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About This Calculator
Model the growth of a lump-sum investment or recurring contributions over time using a specified annual rate of return. The calculator illustrates how reinvested earnings generate their own returns, demonstrating the powerful effect of compounding. Adjust the time horizon and contribution amounts to see how different strategies affect your final portfolio value.
Quick Tips
- 1 Low-cost index funds consistently outperform most actively managed funds over 10+ years.
- 2 Reinvest dividends automatically to harness compound growth on your returns.
- 3 Diversify across stocks, bonds, and real estate to reduce overall portfolio risk.
Example Calculation
An investor puts $10,000 initially and adds $500/month for 20 years at 8% annual return.
Total contributions: $130,000 | Investment growth: $164,413 | Final balance: $294,413
How Compound Interest Works
Compound interest generates earnings on both your initial principal and on all previously accumulated interest. Unlike simple interest, which calculates returns only on the original amount, compounding creates exponential growth over time. A $10,000 investment growing at 8% annually reaches approximately $21,600 after 10 years, $46,600 after 20 years, and $100,600 after 30 years, illustrating how returns accelerate dramatically in later years.
The Impact of Regular Contributions
Making consistent monthly contributions amplifies the power of compound interest significantly. Investing $300 per month at an 8% annual return accumulates roughly $178,000 over 20 years, of which only $72,000 represents your actual contributions. The remaining $106,000 comes from investment returns. Automating regular contributions also removes emotion from the investment process and ensures you invest consistently through both market highs and lows.
Investment Returns by Asset Class
Different asset classes have delivered varying historical returns over time. U.S. large-cap stocks (S&P 500) have averaged approximately 10% annually over the past century, while bonds have returned roughly 5%. Real estate investment trusts (REITs) have historically returned 8-12%, and high-yield savings accounts currently offer around 4-5% APY. A diversified portfolio blending these asset classes can balance growth potential with risk management.
Dollar-Cost Averaging Explained
Dollar-cost averaging (DCA) is the strategy of investing a fixed amount at regular intervals regardless of market conditions. When prices are high, your fixed amount buys fewer shares; when prices drop, it buys more. This approach reduces the risk of investing a large sum at a market peak and tends to lower your average cost per share over time. Studies show DCA is particularly effective for investors who might otherwise hesitate to invest during volatile markets.
Frequently Asked Questions
Compound interest means you earn returns on both your original investment and on the returns already earned. Over time, this creates exponential growth — often called the "eighth wonder of the world."
Historical averages: stocks ~10%, bonds ~5%, savings accounts ~4%. A diversified portfolio of 80% stocks/20% bonds has historically returned about 8–9% annually before inflation.
Enormous. Investing $300/month starting at age 25 at 8% returns yields ~$1.05 million by age 65. Starting at age 35 yields only ~$447,000 — less than half — despite contributing only $36,000 less.
This calculator shows pre-tax growth. In a tax-advantaged account (401k, IRA), the growth matches closely. In a taxable account, actual returns will be lower due to capital gains tax.