Market range

Your forecast holds markets steady at your plan’s rates. Real markets swing, so the market range is a Monte Carlo analysis of the same plan: it runs it through 500 market paths, each a sequence of what markets did in past years, and counts how many your investable savings last to the end of your plan in, such as “in 498 of 500 market paths (99.6%)”.

Your investable savings are what’s in your cash, investment, retirement, education and health accounts. Your home, other property, a business and loans aren’t part of them. Savings last on a path when they stay above zero through the last year of your plan. If your plan starts with no investable savings, the range shows net worth only.

That’s a count of replayed markets: it shows how your plan holds up in markets like those of the past, and it doesn’t predict what markets will do.

The paths come from what US markets and prices did each year from 1928 to 2025: the S&P 500 with its dividends, ten-year Treasury bonds, three-month Treasury bills, consumer prices and home prices. Each path uses those markets in place of your plan’s rates.

The swings are history’s, the middle is LongSpan’s. Each series keeps every year’s rise and fall exactly, with its average over the whole record moved to LongSpan’s long-term rates: stocks 4.0% a year above inflation, bonds 2.0%, cash 0.75%, inflation 2.4% and homes flat after inflation. History’s own averages carry a fall in interest rates and a rise in stock prices that can’t repeat from here, so they would make every path too hopeful.

Your forecast uses your rates, so it can sit anywhere in the range: near the middle on the default rates, near the top if your rates are hopeful, lower if they’re cautious. Where it sits measures your rates against LongSpan’s. The market range page says which percentile your forecast is at when you retire.

• Years come in runs, about 10 in a row on average, so a crash keeps the years around it, the way you’d live through it.
• Every value in a year comes from the same past year: a path that replays 1974 replays its stocks, bonds, bills, prices and housing together.
• Every run is the whole forecast: taxes, required withdrawals, the order your accounts are drawn from and Roth conversions all respond to each path’s markets.
• The paths are the same every time, so your range changes only when your plan does.

Each past year gives your plan what that year’s markets and prices did, at LongSpan’s long-term levels. The stock, bond and cash rates you set aren’t used on a path; how each account is invested is.

• Your retirement, investment, education and health accounts earn what stocks, bonds and cash returned, in the shares each holds. An account on your plan’s mix holds that year’s lifetime mix: by default 90% stocks while retirement is 25 or more years away, 50% at retirement and 30% from 7 years after. An account on a fixed mix holds its own share of stocks every year.
• Your cash earns what Treasury bills did if it earns interest, and nothing if it doesn’t.
• Your property earns what home prices did, unless you set it to hold its value, when it holds still.
• Your equity grants earn what stocks did.
• Your costs, contributions, pay, Social Security and the tax brackets move with inflation. A pension, an annuity or rent pays what you said it would.

Loans, fees, dividends and business stakes follow your plan on every path. In today’s dollars, each path is read at its own prices.

A crypto account grows at the rate you set on every path. Crypto has traded only since about 2010, so there are no crypto years to replay beside 1929, 1974 or 2008. In reality it has swung several times as much as stocks, so the range shows less spread for a plan holding crypto than that plan would really meet.

Each year, the 500 paths are ranked by net worth. The darker band runs from the 25th to the 75th percentile, and the lighter band from the 10th to the 90th. The line is your forecast, which can sit anywhere among them.

The bands are worked out year by year, so they don’t follow the same paths across the chart, and a single path can move between them: a path near the top one year can be near the bottom later.

• The outcomes table reads your net worth at retirement at the 10th percentile, the median and the 90th percentile of the paths.
• “Your forecast at retirement is at the 44th percentile: 222 of the 500 paths end lower, 278 higher” counts the paths whose net worth that year ends below and above your forecast’s. A path level with it counts as half of each. The 50th percentile is the middle.

On the paths where your investable savings reach zero before the end of your plan, LongSpan notes the first year they do. It shows the age the earliest of them runs short at and, where at least 10 paths do, the age half of them have run short by. The details on the market range page count the paths running short year by year.