How this instrument works
A FIRE number is not a projection of what an account will grow into — it is the reverse question, worked backward from spending. Divide the annual amount retirement is expected to cost by a withdrawal rate expressed as a decimal fraction, and the result is the lump sum that spending could draw down at that rate without, on a historical basis, running out. Halve the withdrawal rate and the target doubles, because dividing by a smaller fraction always returns a bigger number.
The 4% figure most people plug in traces back to research published by three Trinity University finance professors in 1998, who tested historical stock and bond returns to see which fixed withdrawal rates a portfolio could sustain across multi-decade retirements without depleting to zero. A 4% withdrawal rate is mathematically identical to a target of 25 times annual expenses, since one divided by 0.04 equals 25 — the 'multiply by 25' shortcut FIRE forums repeat is this same division performed in reverse.
What the arithmetic leaves out matters as much as what it includes. The formula assumes level, inflation-adjusted spending and a withdrawal rate that already reflects your chosen asset mix, retirement length, and tolerance for the plan failing in a bad decade of returns — it does not compute that rate for you, and it takes no account of Social Security, pensions, taxes on withdrawals, or healthcare costs before Medicare eligibility. The result is a savings target, not a guarantee, a schedule, or advice about which rate is actually right for your situation.
- Enter what a year of retirement will cost under Expected annual expenses in retirement, $ — a realistic, all-in figure rather than a rough guess.
- Set Safe withdrawal rate, % to the annual percentage you plan to draw down, commonly cited as 3 to 4.
- Read FIRE number (retirement savings target) for the lump sum that spending and rate together imply.
- Lower the withdrawal rate to see how much a more conservative assumption raises the target, or change expenses to see the target scale directly with spending.
Worked example — $48,000 a year at a 4% withdrawal rate
Take Expected annual expenses in retirement, $ at 48,000 and Safe withdrawal rate, % at 4. The rate becomes 0.04 as a decimal, and 48,000 divided by 0.04 returns FIRE number (retirement savings target) of $1,200,000 — the widely cited '25x expenses' figure, since 1 divided by 0.04 equals 25 and 48,000 times 25 lands on that same $1,200,000.
Drop the withdrawal rate to a more conservative 3%, a figure some planners favor for retirements expected to run 40 years rather than the traditional 30, and the same $48,000 of spending implies a $1,600,000 target instead: a third more savings for an identical lifestyle, because the target and the rate move in inverse, not linear, proportion.
Questions
Why divide by the withdrawal rate instead of just multiplying expenses by 25?
Multiplying by 25 and dividing by 4% (as 0.04) are the identical operation — 25 is exactly 1 divided by 0.04. The calculator divides directly so the target updates correctly the moment the rate changes; the '25x' figure is only ever correct at exactly a 4% rate, and reusing it at any other rate quietly returns the wrong number.
Where does the 4% withdrawal rate actually come from?
It comes from a 1998 study by three Trinity University finance professors who tested fixed withdrawal rates against historical U.S. stock and bond returns from 1926 to 1995, across portfolios holding 25% to 100% stocks, and found that roughly 4% held up across nearly every multi-decade period they examined. It describes past U.S. market outcomes, not a rate guaranteed to hold in the future.
Does this calculator account for taxes or Social Security?
No. FIRE number (retirement savings target) divides raw spending by a raw withdrawal rate — it does not net out Social Security or pension income, does not gross up expenses for taxes owed on withdrawals, and does not price in healthcare costs before Medicare eligibility. Build those adjustments into the expenses figure yourself, or treat the result as a starting point rather than a finished plan.
Is a 3% withdrawal rate always safer than 4%?
A lower withdrawal rate does reduce the historical odds of running out of money, which is why some planners favor 3% to 3.5% for retirements expected to last 40 years or more rather than a standard 30. But safety also depends on asset allocation, spending flexibility in down markets, and how early returns actually unfold — a rate this sheet takes as a given input, not one it evaluates for you.
How is this different from a 401(k) or compound-interest calculator?
Those calculators start from savings and a return rate, then project forward to a future balance through compounding growth over time. This one starts from spending and works backward, with no growth rate, no time horizon, and no compounding anywhere in the arithmetic — a single division answering how much would need to already sit in the account, not how contributions will grow.
Does the target change if I plan to retire earlier or later?
Not directly — retirement age and time horizon do not appear anywhere in the formula. They matter indirectly, through the withdrawal rate: a retirement expected to last 50 years typically calls for a lower, more conservative rate than one lasting 20, and a lower rate raises the required target for identical spending. Adjust Safe withdrawal rate, % to reflect your expected retirement length before reading the result.
References
- Consumer Financial Protection Bureau — Planning for retirement
- SEC Investor.gov — Employment to Retirement
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.