How To Calculate Your FIRE Number

The financial independence number is one division away from your budget, which is exactly why it deserves suspicion. Everything meaningful is hidden inside the numerator. Annual spending here cannot mean what you spend now if your working-life budget includes a mortgage that ends, commuting that ends, and payroll taxes that end — and simultaneously excludes healthcare that begins and travel that expands. The useful input is a projected retirement budget built line by line, not today's bank feed inflated by a couple of percentage points.

The denominator carries equal weight in the opposite direction. The four percent figure originates in portfolio survivability research asking how large a first-year withdrawal could be, indexed to inflation thereafter, without exhausting a stock-heavy portfolio over a thirty-year window. Its reciprocal is twenty-five, which is where the familiar "25 times your expenses" rule comes from. There is nothing magical about the multiple; it is 1 divided by 0.04, and switching to a 3.5% rate — which many people prefer for longer horizons — changes the required multiple to 28.57 times, which is why $40,000 of spending moves from $1,000,000 to $1,142,857.

Notice how violently small changes in the denominator move the answer. Going from 4% to 3% multiplies your target by a third: the multiple goes from 25 to 33.33, and the required portfolio for the same $40,000 of spending goes from $1,000,000 to $1,333,333. That $333,333 gap is larger than most people's entire annual salary, and it comes entirely from how conservative you are willing to be. Choosing a withdrawal rate is therefore not a technical footnote — it is the second-largest decision in the calculation after picking the spending number itself.

Taxes are the most commonly omitted item in the numerator, and they are not optional. A withdrawal from a traditional tax-deferred account is ordinary income; a withdrawal from a taxable account may realise capital gains. If your target is funded entirely by pre-tax accounts and you need $40,000 to live on plus roughly $6,000 of tax, the honest numerator is $46,000 and the target becomes $1,150,000. The same reasoning applies to healthcare before public coverage begins, which for a couple retiring in their forties can add five figures a year for a decade or more.

The second half of the question — how long until you get there — turns out to depend almost entirely on one variable: your savings rate as a share of take-home pay. Because spending determines both how much you contribute and how large your target must be, income cancels out of the algebra. At a 5% real return with a 25x target, years to independence is ln(1 + 25r(1-s)/s) / ln(1 + r), where s is the savings rate and r the real return. At a 10% savings rate that is 51.35 years; at 20%, 36.72; at 30%, 27.98; at 50%, 16.62; and at 70%, 8.79. The relationship is sharply non-linear because raising the rate does two things at once.

That non-linearity is the genuinely actionable insight in this whole framework, and it explains why the community obsesses over savings rate rather than income or returns. Going from 10% to 20% cuts twenty years off the timeline, while going from 50% to 60% removes only four. Return assumptions pale by comparison: the entire choice between 4% and 5% real returns moves a thirty-year plan by a handful of years, whereas a ten-point savings rate improvement moves it by a decade.

Several important things sit outside the arithmetic. Any government pension or employer benefit arriving at a later age reduces the amount your portfolio must fund in the intervening years, and a Social-security-style bridge changes the calculation substantially for people in their forties versus their sixties. Sequence risk remains: the thirty-year framing comes from historical data, and the first decade of returns determines outcomes far more than the average does.

Which is why the honest way to use a FIRE number is as a range rather than a target date. Run it at two withdrawal rates and two spending levels, and plan against the uncomfortable corner. A number that assumes optimistic spending, an aggressive withdrawal rate and fifteen more years of above-average returns is not a plan — it is a mood.