What a FIRE number is, and why it depends on spending rather than income
Your FIRE number is the size of portfolio at which the withdrawals it can sustain match what you spend. Past that point, employment income becomes optional rather than structural. The whole calculation is one division: take the amount you expect to spend in a full year of retirement and divide it by the fraction of the portfolio you are willing to draw in the first year.
Notice what is absent. Your salary does not appear. Neither does your job title, your net worth including the house you live in, nor the amount your neighbour has saved. Financial independence is defined entirely by the gap between a portfolio's sustainable output and your outgoings, which is why two people with identical incomes can have targets that differ by a factor of three. The person who spends $40,000 needs $1,000,000 at a 4% rate; the person who spends $120,000 needs $3,000,000.
This also explains the peculiar leverage that spending has in early-retirement planning. Cutting $5,000 a year of recurring spending does two things at once: it reduces the target by $125,000 at a 4% rate, and it raises the amount you save each year by $5,000. Cutting spending moves the finish line towards you while speeding you up. A raise only speeds you up. Once you have your number here, the years to financial independence calculator turns it into a date.
One clarification about the assets that count. The number produced here is an invested portfolio: index funds, bonds, and anything else that produces a return you can spend. Home equity in the house you live in does not belong in it, because you cannot withdraw 4% of your kitchen. Rental equity is a judgement call — if you count the equity as portfolio, you must also count the net rental income as a withdrawal rather than as extra income on top.
The formula, and where the 25× rule comes from
The identity is F = E ÷ w, where E is annual spending and w is the withdrawal rate as a decimal. Because dividing by a fraction is multiplying by its reciprocal, the same statement is F = E × (1 ÷ w). At w = 0.04 the reciprocal is 25, which is the entire content of the famous “25 times your expenses” shorthand. At 3.5% it is 28.6; at 3% it is 33.3; at 5% it is 20.
The reciprocal relationship is worth internalising because it is not linear. Moving from 4% to 3.5% — half a percentage point — raises the target by 14.3%, from 25× to 28.6×. Moving from 3.5% to 3% raises it by another 16.7%. Each further half point of caution costs more than the last. That convexity is why the argument between a 4% and a 3.25% planner is not a rounding dispute: on $60,000 of spending it is the difference between $1.5 million and roughly $1.85 million, or several extra working years.
The 4% figure is not a law of finance. It comes from William Bengen's 1994 Journal of Financial Planning paper, which tested rolling 30-year retirements against actual US market and inflation history and asked which initial withdrawal rate, raised annually with inflation, never exhausted a portfolio of 50–75% stocks. Roughly 4% was his answer, and he named it SAFEMAX. The Trinity study — Cooley, Hubbard and Walz, 1998 — extended the approach across several stock and bond mixes and several horizons and reported portfolio success rates rather than a single safe rate.
Two things follow. First, 4% was calibrated to a 30-year horizon; someone retiring at 40 is planning for 50 or more, and the research does not directly support 4% there. Second, both studies are US-specific and backward-looking. If you want to interrogate the rate itself rather than accept it, the safe withdrawal rate calculator is the tool for that, and this page simply takes whatever rate you decide on.
The inflation adjustment
Your target is quoted in today's dollars, but you will reach it years from now, and the same lifestyle will cost more then. So the second formula restates the target in the money of your FI year: Fₙ = E(1 + π)ⁿ ÷ w. This is not double-counting inflation with your investment returns, because the withdrawal-rate research already assumes withdrawals rise with prices after retirement. What the adjustment fixes is the arrival problem: the finish line drifts away from you at the rate of inflation for as long as you are still walking towards it.
Worked example: $60,000 of spending, 4%, twelve years away
You spend $5,000 a month, all-in — $60,000 a year including rent, health insurance, tax on withdrawals and holidays. You accept the 4% rule, and you think you are about twelve years from the target. You assume 2.5% inflation.
- Convert the rate. w = 4 ÷ 100 = 0.04.
- Take the reciprocal. 1 ÷ 0.04 = 25. That is your multiple.
- Divide. F = $60,000 ÷ 0.04 = $1,500,000. Equivalently $60,000 × 25.
- Build the inflation factor. (1 + 0.025)12 = 1.02512 = 1.34489. Prices are about 34.5% higher after twelve years at 2.5%.
- Restate the spending. $60,000 × 1.34489 = $80,693 a year, which is $6,724 a month.
- Restate the target. $80,693 ÷ 0.04 = $2,017,333. Or, identically, $1,500,000 × 1.34489.
So the honest answer is not “$1.5 million”. It is “$1.5 million of today's purchasing power, which will be priced at about $2.02 million by the time I get there.” Both numbers are correct and they answer different questions. Use the today's-dollars figure to compare with other people's targets and to sanity-check your lifestyle; use the inflated figure only if the balance you are projecting forwards is in nominal dollars too. If your projection uses a real (inflation-adjusted) return, compare it against the $1.5 million and ignore the inflated figure entirely — mixing a real growth rate with a nominal target is the single most common error on this page.
Now the variants. At a lean level of 65% you would spend $39,000, needing $39,000 ÷ 0.04 = $975,000. At a fat level of 175% you would spend $105,000, needing $2,625,000. Those three numbers — $975,000, $1,500,000 and $2,625,000 — bracket the decision. Many people find the useful insight is that the lean number arrives many years before the fat one, and that a phased exit somewhere in between is available long before the headline target is.
How to read your number without fooling yourself
Start by testing the input, not the output. A FIRE number is only as good as the spending figure it divides, and the spending figure is where nearly all the error lives. Pull twelve months of actual bank and card statements and total them. Then add the things a working year hides: health insurance you currently get through an employer, the full income tax on portfolio withdrawals, car replacement amortised across its life, roof and boiler replacement if you own, and the dental work you have been postponing. A budget assembled from memory typically misses the lumpy items entirely, and those are the ones that break a plan.
Second, check the horizon your withdrawal rate is calibrated to. A 4% rate came from 30-year tests. If you are 38 and planning for 55 years, the same historical data supports a lower rate; many long-horizon planners settle between 3.0% and 3.5%. The table below shows exactly what that costs you in target size.
Third, treat the number as a range with a floor, not a threshold. Two portfolios of identical size are not equally safe: one that starts with a 40% drawdown in its first three years faces a very different arithmetic from one that starts with a bull market, even at the same withdrawal rate. That is sequence-of-returns risk, and it is why practitioners hedge with flexibility rather than precision. A plan that can cut spending by 10% in a bad year shrinks the portfolio it needs by exactly 10% — $150,000 off a $1.5 million target — and it does so in the years when markets have already fallen, which is when the extra capital would have been hardest to accumulate.
Fourth, remember what is not in the number. Social Security, a pension, or an inheritance reduces the portfolio you need, but only from the year it starts paying. If you claim at 67 and stop work at 50, the portfolio has to carry seventeen years alone and then carry a reduced load. Modelling that properly means bridging: use the Social Security claiming age benefit calculator to size the later income and the retirement withdrawal longevity calculator to test whether the bridge holds.
Portfolio multiple and target by withdrawal rate
| Withdrawal rate | Multiple of spending | Target on $60,000 | Change vs 4% |
|---|---|---|---|
| 2.50% | 40.0× | $2,400,000 | +60.0% |
| 3.00% | 33.3× | $2,000,000 | +33.3% |
| 3.25% | 30.8× | $1,846,154 | +23.1% |
| 3.50% | 28.6× | $1,714,286 | +14.3% |
| 4.00% | 25.0× | $1,500,000 | — |
| 4.50% | 22.2× | $1,333,333 | −11.1% |
| 5.00% | 20.0× | $1,200,000 | −20.0% |
| 5.50% | 18.2× | $1,090,909 | −27.3% |
| 6.00% | 16.7× | $1,000,000 | −33.3% |
Every figure here is arithmetic on 1 ÷ rate, not a claim about safety. A 6% rate produces a target of 16.7× spending; whether a portfolio survives 30 years at 6% is a separate question the withdrawal-rate literature answers far less favourably than it answers 4%.
Lean, standard and fat FIRE have no official definitions
These labels are community shorthand, not defined terms, and you will see wildly different dollar thresholds quoted for each. That is why this calculator asks you for the lean and fat spending levels as percentages of your base budget rather than imposing a threshold. The defaults — 65% and 175% — are starting points, not standards. Set them to two budgets you could actually describe: the one you would live on if markets disappointed, and the one you would choose if they did not.
Mistakes that make a FIRE number wrong
- Using take-home pay minus savings as your spending figure. It silently excludes employer-paid health insurance and payroll-deducted items, and it includes work costs you will drop. Count actual outgoings instead.
- Forgetting tax on withdrawals. Money coming out of a traditional 401(k) or IRA is ordinary income. If you need $60,000 to spend, you may need to withdraw appreciably more than $60,000. Taxable-account withdrawals are taxed only on the gain portion, so your account mix matters here.
- Comparing a nominal projected balance against a today's-dollars target. Pick one basis. Real return against today's target, or nominal return against the inflated target — never one of each.
- Counting home equity in the portfolio. Your primary residence produces shelter, not withdrawals. If you plan to downsize, count only the equity you will actually release, and only from the year you release it.
- Applying a 30-year withdrawal rate to a 50-year retirement. The horizon the rate was tested against is part of the rate. Longer retirement, lower rate.
- Treating the target as a cliff. Nothing changes on the day you cross it. Flexibility about spending, a few years of part-time income, or a cash buffer for the first bad market all matter more than the last $50,000.
Where this sits among the other financial-independence tools
This calculator answers “how much?” and nothing else. Three companion questions have their own tools. “How long?” is savings-rate arithmetic, handled by the years to financial independence calculator. “Can I stop saving now and let compounding finish the job?” is the Coast FIRE question, handled by the Coast FIRE calculator, which discounts your target back to today at your expected return. “Will the money actually last?” is a depletion question that needs a return sequence, not a multiple.
There is also a structural alternative to the multiple approach worth knowing about. Instead of asking what portfolio supports your spending at a fixed rate, you can ask what it would cost to buy that income — the price of an inflation-linked annuity or a ladder of Treasury Inflation-Protected Securities covering your spending to a chosen age. That approach gives up upside and bequest value in exchange for removing sequence risk entirely, and for a floor of essential spending many planners prefer it. The multiple approach dominates FIRE discussion because it preserves the portfolio and its optionality, but it does so by accepting the market risk the annuity removes.
Finally, treat this number as a live figure rather than a monument. Your spending will change when children arrive or leave, when you move, and when health costs enter. Recompute annually with your actual trailing-twelve-month spending. A target built on a real number you measured beats a target built on a number you hoped for, however carefully you divided it.
Key terms
- Safe withdrawal rate (SWR)
- The percentage of the starting portfolio withdrawn in the first year of retirement, with the dollar amount then raised annually with inflation. It is not recomputed against the current balance each year.
- SAFEMAX
- Bengen's term for the highest initial withdrawal rate that survived every historical rolling period he tested at a given horizon and asset allocation.
- Sequence-of-returns risk
- The risk that poor returns arrive early in retirement, when the portfolio is largest and withdrawals bite deepest. Two retirements with identical average returns can end very differently depending on their order.
- Coast FIRE
- The point at which existing invested assets will grow into your full FIRE number by your target age with no further contributions. It always arrives before the FIRE number itself.