The FIRE (Financial Independence, Retire Early) community's central rule of thumb is simple to state: save 25 times your annual expenses, and you can withdraw from that portfolio indefinitely. It's a genuinely useful starting point, but the number is a compressed summary of a specific piece of research with specific assumptions worth understanding before you treat it as gospel.
Where "25x" and "4%" actually come from
25x annual expenses is just the inverse of a 4% withdrawal rate (1 ÷ 0.04 = 25). The 4% figure traces back to research — most famously the Trinity Study and William Bengen's earlier work — that backtested various withdrawal rates against historical US market returns across rolling 30-year periods, including some genuinely bad starting points like the late 1960s and the Great Depression era. The finding: a 4% initial withdrawal rate, adjusted for inflation each subsequent year, survived essentially all 30-year historical periods studied without the portfolio running out.
The assumptions baked into that number
A few things are easy to gloss over: the underlying research used a specific asset allocation (typically a stock/bond mix, not 100% equities or 100% bonds), a 30-year time horizon (not the 40-50+ year horizon many FIRE seekers retiring in their 30s or 40s are actually planning for), and US historical market returns specifically, which may or may not be representative of future returns or of returns in other countries. More recent research analyzing longer horizons has generally suggested a somewhat more conservative withdrawal rate — often discussed in the 3-3.5% range — may be more appropriate for a 40+ year retirement, precisely because a longer horizon gives more time for a bad sequence of returns to do damage.
Sequence-of-returns risk: the nuance the round number hides
Two retirees with the identical average annual return over 30 years can end up with wildly different outcomes depending on the order those returns happen in. A significant market decline in the first few years of retirement, while you're actively withdrawing, does far more damage than the same decline happening in year 25 — because you're selling a larger share of your portfolio at depressed prices early on, permanently reducing what's left to compound afterward. This is why the 4% rule's real strength is that it was tested against genuinely bad historical sequences, not just average returns.
Lean, Fat, Coast, and Barista — the FIRE number isn't one number
- Lean FIRE — a smaller number, based on a genuinely bare-bones expense budget.
- Fat FIRE — a larger number that preserves a more comfortable, lifestyle-inflation-inclusive spending level in retirement.
- Coast FIRE — front-load your savings early enough that compound growth alone, with no further contributions, will reach your traditional retirement-age FIRE number — after which you can stop aggressively saving and simply cover current living expenses. This is a different milestone than full FIRE: it's the point where you're financially "coasting" toward retirement rather than already there.
- Barista FIRE — a portfolio large enough to cover most, but not all, expenses, with a part-time income covering the rest — reducing the withdrawal rate needed from savings.
What the 4% rule doesn't include
Healthcare costs are the most commonly underestimated line item for early retirees in countries without universal healthcare, particularly for the years before any public healthcare eligibility age kicks in. Major one-off expenses (home repairs, family emergencies) also aren't smoothed into a simple annual-expenses figure. Many FIRE plans also don't fully account for taxes on withdrawals, which vary significantly depending on which account types (taxable, tax-deferred, tax-free) the withdrawals come from.
Geographic arbitrage changes the number more than almost anything else
Because the FIRE number is a multiple of annual expenses, moving somewhere with a meaningfully lower cost of living — including relocating within your home country or internationally — proportionally lowers the number you need to reach. This is a major reason geographic flexibility features so heavily in FIRE planning specifically, more so than in traditional retirement planning at a fixed retirement age.
How to use this practically
Treat 25x (a 4% withdrawal rate) as a reasonable, historically-grounded starting point for a traditional-length retirement, and lean toward a larger multiple (a lower withdrawal rate, in the 3-3.5% range) the earlier you plan to retire and the longer your expected retirement horizon is. Model your own number against your actual expenses, not a generic average, and stress-test it against a bad early sequence of returns rather than just an average expected return.