Will my money last? A Monte Carlo simulation of your wealth

What this tells you

Nobody knows what markets will do. A single projection that assumes the same return every year hides that. This calculator runs your money through hundreds of different possible market histories instead. Each history is called a run. Some runs have good years early, some have bad years early, and most are somewhere in between.

The headline is the chance the money lasts: the share of runs in which your savings covered every year of spending for the whole period. If it says 85%, then in 85 of every 100 runs the money did not run out. In the other 15 it did.

The chart draws every run as a faint line. Where many lines overlap, the chart is darker, so the dark band is where outcomes cluster. Three solid lines summarise them. The median is the middle run: half ended higher and half lower. The 10th percentile is the run that did better than only one in ten. The 90th percentile did better than nine in ten.

Reading the chart

Some runs grow very large, which squeezes everything else against the bottom of the chart. Choose Log above the chart to switch to a logarithmic scale. On that scale each gridline is ten times the one below it, so a run that doubles climbs the same height whether it starts at $100,000 or $1 million. Runs that reach zero drop to the floor of the chart.

Choose today's dollars to remove inflation. Inflation is the general rise in prices. A dollar thirty years from now will buy much less than a dollar today, so large future amounts can look better than they are. In today's dollars, every amount is shown as what it would buy now.

The other figures

• Ending wealth shows the bottom 10%, median and top 10% of runs at the end of the period.
• Realized yearly return is the steady yearly rate that would have turned your investments' starting value into their ending value. It works like an APY (annual percentage yield) on a savings account, but it is measured after all the ups and downs. Money you add or spend does not change it, so it describes the market each run saw rather than your budget.
• Largest drop from a peak is the biggest fall in your wealth from its highest point so far. People call this a drawdown. It includes both market losses and what your spending took out. A drop of 100% means the money ran out.
• Typical year the money runs out is the middle year among the runs where it did.
• Starting withdrawal rate is your first year's spending, minus income, as a share of what you start with.
• Buying power lost to inflation is how much less a dollar buys at the end than today.

Why the average return is not what you get

If a portfolio loses 20% one year and gains 20% the next, it does not end up where it started. It ends 4% lower, because the gain applies to a smaller amount. The more returns swing, the more this costs. This is sometimes called volatility drag. Volatility is how widely a single year's return swings around the average.

So an average return of 8% a year, with typical swings, compounds at closer to 7% in the middle run. From 1928 to 2025, US stocks averaged about 11.9% a year but compounded at about 10.0%. The notes under the result give the figure for your inputs.

When the bad years arrive matters

Two runs can have the same average return and very different endings. If big losses come early, while you are also withdrawing money, you sell more of your investments at low prices. Less is left to recover when markets rise. If the same losses come late, far less damage is done. This is called sequence of returns risk. It is the main reason a single steady-return projection can be misleading, and the main thing a simulation like this shows.

How it works

1. Each year, your income comes in and your spending goes out. Both start at today's amounts and rise with inflation.
2. What is left is invested for the year at a return drawn at random. The draws average out to your average yearly return and swing by your volatility.
3. If your savings plus income cannot cover a year's spending, that run has run out of money. It stays at zero from then on.
4. This repeats for every year and every run.

The random draws come from a fixed starting number, the scenario set. The same inputs always give the same result, and a shared link shows exactly what you saw. Change the scenario set to draw a different group of market histories. If the chance of success moves a lot between sets, try more runs.

What this does not include

• Taxes on withdrawals, dividends or gains, and investment fees. Both lower real returns.
• Real market behaviour beyond random yearly swings. Real markets have long winning and losing streaks, sudden crashes, and years that are worse than this model expects.
• Changes you might make along the way, such as spending less after a bad year, working longer, or receiving an inheritance.
• Social Security, pensions that start later, one-off expenses, or a home you could sell. Add regular income from the first year in yearly income. There is no way to start it later.
• Inflation that changes over time.

A result is a range of possibilities under these assumptions, not a forecast.

Common questions

Why does my result change when I change the scenario set?

Each set is a different group of possible market histories. With a few hundred runs, the percentages move a little from set to set. More runs make them steadier.

What does a 0 at the bottom 10% mean?

At least one run in ten ran out of money before the end. The chance the money lasts tells you how many.

Why does moving a slider not reshuffle the lines?

Each run keeps its own market history when you change your savings, spending, the number of years or the number of runs. That way a change you make shows its own effect, not a new roll of the dice.

What return and volatility should I use?

That depends on how your money is invested, and no one knows future returns. US stocks averaged about 11.9% a year from 1928 to 2025, swinging by about 19% a year. A mix with bonds usually averages less and swings less. Trying a few different values shows how sensitive the result is.