Monte Carlo Investment Simulator
Run thousands of scenarios with geometric Brownian growth. Enter your return assumptions, see the distribution of outcomes. Runs 100% in your browser.
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Added at the start of each year. Set to 0 for a lump-sum simulation.
Historical US equity ≈ 7% real, 10% nominal. Bonds ≈ 2–4%. Mixed ≈ 5–7%.
US equity ≈ 15–20%. 60/40 portfolio ≈ 9–12%. Bonds ≈ 5–7%.
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Monte Carlo Simulation
Median terminal value
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Mean terminal value
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10th percentile
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90th percentile
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Total contributed
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Portfolio value over time (percentile bands)
Terminal value distribution
5–95% band 20–80% band Median Target
Click Run simulation to see results.
What is a Monte Carlo simulation?
Instead of projecting one "expected" growth path, Monte Carlo runs thousands of randomized scenarios and shows the distribution of possible outcomes. It answers "what could happen?" rather than "what will happen?"
How this simulator works
- Each year, the portfolio grows by a random return drawn from a normal distribution with the mean and std dev you enter.
- Contributions are added at the start of each year, before that year's return is applied.
- This is geometric growth — compounding year-over-year.
- We run N trials in parallel, then compute percentiles across all of them at each time step.
Limitations worth knowing
- Normal-distributed annual returns understate tail risk. Real markets have fatter tails.
- No inflation adjustment. Enter a real (inflation-adjusted) return if you want real dollars.
- No taxes, fees, or behavioral sequence-of-returns analysis.
- Past performance does not predict future performance. This is a thinking tool, not a crystal ball.