compound interest is interest calculated on both your initial principal AND the accumulated interest from previous periods. Unlike simple interest (calculated only on the principal), compound interest creates exponential growth — your money earns returns, and those returns earn returns.
Albert Einstein reportedly called it the "eighth wonder of the world." Whether or not he said it, the math is real and powerful.
See It in Action: Use our free Compound Interest Calculator to visualize exactly how your money grows over time.
$10,000 invested at 8% for 20 years:
| Year | Simple Interest | Compound Interest (Annual) | Compound Interest (Monthly) |
|---|---|---|---|
| 1 | $10,800 | $10,800 | $10,830 |
| 5 | $14,000 | $14,693 | $14,898 |
| 10 | $18,000 | $21,589 | $22,196 |
| 20 | $26,000 | $46,610 | $49,268 |
After 20 years, compound interest created $20,610 more than simple interest. The monthly compounding created an additional $2,658 over annual compounding — just from more frequent calculation.
A = P × (1 + r/n)^(nt)
Where:
| Frequency | n value | $10,000 at 8% for 20 years |
|---|---|---|
| Annually | 1 | $46,610 |
| Semi-annually | 2 | $47,799 |
| Quarterly | 4 | $48,451 |
| Monthly | 12 | $49,268 |
| Daily | 365 | $49,530 |
The Rule of 72 lets you estimate how long it takes money to double:
Years to Double = 72 ÷ Annual Interest Rate
Examples:
This also works in reverse for debt or inflation.
The same exponential growth that builds wealth destroys it when applied to debt. A credit card at 22% APR compounds monthly:
If you invest instead of paying off high-interest debt, you're betting on 12% investment returns while paying 22% interest — a guaranteed loss.