The most useful tools are honest about what they can't see. This one projects one thing clearly. Everything outside that frame is yours to hold.
That bet — that the future rhymes with the past — is the only way to run scenarios at all. You can't simulate a future that has no relationship to anything that has happened before.
William Bengen's original research — the study that produced the 4% rule — wasn't a discovery the way a law of physics is. It was a careful reading of historical data: a pattern that held across every thirty-year period on record, including the Great Depression and the 1970s stagflation. That's not a guarantee. It's evidence — and evidence, carefully examined, is more honest than intuition.
So the tool runs thousands of scenarios drawn from historical return distributions, stress-tests them against different sequences of good and bad years, and reports a probability. A number that claimed certainty would be lying.
From a handful of inputs, the model projects a range of outcomes. That's what it does — and it does it on purpose, because a calculator that tried to model everything would be wrong about everything.
Your savings balance, contribution rate, target spending, planned retirement age, and Social Security estimate — and the range of outcomes those imply over time.
Your effective tax rate, estate plan, long-term-care exposure, a concentrated single holding, your mortgage, your children's or parents' needs — and how you'll actually behave when markets fall 40% and stay there.
That isn't a flaw to be fixed in a later version. It's the nature of a focused tool. This one models one thing — the relationship between your savings, your spending, and time — and tries to be right about that. It's one input to a plan, not the plan itself.
A good financial planner knows the number on the screen isn't the decision. The decision is whether you can sleep at night with that number. Whether your spouse agrees with the trade-offs it implies. Whether you're the kind of person who will hold through a bear market, or quietly start spending down the moment things feel uncertain.
Those things don't live in a spreadsheet. They live in a conversation. The model can tell you your work-optional age is projected to be 61 under these assumptions. It can't tell you whether 61 feels like relief or like giving up something you love. That's a different question — and the more important one.
If you're making significant decisions based on what you find here, a fee-only fiduciary advisor is worth the conversation. Not because the numbers are wrong, but because the numbers are only part of what you're deciding.
Most people avoid thinking carefully about retirement finances because the numbers feel abstract and the stakes feel paralyzing. Running the model makes it concrete: here is what a given amount of savings, a given spending level, and a retirement at a given age actually imply. Here is the probability. Here is where the plan is fragile and where it isn't.
That clarity — even imperfect, even hedged, even built on assumptions that will turn out partly wrong — is more useful than the alternative, which is not running the numbers at all. A map is not the territory; but you need a map to navigate the territory. This is a map.