Monte Carlo Retirement Simulator
See how your portfolio might grow across thousands of randomized market scenarios — not just one lucky average. Share the result with a link.
Your plan
How this Monte Carlo simulator works
Most retirement calculators assume your portfolio earns the same return every single year. Real markets don't work that way — some years are up 25%, some are down 20%, and the order those years arrive in matters. A Monte Carlo simulation captures that uncertainty.
For each simulated future, and for each year in it, the tool draws a random annual return from a normal (bell-curve) distribution centered on your expected return with a spread set by your volatility. Each year it applies that return to your balance and then adds your contribution:
next balance = balance × (1 + random return) + annual contribution
It repeats this for the number of years you set, records the whole path, and runs the entire experiment hundreds or thousands of times. Sorting the results by year produces the percentile bands you see: the 10th percentile is a pessimistic path, the 50th is the median, and the 90th is an optimistic one.
Reading the chart
- The median line is the middle outcome — half of simulations did better, half did worse.
- The shaded bands show the range: the wide band spans the 10th to 90th percentile, the inner band the 25th to 75th.
- A wider fan means more uncertainty — driven mostly by higher volatility and a longer time horizon.
Assumptions & limits
- Returns are drawn independently each year from a normal distribution. Real returns have fatter tails and some autocorrelation, so treat the extremes as illustrative.
- Results are nominal. For today's dollars, subtract your inflation assumption from the expected return.
- Taxes, fees, and withdrawals are not modeled. This is an accumulation (saving) projection, not a withdrawal (spend-down) plan.
- Nothing is saved server-side; everything runs in your browser, and the same link always reproduces the same chart.
Frequently asked questions
What is a Monte Carlo retirement simulation?
It runs your plan through thousands of possible futures. Instead of assuming the same return every year, it draws a random return each year from a bell curve set by your expected return and volatility. Many paths together reveal the full range of outcomes rather than one misleading average.
Why not just use a fixed average return?
A fixed average hides risk. Two portfolios with the same average return can end up far apart depending on the order and size of good and bad years — “sequence of returns” risk. Monte Carlo makes that visible as the spread between the pessimistic and optimistic bands.
What return and volatility should I use?
That depends on your asset mix, and it's your assumption to make — not advice. As rough historical reference points, a broad US stock index has had long-run nominal returns in the high single digits with volatility around 15–18%, while a stock/bond blend is lower. Try a range to see how sensitive your plan is.
Are these results in today's dollars?
No — the projection is nominal by default. For today's purchasing power, subtract your inflation assumption from the expected return (e.g. enter 4% instead of 7% for a ~3% real return).
Can I share my simulation?
Yes. Every input lives in the URL and the simulation uses a fixed seed, so a shared link reproduces the exact same chart. Use “Copy shareable link”.
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