Why most FIRE calculators use unrealistic return assumptions

The problem with one number

A single projected return implicitly assumes every year between now and your FIRE date behaves like the average. Markets don't work that way — a portfolio that returns 9% a year on average might do it through some combination of +25% years and -15% years, and which sequence you get, and when, changes your actual outcome substantially. A downturn early in accumulation costs you far less than the same downturn right before you'd planned to retire. A single-line calculator can't show you that risk because it was never modeling a range of futures — it was drawing one.

What we do instead

FirePath projects four scenarios in parallel, not one: Bullish (12%/yr), Base Case (9%/yr), Bearish (5%/yr), and Conservative (3%/yr). You see all four against your real current balance and real monthly contribution, so your FIRE date isn't a point — it's a range, and you can see how much your plan actually depends on the market cooperating.

On top of the four scenarios, FirePath also runs a Monte Carlo simulation seeded from your own portfolio's actual return history, rather than a generic market assumption — so the probability it shows you reflects how volatile your specific portfolio has actually been, not a textbook average.

Why this matters more than it sounds

We've had to correct our own default assumptions before after realizing they leaned too optimistic — which is exactly the trap a single-number calculator makes easy to fall into and hard to notice. A calculator that only ever shows you the outcome where things go well isn't lying to you outright, but it isn't showing you the plan, either — it's showing you the best case and calling it the plan.