Fan charts are easy to misread because they look like weather maps. A cone of outcomes is not a forecast of your next thirty years with an error bar. It is 1,000 (or 5,000) complete lives, stacked by year, with a percentile taken across lives at each year. The median line is the middle life at that date, not the path you will follow. The p10 line is the 100th-worst life at that date in a 1,000-path sample, not "the crash."
This page uses the same B4_RETIREE freeze as the 1,000-lifetimes piece: $1,000,000, age 65, rigid 4%, 30 years, seed 20260622, 1,000 paths, real (today's-dollar) fan. The treatment is which percentile you read. The household does not move.
How to read a cone of outcomes
Start in the middle. The p50 line on this freeze stays near a million of today's dollars for most of the horizon and ends at $1,083,603. That is not growth. That is a withdrawal program that roughly held real capital in the typical world. If you expected a 7% machine to triple the pot, you were reading a different chart. The dashed 7% path on the sibling article is that other chart.
Then read the top. p90 at year 10 is $2,305,367; at year 30 it is $5,692,838. Kind sequences, same spending, same mix. Those lives are real in the model. They are not a plan. Planning to the p90 is how a household spends a future it has not been issued.
Then read the bottom. p10 at year 10 is $420,744; at year 20, $168,407; at year 30, $0. The line did not "choose" zero. Enough lives had already failed that the tenth percentile is the floor. 10.4% of the sample hit zero. A cone that kisses the axis is telling you the left tail is not a thin scrape. It is a mass.
Median, best 10%, worst 10%
Language fails here. "Best 10%" sounds like a prize. It is the 100th-best of 1,000 modelled lives at that year, or the 90th percentile, depending on how the caption is written. This chart uses p90 as the upper line: 90% of lives are below it. "Worst 10%" is p10: 10% of lives are below it. Neither line is a scenario you can bookmark as "if there is a crash." Crashes happen at different dates. A life that is p10 at year 8 may not be p10 at year 22. Percentiles are not identities. They are snapshots.
That is the most common cone mistake. People pick the p10 line with their eyes and imagine riding it. They cannot. Next year they will be re-sorted. A household that is unlucky early and then average will jump from the lower edge toward the middle. A household that is average and then unlucky will fall. The cone does not show those identities. Showing them would be a spaghetti plot of 1,000 polylines, which is honest and unreadable. The cone is the readable lie. It is a useful lie if you know what it dropped.
p25 and p75, not drawn as the main series here, sit at $346,431 and $2,672,674 at year 30. The interquartile range is the "usual" band if you insist on that word. Usual still includes a four-fold gap. Usual is not safe. Usual is a width.
What the cone is not
It is not a confidence interval around a forecast. A 90% cone is not "we are 90% sure you will land in here." It is "90% of modelled lives landed in here, given this generator, this household, this seed." Change the generator and the cone moves. The overstatement article is that warning. More paths shrink the wiggle in the percentile estimates. They do not make the generator true.
It is not history. Q3 was not run. 1966, 1973, 2000 and 2008 are not drawn on this fan. A historical fan would be overlapping windows, with tiny independent n. Do not point at this picture and say "the twentieth century."
It is not advice. The cone does not tell you to spend less. It tells you what this spending did in this model. If the p10 line bothers you, that is a preference. Translate the preference into a different rate or a different rule, then freeze again. Do not squint at the ink until it looks kinder.
Scope
Q1 run. Q2 not run: the cone is inside this calibration. Q3 not run: not a historical cone. Q4: 5 bps, tax off, real fan. Teaching identity. Information set: observation before the step's return. What would falsify: p10, p50 and p90 collapsing onto one line, or p10 being a single path rather than a cross-section. The freeze is a cone, not a thread.
Year by year, in dollars
Year 1: p10 $817,725, p50 $1,006,614, p90 $1,241,468. The cone is already open after one year because the first year is already a draw. Year 8: p10 $462,678, p50 $982,746, p90 $1,995,040. Year 16: p10 $251,366, p50 $991,668, p90 $3,094,493. You can watch the lower edge walk toward the floor without inventing a story about which crash did it. The freeze will not name a crash. It will name a percentile.
The p25 line at year 30 is $346,431. That is still a surviving household in the model, thinner than the start, not ruined. The p10 line is ruined. The gap between p25 and p10 is the difference between "tight" and "done." A cone that only drew p25 and p75 would have hidden the floor. We drew p10 because the floor is the point of the chart.
If you take one habit from this page, take this: when someone shows you a cone of outcomes, ask whether the lines are percentiles across lives or error bars around a forecast. If they cannot say, they do not know what they plotted. This freeze is percentiles across 1,000 lives. It is not a confidence interval around $1,083,603.
Read one on a tool
The guide how to read a Monte Carlo is the companion walkthrough. The Monte Carlo retirement calculator is the cone on numbers you type. When that tool draws three lines, name them out loud: tenth percentile, median, ninetieth percentile, across lives, at each year, in today's dollars if the caption says real. If the tool cannot say those words, it drew decoration. This freeze can say them: p10 $0, p50 $1,083,603, p90 $5,692,838 at year 30, with 10.4% of lives already at the floor. That sentence is the whole literacy this page is for.
What this page refuses to say
It refuses to say the p10 line is your crash plan. It refuses to say the p90 line is available if you stay optimistic. It refuses to say the median of $1,083,603 is what you will have. It refuses to call the cone a 90% confidence interval. It refuses to draw 1,000 spaghetti paths and then pretend the cone was those identities. It refuses to drop p10 because it looks rude. Rude is the point of p10. The floor at year 30 is the result 10.4% of lives already named as ruin.
It refuses a historical caption. 1973 is not on this fan. It refuses an ERP stress caption. A lower premium would pull every line down. We did not run that here. It refuses advice: the cone does not tell you to spend 3%. It tells you what 4% did in this model. Translate a preference into a new card, then freeze again.
It refuses to treat year 0 as a finding. Year 0 is $1,000,000 on every line because that is the start. The finding is the opening of the band and the pin at the floor. If a chart starts the cone at year 10, ask what happened in the first decade. On this freeze the first decade already opened p10 to $420,744 and p90 to $2,305,367.
Replication spec
Shared freeze with 1000-simulated-lifetimes. Real yearly p10/p50/p90 are sampled from the monthly fan at months 0, 12, 24, ..., 360. Year 4: p10 $624,152, p50 $997,106, p90 $1,544,449. Year 12: p10 $353,258, p50 $987,505, p90 $2,490,492. Year 24: p10 $103,319, p50 $1,000,947, p90 $4,055,607. Year 28: p10 $31,372, p50 $1,038,570, p90 $4,804,034. The p10 line is already thin by year 24 ($103,319) and is at the floor at year 30.
p25 and p75 at year 30 ($346,431 and $2,672,674) are the interquartile band. They are not drawn as the main series because the floor is the lesson, and p25 still hides it. Ruin share 10.4% is the count behind the p10 pin. 1,000 paths, seed 20260622, rigid 4%, real dollars. CSV: /data/studies/how-to-read-a-cone-of-outcomes.csv.
If you rebuild the fan and p50 at year 10 is not $1,005,962, you are not on this freeze. Do not keep this caption. Percentiles are not identities: a life that is p10 at year 10 is not required to be p10 at year 20. The cone never said it was.
Teaching identity. No recommended spending. No historical cone. No ERP stress. 5 bps, tax off. The matching guide is how to read a Monte Carlo. This page is the picture that guide describes, with this specimen's dollars on the axis.
Last recap: a cone of outcomes is p10, p50, p90 across lives, not a tunnel. On this freeze those three at year 30 are $0, $1,083,603, and $5,692,838. The floor on p10 is the 10.4% ruin mass, not a plotting bug. Median is not a promise. p10 is not the crash. p90 is not a plan. If you remember only that, you can read every fan chart on this site without inventing a life you were not issued.
Notes. Shared freeze with 1000-simulated-lifetimes. B4_RETIREE rigid 4%, seed 20260622, 1,000 paths, real p10/p50/p90 yearly from the monthly fan. Q1 run. Q2/Q3 not run. Q4: 5 bps, tax off. CSV: /data/studies/how-to-read-a-cone-of-outcomes.csv. Card dated 2026-08-28. This article is educational analysis, not investment advice, and does not recommend any security or strategy.