How the FIRE Number Is Calculated: Where the 25x Rule Comes From

Ask anyone in the FIRE community how much they need to retire, and you'll almost always hear some version of "25 times my annual expenses." That number shows up so often it can feel like a rule handed down from nowhere in particular, but it comes from a specific piece of math, rests on a specific set of assumptions, and — like most rules of thumb — works better in some situations than others.

Where the 25x Number Comes From

The 25x rule is simply the mathematical inverse of the 4% safe withdrawal rate, which is the annual percentage of your portfolio that research on historical market returns (most famously the Trinity Study) suggests you can withdraw each year, adjusted for inflation, with a high probability of your money lasting 30 years or more. If 4% of your portfolio covers a year of expenses, then your full portfolio must be 25 times those annual expenses, since 1 ÷ 0.04 = 25. The two numbers are really one idea expressed two different ways: a withdrawal rate and a portfolio multiple.

The Math, Step by Step

The calculation itself only has one real input: your expected annual expenses in retirement, not your current expenses, not your income, and not your current lifestyle unless you expect it to stay exactly the same. Multiply that annual expense figure by 25 and you have your target portfolio size. Everything else in a FIRE plan — savings rate, timeline, investment return — exists to answer the follow-up question of how you get from your current portfolio to that number.

A Worked Example With Real Numbers

Say your realistic annual spending in retirement, after accounting for the things that change (no more commuting costs, but new travel or hobby spending, for instance), comes out to $45,000. Your FIRE number is 25 × $45,000 = $1,125,000. If your current portfolio is $200,000 and you're adding $2,500 a month at a 6% expected real return, the FIRE Calculator above would show you're roughly 16-17 years away from that target. Change any one input — a higher monthly contribution, a lower expense estimate, a different expected return — and you can immediately see how much it moves that timeline, which is usually more instructive than the 25x figure on its own.

Why 25x Is a Starting Point, Not a Guarantee

The 4% figure the 25x rule is built on comes from historical backtests over rolling 30-year periods, mostly using U.S. stock and bond market data. Historical performance isn't a guarantee of future performance, and a 30-year time horizon may be too short for someone retiring at 35 who could plausibly need the portfolio to last 50-plus years. The original research also assumed a specific asset allocation (a mix of stocks and bonds) and specific withdrawal behavior (adjusting only for inflation, not cutting back in down markets), both of which real retirees may deviate from. None of this invalidates 25x as a planning starting point — it's simply a reason to treat it as a well-researched estimate rather than a mathematical certainty.

Adjusting the Multiple for Your Own Risk Tolerance

Because the 25x figure is sensitive to how long your money needs to last and how much safety margin you want, many people in the FIRE community adjust it up or down from the default. Someone retiring in their early 30s with a very long time horizon might target 30x-33x expenses (equivalent to a roughly 3-3.3% withdrawal rate) for extra safety margin. Someone comfortable with more flexibility — willing to cut spending or pick up part-time work in a bad market — might be comfortable targeting 22x-23x (a roughly 4.3-4.5% withdrawal rate). Neither choice is objectively correct; it's a trade-off between a longer accumulation phase and a smaller safety margin.

Common Mistakes When Applying the 25x Rule

The single most common mistake is applying the multiple to current expenses instead of projected retirement expenses, which usually understates the number if healthcare, travel, or hobbies will cost more, or overstates it if commuting, work clothes, and childcare costs will disappear. A second common mistake is treating the 25x figure as fixed once calculated rather than revisiting it as spending assumptions change over the years leading up to retirement. A third is ignoring taxes on withdrawals — the 25x figure is typically calculated on the amount you need to actually spend, so if withdrawals will be taxed, the pre-tax portfolio target needs to be somewhat larger.

Putting It Into Practice

The most useful way to apply the 25x rule isn't as a single fixed number to hit, but as a formula to recalculate periodically as your expense estimate gets more accurate and your risk tolerance becomes clearer. Use the calculator above with your best current estimate of retirement expenses, check the resulting FIRE number and timeline, and revisit both every year or two — the number will move as your life and plans do, and treating it as a living target rather than a one-time calculation tends to produce a more realistic plan.