Compound Interest Calculator
Compound interest is the most powerful force in personal finance — your money earns returns on both the original principal and the accumulated interest from previous periods. This calculator shows how regular contributions and time can grow your savings significantly.
Calculate Compound Growth
This tool is for educational purposes only. It is not financial advice. Consult a qualified financial professional for personalized guidance.
How Compound Interest Works
With simple interest, you earn returns only on your original principal. With compound interest, you earn returns on both your principal and all previously accumulated interest. Over long periods, this difference is dramatic.
| Factor | Effect on Growth | Example |
|---|---|---|
| Starting Principal | Higher starting amount means more compounding from day one | $10,000 at 7% for 30 years = $76,123 |
| Regular Contributions | Consistent additions accelerate growth dramatically | $500/month at 7% for 30 years = $566,765 |
| Interest Rate | Even 1-2% higher rate makes a large difference over decades | $10K at 5% vs 7% for 30 years: $43K vs $76K |
| Time | The most powerful factor — longer timeframes create exponential growth | $10K at 7%: 10 yrs = $20K, 20 yrs = $39K, 30 yrs = $76K |
| Compounding Frequency | More frequent compounding produces slightly higher returns | Monthly vs annual on $10K at 7%: ~0.3% more over 10 years |
Common Return Rate Benchmarks
| Investment Type | Historical Average Annual Return | Risk Level |
|---|---|---|
| High-yield savings account | 4–5% (current rates; historically 1–2%) | Very low |
| Certificates of deposit (CDs) | 4–5% (current); historically 2–3% | Very low |
| Government bonds (10-yr Treasury) | 4–5% | Low |
| Corporate bonds (investment grade) | 5–6% | Low-moderate |
| Balanced fund (60/40 stocks/bonds) | 7–8% | Moderate |
| S&P 500 index fund | 10–11% (nominal); ~7% after inflation | Moderate-high |
| Small-cap stocks | 11–12% (nominal) | High |
Note: Past performance does not guarantee future returns. All investments carry risk of loss.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes for an investment to double. Divide 72 by the annual return rate. At 7% annual returns, your money doubles approximately every 10.3 years (72 ÷ 7 = 10.3). At 10%, it doubles every 7.2 years.
Should I use the nominal return or inflation-adjusted return?
For long-term planning, use inflation-adjusted (real) returns for a more realistic picture of purchasing power. The S&P 500 has returned about 10% nominally but roughly 7% after inflation over the long term. For comparing investment options, nominal rates are fine as long as you compare consistently.
How much should I be saving monthly?
A common guideline is to save 15–20% of gross income for retirement. If you start at 25, saving 15% of income with a 7% return can build a substantial nest egg by 65. Starting later requires a higher savings rate. The most important step is to start — even small amounts benefit from compounding over time.
Does compounding frequency matter?
It matters, but less than most people think. The difference between monthly and daily compounding on $10,000 at 7% over 10 years is about $17. The difference between annual and monthly compounding is about $200 on the same amount. Focus more on the rate and your contribution amount than on compounding frequency.