How many scenarios should I run?

This simulator runs 10,000 trials by default. More trials give more accurate percentile estimates, but 10k is plenty for most financial planning questions.

What inputs do I need?

Initial portfolio value, expected annual return, return volatility, annual contribution amount, and time horizon in years. The simulator uses geometric, log-return style compounding with independent annual shocks.

Is this investment advice?

No. This is an educational tool to visualize a range of outcomes. It does not account for taxes, fees, inflation, or your personal risk tolerance. Consult a financial advisor before making real investment decisions.

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.

Best for Understanding a range of possible investment outcomes instead of relying on one forecast.

Use when You need percentile bands, downside cases, or probability of reaching a target under uncertain returns.

Output includes Median and mean terminal value, 10th/90th percentiles, success probability, and two charts.

Initial investment

Annual contribution

Added at the start of each year before that year's simulated return. Set to 0 for a lump-sum simulation.

Years

Expected annual return (%)

The long-run average return assumption used in simulated log-return compounding. It is not a guaranteed return.

Annual volatility — std dev (%)

Annual return uncertainty applied independently in every year and every trial.

Target amount (optional)

We'll show % of simulations that hit this. Leave blank to skip.

Number of trials

Presets

Quick starting points. Adjust any assumption after applying a preset.

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 uses an independent random annual shock and geometric, log-return style compounding.
Contributions are added at the start of each year, before that year's simulated return is applied.
The output is a simulated range of outcomes, not a guarantee or forecast.
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.

Pair with related tools

Deepen your investment analysis with:

NPV & IRR Calculator — evaluate project returns and discount rates.
Break-Even Calculator — understand your downside scenarios and coverage ratios.