Ask most people pursuing financial independence how they got their number, and a common answer sounds something like this: "I spend roughly $4,000 a month, so I multiplied by 12, multiplied by 25, and that's my FI number." It's a real, common shortcut — and the two moves inside it, the "roughly" and the 25, are exactly where the number goes wrong.
The 25x itself isn't the problem. It's just the inverse of a 4% withdrawal rate — a figure that traces back to the Trinity Study, 1990s research testing how withdrawal rates held up against historical market returns — if you can live on 4% of a portfolio each year, the portfolio has to be 25 times your annual spending (1 divided by 0.04 equals 25). That part of the math is sound arithmetic, not a myth to debunk. The actual weak point is what gets fed into it: a remembered, rounded, mental estimate of "roughly $4,000 a month" instead of a real accounting of what a household actually spends.
Memory is a bad instrument for this. It compresses the boring recurring stuff — groceries, utilities, the subscription that renews quietly every March — and it drops anything that doesn't happen monthly entirely. The fix isn't a better guess. It's categorized tracking of 12 to 24 months of actual transactions: every card statement, every bank transfer, sorted into consistent categories (housing, food, transportation, insurance, healthcare, discretionary) rather than left as a single lump sum. Twelve months captures a full seasonal cycle — heating bills in January, travel in July. Twenty-four months is better still, because it catches the expenses that don't show up every year at all, which is the part most people miss.
That's the second failure point in the shortcut: irregular, lumpy costs. A monthly average built from bank statements will faithfully capture the electric bill and completely miss the $6,000 roof repair, the new transmission, the annual property tax bill, or the once-a-decade wedding and funeral travel. These costs are real, they're not rare in the sense of never happening, they're just rare in the sense of not happening every month — which means a naive monthly average either understates them badly or ignores them outright. The correct handling isn't to skip them because they're irregular; it's to look back over your full tracking window, add up every lumpy or annual-ish cost you can identify, and divide by 12 to fold it into your monthly baseline as its own line item — something like "annualized irregular costs: $300/month" — rather than pretending the number doesn't exist because it didn't happen last Tuesday.
Healthcare deserves its own callout here, and it's a category tracking alone won't fix. If your expense history was built while you had employer coverage, it captures your co-pays and deductibles, not what you'll actually pay once you're buying your own plan. That gap has widened sharply: ACA premium payments for subsidized enrollees were projected, as of mid-2026, to more than double in 2026 as enhanced tax credits expire. Historical tracking tells you what you spent. For healthcare specifically, an early retiree also has to model what they will spend, since the employer subsidy disappears the day the paycheck does.
Once you have a real, categorized, irregular-cost-inclusive annual spending figure, the second decision is the multiplier — and this is where a lot of FIRE discussion quietly assumes 25x is a law rather than a choice. The multiplier is just the reciprocal of whatever withdrawal rate you're willing to bet on. Choose 4%, and the multiplier is 25. Choose something more conservative, and the multiplier climbs. Morningstar's December 2025 research, for instance, put the highest safe starting withdrawal rate for a 30-year retirement horizon at 3.9%, citing elevated equity valuations and modest bond yields — which works out to a multiplier of roughly 25.6x rather than 25x. That's not a rounding difference in practice; on real spending it moves the target by tens of thousands of dollars, and the "correct" withdrawal rate is a genuinely open question in the research, not a settled constant — the 4% figure comes from historical U.S. market returns that a given early retiree's 30-, 40-, or 50-year retirement window may or may not resemble.
Here's how the full method runs end to end. Say categorized tracking across 24 months of actual transactions shows $4,200 a month in regular recurring spending — housing, food, insurance, transportation, subscriptions, discretionary. Reviewing that same window for irregular costs — car repairs, a medical deductible year, home maintenance, gifts and travel that don't recur monthly — adds $300 a month once annualized in this household's case; another household's figure here could easily run higher or lower, which is precisely why the tracking has to be done rather than assumed. That's $4,500 a month, or $54,000 a year, in real tracked spending. At a 4% withdrawal rate, the 25x multiplier puts the FI number at $1,350,000. At Morningstar's more conservative 3.9%, the same $54,000 in spending requires closer to $1,384,615. Same household, same spending, two different numbers — and the gap between them is entirely a function of which withdrawal-rate bet you're willing to make, not a disagreement about what the household actually spends.
That's the part worth sitting with: the tracking exercise is the part with a right answer. Twelve to twenty-four months of categorized real transactions, irregular costs annualized rather than ignored, healthcare re-modeled rather than extrapolated from an employer-subsidized past — that produces a spending figure that's genuinely accurate to your life. The multiplier applied to that figure is a judgment call about future market returns, and reasonable, informed people land in different places on it. Treating the tracking as approximate and the multiplier as fixed gets the difficulty exactly backwards.
The number worth trusting isn't the one you can produce fastest from memory. It's the one built from a spreadsheet you were honest with for two years, multiplied by whichever withdrawal rate you're actually prepared to defend if the first decade of your retirement turns out to be a bad one.