📈 Investment Growth Simulator
See exactly what your investment portfolio becomes over time. Account for compound growth, regular contributions, and the impact of market volatility on your long-term returns.
Investment Growth Trajectory
What is the Investment Growth Simulator?
The Investment Growth Simulator is built to answer one powerful question: "What will my portfolio actually be worth in 20 years?" Most people have a rough sense that "compound interest is powerful" but no concrete picture of the dollar value. This simulator turns that abstract idea into a vivid, personalized projection.
Unlike a simple compound interest calculator, this tool layers in realistic investment features: expected returns adjusted for volatility, dividend reinvestment, and time horizon effects. The result is a much more honest projection of what stock market investing actually delivers.
The Math Behind It
The core formula uses the future value of an annuity with compound growth:
Where P is the initial investment, r is the periodic (monthly) return, t is the number of periods, and PMT is the monthly contribution. We layer in dividend yield as additional compounding, and apply a "volatility haircut" that reduces the effective return slightly to account for sequence-of-returns risk and behavioral drag.
How to Use This Simulator
- Enter your initial investment amount and how much you'll add each month.
- Set your expected return. Stocks have historically returned 7–10% nominal; bonds 3–5%.
- Add volatility (the standard deviation of returns) to model real-world risk.
- Include dividend yield if your investments pay distributions (S&P 500 averages ~2%).
- Set your time horizon and click "Run Simulation" to see the full projection.
Benefits of Visualizing Investment Growth
Numbers on a page become real when you see them on a chart. A projection showing $400,000 in 20 years hits different from a vague idea that "I should invest more." It also shows the cost of waiting: starting at 25 versus 35 typically doubles the end result, even with the same monthly contribution.
The simulator also helps with realistic expectations. By factoring in volatility and dividends, you get a more honest projection than the rosy "10% annual return" sales pitch. Use it to stress-test your retirement plan or to set specific dollar-amount goals.
Frequently Asked Questions
What return should I use for stocks?
Historical S&P 500 nominal return is ~10%. After inflation, real return is ~7%. For a 20+ year horizon, 7–8% is a reasonable expected return, and accounts for the volatility haircut.
Does volatility reduce my expected return?
Volatility itself doesn't reduce geometric average return in a random-walk model, but it creates sequence-of-returns risk — withdrawing during a bad year hurts much more than withdrawing during a good one. For accumulation, volatility matters less.
Should I include dividends?
Yes. Dividend reinvestment is a significant contributor to long-term total return. The S&P 500's ~2% dividend yield compounds meaningfully over decades.
What about taxes?
This is a pre-tax projection. In a taxable account, drag from capital gains and dividend taxes can reduce returns 0.5–1.5% annually. Tax-advantaged accounts (401k, IRA) avoid most of this.
Can I model bear markets?
The simulator uses expected returns averaged over time. A real 20-year period will include 2–3 major bear markets. The long-term average smooths these out, but be prepared for year-to-year volatility of ±15%.
Related Simulators
- Future Wealth Simulator — total net worth picture.
- Savings Growth Simulator — for guaranteed-rate accounts.
- Retirement Projection Simulator — long-term nest egg planning.
- Time Value Simulator — see what your time is worth.