The one-line formula
Your FIRE number is the portfolio size at which work becomes optional. The math fits on a napkin:
FIRE Number = Annual Expenses × 25
That's it. If you spend $40,000 a year, your FIRE number is $1,000,000. If you spend $80,000, it's $2,000,000.
The 25x multiplier comes from the 4% rule: research on historical market data (most famously the Trinity study) found that withdrawing 4% of a diversified portfolio in year one, then adjusting for inflation, survived nearly every 30-year retirement period in U.S. market history. Invert 4% and you get 25.
Simple formula. But every part of it hides a decision.
Expenses dominate everything
Here's the property of the formula most people miss: your income is not in it. Your FIRE number depends entirely on what you spend, not what you earn.
And the multiplier makes every expense enormous:
- $100/month of recurring spending = $1,200/year = $30,000 more you need to save
- A $500/month car payment = $150,000 added to your FIRE number
- Cutting $10,000/year of expenses = $250,000 you no longer need
This is why the FIRE community obsesses over expenses. A dollar of reduced spending is worth 25 dollars of portfolio — and it works from both ends, since spending less also means you save more each month.
The withdrawal rate is a dial, not a law
The 4% rule is a historical observation, not a guarantee. Move the dial and the multiplier moves with it:
- 4.0% SWR → 25x expenses
- 3.5% SWR → ~28.6x expenses
- 3.0% SWR → ~33.3x expenses
Why would you choose a lower rate? The original research modeled 30-year retirements. If you retire at 40, your money needs to last 50+ years, and many early retirees prefer 3.25–3.5% for that horizon. The cost of that caution is real: dropping from 4% to 3% on $40K of expenses raises your target from $1M to $1.33M — potentially years of extra work.
There's no objectively correct rate. There's a tradeoff between retiring sooner and sleeping better.
Lean, Regular, and Fat FIRE
One expense number implies three different targets, because your retirement lifestyle doesn't have to match your current one:
- Lean FIRE — a stripped-down retirement at roughly 60% of your current spending. Smaller target, arrives years earlier, but with little slack in the budget.
- Regular FIRE — your current lifestyle, continued. The standard 25x calculation.
- Fat FIRE — an upgraded retirement at roughly 150% of current spending: more travel, more buffer, more generosity. The target is much larger, but so is the margin of safety.
These aren't just labels — they're a sensitivity analysis. If your Lean number is reachable in 8 years and your Fat number in 20, you know the shape of your decision space.
Where most estimates go wrong
The formula is only as good as the expense number you feed it, and most people feed it a bad one. Common mistakes:
Using current expenses without adjustment. Retirement removes some costs (commuting, work clothes) and adds others. The big one for Americans: health insurance. If your employer currently covers it, add real money — often $10K–$20K/year for a household — to your retirement expense estimate.
Forgetting lumpy expenses. A roof every 20 years, a car every 10, a medical deductible in a bad year. Annualize them. If you only count smooth monthly spending, your FIRE number is understated.
Ignoring taxes. Withdrawals from traditional 401(k)s and IRAs are taxable income. If your $40K of spending requires $45K of gross withdrawals, your expense number is $45K, not $40K.
Anchoring on income. "I make $120K so I need to replace $120K" is the retirement-industry framing, and it's wrong. If you save half your income, you live on $60K — that's the number that matters.
Progress feels slow, then fast
One more thing the formula doesn't show: the path to it is nonlinear. The first $100K is famously the hardest — it's almost all contributions. By the time you're at 50% of your FIRE number, compounding is doing heavy lifting alongside your savings, and the last stretch often goes faster than the middle. If you're early and discouraged, the math genuinely is on your side later.
Run your own numbers
The FIRE Number Calculator on STWLTH computes all three targets — Lean, Regular, and Fat — from your expenses and withdrawal rate, shows your current progress toward each, and projects your portfolio year by year to the age you'd actually hit them. Move the SWR slider and watch the targets shift; that sensitivity is the whole decision, made visible.