Investment Calculator

A range, not a single number

Most investment calculators ask for a return, say 7%, and print one tidy figure at the end. Real markets don’t deliver an average every year. Two people who save the same amount for the same twenty years can finish tens of thousands of dollars apart, depending on when the good and bad years happened to fall.

This calculator shows that spread. It replays your savings plan through thousands of simulated market histories built from real US returns since 1954. It then shows where the middle of those outcomes lands and how wide the range is around it. Everything runs in your browser; nothing you enter is sent anywhere.

How to read the chart

The solid line is the median. Half of the simulated paths end above it and half below. It is the closest thing to a “typical” result, but it is not a prediction.

The darker band is the middle half of outcomes. One path in four ends above it and one in four below.

The lighter band holds 9 in 10 outcomes. Its bottom edge is the pessimistic case in the result header: only one simulated path in ten does worse. Its top edge is the optimistic case.

The dashed line grows at a fixed 7% a year before inflation, the number many calculators assume. The dotted line is the median from plain history, with no adjustment for today’s market. Compare the solid and dotted lines to see how much today’s conditions change the picture.

Notice how the bands fan out. Early on, your contributions dominate and the range is narrow. Later, market returns dominate and the range widens fast. That widening is the honest answer to “what will I have?”

Today’s money or future dollars

By default the results are in today’s money: what the balance would buy at today’s prices. That is usually what people mean when they ask whether they will have enough. Switch to future dollars to see the larger number your statement will actually show. At 3% inflation, a future dollar buys about half as much after 23 years.

With Raise contributions with inflation on, your monthly amount keeps its buying power: at the historical inflation rate, $500 today becomes about $1,000 a month in 20 years. In today’s money that is still $500 a month, which is why “Total paid in” stays simple.

What “market stage” means

The tool reads the US market each day and sorts it into one of four stages, using public data from the Federal Reserve Bank of St. Louis:

• Expansion: prices are above their 10-month average, near their high, with no warning signs from bonds or credit.
• Late-cycle: still near the high, but the yield curve has inverted in the past year, corporate credit spreads are unusually wide, or prices have slipped below trend.
• Contraction: prices are well below their high (10% or more) and still below trend.
• Recovery: prices are still well below their high but have turned back above trend with rising momentum.

A new stage is only confirmed when the readings agree for two months running, so one noisy month does not flip it. The panel under the chart shows the current stage, each reading behind it, and the date of the data.

Be clear about what the stage can and can’t do. It barely predicts the next 12 months. Nobody’s signal does that reliably. What it changes is the shape of risk: historically, returns in contractions were more volatile and recoveries were unusually strong. In the simulation, each path starts in today’s stage, draws months from that stage’s history, and moves between stages as history did. Over a decade or two those differences mostly wash out.

Why valuations matter more

The bigger adjustment comes from how expensive stocks are when you start. The measure is the cyclically adjusted price-to-earnings ratio (CAPE): price divided by ten years of average earnings, after inflation. Historically, a high starting CAPE was followed by lower returns over the next ten years, and a low CAPE by higher ones. The relationship is real but loose, so the tool uses only half of what it implies and moves the rest back toward history.

In October 2026, CAPE was about 41, higher than 99% of readings since 1881. After that halving, the model lowers expected stock returns by about 3.6 percentage points a year for the first ten years. The adjustment then fades out over the following ten. Bonds get a similar, smaller adjustment based on today’s 10-year Treasury yield minus inflation.

A worked example

$10,000 to start, $500 a month rising with inflation, 20 years, a Balanced 60/40 mix, 0.2% in annual fees. Results in today’s money, using market data from October 2026:

Total paid in: $130,000.
Historical average: median about $231,900; 9 in 10 paths between $152,600 and $356,500.
Today’s market: median about $194,200; 9 in 10 paths between $133,500 and $288,400.
Fixed 7% before inflation: about $189,400.

Today’s high valuations pull the median about 16% below plain history. With a $250,000 target in today’s money, the chance of reaching it falls from about 41% to about 21%. The pessimistic case moves less than the median, but it ends only just above the $130,000 paid in: in a poor twenty years, steady saving roughly keeps pace with inflation and little more.

The mix matters as you’d expect. In the same market, Aggressive 90/10 gives a median of about $216,700 with a range of $132,300 to $363,700. Conservative 30/60/10 gives about $169,500, from $128,000 to $231,000. More stock raises the middle and the top, but not the bottom.

Your figures will differ slightly from these as the market data updates each day.

Why not just use 7%?

Because a single rate hides the range, and the range is what decides whether a plan works. The rate itself is also slippery. 7% is usually quoted before inflation. After inflation, stocks have returned about 7.5% a year since 1954 and bonds about 1.6%, so a 60/40 mix has averaged roughly 5%, or nearly 9% before inflation. On that history the dashed 7% line sits below the historical median; on today’s valuations it sits close to the market-adjusted median. The dashed line stays on the chart for comparison.

Fees compound too

A fee comes off every month, whether markets rise or fall. Over 30 years, 1% a year in fees leaves a plan like the example above with about 17% less than a fee-free version. On a single lump sum left alone, it’s about 26% less. Try 0.05% (a typical index fund) against 1% (a typical advised account) to see the difference in your own numbers.

How reliable are these ranges?

We tested the model on every start date from 1964 to 2016, rebuilding it each time from only the data available then. Its 9-in-10 range held the real 10-year outcome close to the intended share of the time, within about 6 percentage points at each end. Real outcomes beat the median more often than not, so if anything the model has been cautious. The methodology page has the full results, including where it missed.

What it leaves out

Taxes, withdrawals, and markets outside the US are not modelled. The history it draws on is the US since 1954: one of the most successful markets of the period, which is a reason for some extra caution. It never looks at individual stocks or funds, and nothing here is a recommendation to buy anything.

Educational scenarios, not a forecast or investment advice. Past performance doesn’t guarantee future results.

Frequently asked questions

Is this a forecast?

No. It shows a range of outcomes your plan could have had if the future resembles US market history since 1954, adjusted for today's conditions. The median is the middle of that range, not a prediction, and real results can fall outside the range.

What do the pessimistic and optimistic cases mean?

In 9 out of 10 simulated paths, the balance ends between them. The pessimistic case is the point only one path in ten ends below; the optimistic case is the point only one in ten ends above.

Why is the result lower than other investment calculators?

Two reasons. Results default to today's money, after inflation, while most calculators show future dollars. And when stock valuations are high, as they are now, the model lowers expected stock returns for the next decade. Switch the scenario to Historical average to see the result without that adjustment.

How often is the market data updated?

The market reading refreshes daily from the Federal Reserve Bank of St. Louis (FRED). A change of market stage is confirmed only after two months of agreeing readings. The model behind the simulation is recalibrated about once a quarter.

Why do my numbers change from one day to the next?

The live valuation and market readings change daily, and they feed the simulation. With the same inputs on the same day, the numbers are identical, so a shared link shows the same result.

Does it include taxes or withdrawals?

No. It models saving and growth before tax, with fees taken monthly. Withdrawals in retirement, taxes and account types are not modelled yet.

Is anything I enter stored?

No. The simulation runs entirely in your browser. Your inputs are kept only in the page address, so you can bookmark or share a scenario.