Monte Carlo portfolio sim.

A Monte Carlo Portfolio Simulator is a free online tool that runs thousands of simulated investment paths — each year drawing a random return from a log-normal distribution around a user-set mean and volatility — and shows the odds of reaching a financial goal. It runs entirely in your browser; nothing is uploaded or stored.

Starting balance ($)

Annual contribution ($)

Years (5–50)

Expected return (%/yr)

Volatility (%/yr)

Goal target ($)

Simulations

Inflation-adjusted (2.5%/yr)

Educational toy, not financial advice. Past returns don’t predict the future.

–median final value

–P10 – P90 range

–goal-hit probability

–end below starting capital

Possible futures

P10–P90 bands median (P50) goal sample paths

Where you land

How it works

Each simulated year draws a return from a log-normal distribution centred on your expected return, with your chosen volatility as its spread. Normals come from the Box–Muller transform; your contribution is added at the start of each year, then the year’s growth compounds on top.

After N runs, every year’s values are sorted into P10 / P25 / P50 / P75 / P90 percentiles — half of all simulated futures land between P25 and P75, and 80% between P10 and P90. The median (P50) is the middle path, not the average.

The inflation toggle converts your expected return to a real return at 2.5% annual inflation, so the chart shows purchasing power rather than nominal dollars.

Built by Shipwright Studio · ← all lab demos