Compound interest is often called the “eighth wonder of the world” — and for good reason. It is the mechanism that turns consistent savings into substantial wealth over time. Whether you are saving for retirement, a child’s education, or a major purchase, understanding how compound interest works helps you set realistic goals and choose the right savings strategy.
Use the free Compound Interest Calculator to see how your money can grow.
How Compound Interest Works
Simple interest earns returns only on your original principal. Compound interest earns returns on both your principal AND previously earned interest. Over time, this creates exponential growth.
| Year | Simple Interest ($10K at 7%) | Compound Interest ($10K at 7%) |
|---|---|---|
| 1 | $10,700 | $10,700 |
| 5 | $13,500 | $14,026 |
| 10 | $17,000 | $19,672 |
| 20 | $24,000 | $38,697 |
| 30 | $31,000 | $76,123 |
After 30 years, compounding produces 2.5x more than simple interest on the same principal and rate.
The Compound Interest Formula
A = P(1 + r/n)^(nt)
- A = Final amount
- P = Principal (initial deposit)
- r = Annual interest rate (decimal)
- n = Number of times interest compounds per year
- t = Number of years
Don’t want to do the math yourself? Use the Compound Interest Calculator — it handles all the formulas instantly.
Three Factors That Impact Your Growth
1. Compounding Frequency
The more frequently interest compounds, the faster your money grows. Here’s how $10,000 at 7% APR grows over 10 years with different compounding schedules:
| Frequency | Final Amount |
|---|---|
| Annually | $19,672 |
| Semi-annually | $19,803 |
| Quarterly | $19,871 |
| Monthly | $19,920 |
| Daily | $19,946 |
| Continuously | $19,955 |
2. Time Horizon
Time is the most powerful variable in compound interest. Starting early gives your money more time to compound, which is why financial advisors stress the importance of beginning to save in your 20s.
3. Regular Contributions
Adding regular monthly contributions dramatically accelerates growth. A $10,000 initial deposit with $500/month at 7% grows to:
- 10 years: $107,074
- 20 years: $301,569
- 30 years: $690,438
The Rule of 72
A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 7%, your money doubles every ~10.3 years (72/7). At 10%, every ~7.2 years.
See the real numbers with the Compound Interest Calculator — try different rates, contributions, and timeframes to build your savings plan.
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