What is a Monte Carlo simulation?

Running the same plan many times over with a different random market history each time, then counting how often it worked. It answers "how often does this survive?" rather than "what is the single most likely balance?" — which is the more useful question when the stakes are your whole retirement.

Why does the calculator run to age 100?

Because planning only to your life expectancy leaves roughly half of people short. Age 100 is a deliberately conservative end point that most plans will never need but few people would regret having aimed for.

Why do I get the same answer every time?

The random numbers come from a fixed starting seed, so results are stable across visits, devices and page reloads. Two people entering identical plans see identical odds, and moving a slider back where it was restores the previous answer exactly.

Does it include taxes or Social Security?

No — neither is modeled. Enter spending as the amount you expect to draw from savings on top of Social Security, and keep in mind that withdrawals from traditional accounts are taxable in ways this tool ignores entirely.

Watercolor painting of a lone sailboat on a calm sea at sunset, seen from a clifftop meadow of wildflowers and tall grass

The method

How this calculator works

Every number on this site comes from the same small piece of math. Here is exactly what it does, what it assumes, and where it stops.

One thousand versions of your future

Most retirement calculators grow your money at a single steady rate — 7% a year, every year — and draw one line. Real markets do not behave that way, and a single line hides the thing you most want to know: how much room for error you have.

This site uses a Monte Carlo simulation instead, which means running your plan 1,000 times over with a different randomly drawn market history each time. Every year of every run gets its own return, pulled from a bell curve centred on the return you expect. Some runs get a wonderful first decade; some get a crash the year after you retire. The chart shows the middle half of those outcomes as a shaded band — solid up to the 25th percentile, fading through the median to the 75th — and the headline reports how many of the 1,000 still had money at 100. The random draws start from a fixed seed, so identical inputs always produce exactly the same answer.

Risk is tied to return, not typed in separately

You do not enter volatility, partly because few people have reliable intuition for it and partly because it is not independent of the return you expect. Instead the calculator derives it: roughly 2.5 points of year-to-year swing for every point of expected return above a 3% cash rate.

That formula is calibrated to history. The roughly 10% long-run return of U.S. stocks lands at about ±17.5%, close to the actual standard deviation of S&P 500 annual returns over the last three decades, while a cash-like 3% return carries no swing at all. The practical consequence is that raising the return does not simply improve your odds — it also widens the range of outcomes, sometimes by enough to cancel the gain. The worst any single year can be is −100%; the growth factor is floored at zero.

The 80% bar, today's dollars, and what is missing

"You could retire at 62" means the earliest age at which at least 80% of the 1,000 runs still had money left at 100. Eighty percent is a planning convention rather than a law, and it accepts openly that a fifth of futures would require you to adjust. The today's-dollars switch re-expresses the whole plan in constant purchasing power using the Fisher relation — a real return of (1 + nominal) ÷ (1 + inflation) − 1 — so every dollar on screen means what a dollar means now.

Deliberately absent: taxes of any kind, Social Security, pensions, part-time work, home equity, spending that changes through retirement, and market models with fatter tails than a bell curve. The spending you enter is what you draw from savings on top of Social Security, not your whole budget. This is a tool for exploring how the pieces of a plan interact with one another. It is not financial advice, and it does not know anything about you.

Questions people ask

What is a Monte Carlo simulation?: Running the same plan many times over with a different random market history each time, then counting how often it worked. It answers "how often does this survive?" rather than "what is the single most likely balance?" — which is the more useful question when the stakes are your whole retirement.
Why does the calculator run to age 100?: Because planning only to your life expectancy leaves roughly half of people short. Age 100 is a deliberately conservative end point that most plans will never need but few people would regret having aimed for.
Why do I get the same answer every time?: The random numbers come from a fixed starting seed, so results are stable across visits, devices and page reloads. Two people entering identical plans see identical odds, and moving a slider back where it was restores the previous answer exactly.
Does it include taxes or Social Security?: No — neither is modeled. Enter spending as the amount you expect to draw from savings on top of Social Security, and keep in mind that withdrawals from traditional accounts are taxable in ways this tool ignores entirely.

Keep exploring

The assumptions behind these numbers

Every figure comes from 1,000 simulated market paths assuming a 6% average yearly return in retirement (10% while still saving) with year-to-year swings scaled to those returns, and 3% inflation — so every dollar reads as today’s dollars, and “works” means the money lasts to age 100 in at least 80% of paths.

Social Security, pensions, and taxes aren’t modeled. If you expect income from those, your real answer is better than the one on this page. For exploring, not financial advice.