How to Calculate Your FIRE Number (And What It Actually Means)

$40,000 a year. That’s what a $1 million portfolio can safely produce, if you believe the research, trust the assumptions, and don’t retire into the worst sequence of returns in history.

Most people asking “how much do I need to retire?” are really asking: what’s my number? The answer is a formula. Whether the formula holds for your specific situation is a different question.

The 4% rule

In 1994, financial planner William Bengen analyzed 75 years of U.S. market history and found that a retiree who withdrew 4% of their portfolio in year one, then adjusted that amount for inflation each year, never ran out of money in any 30-year period he tested. Four years later, researchers at Trinity University ran a broader study and confirmed the finding.

The 4% rule became the standard.

It’s not a law. It’s a finding from historical data: hold a diversified portfolio of stocks and bonds, withdraw 4% per year adjusted for inflation, and you’ve historically had a very high probability of not outliving your money over 30 years.

The 25x multiplier

Flip the 4% rule and you get the FIRE number formula:

The multiplier comes directly from the math: 1 ÷ 0.04 = 25. The variable you actually control isn’t the withdrawal rate. It’s the expenses.

Variants: Lean, Fat, Coast, and Barista FIRE

The same 25× formula produces very different targets depending on the lifestyle you are funding and how you get there. Four labels come up constantly:

Coast FIRE is the one worth a worked example because it changes what you do today. Suppose your full number is $1,000,000 and you are 35, planning to let your investments grow untouched until 65. Assuming about a 5% return after inflation (the calculator uses a 7% return against 2% inflation, which is close to 5% real), money grows by a factor of roughly 4.3 over 30 years, so the amount you would need invested today is about $1,000,000 divided by 4.3, or roughly $230,000. At those inputs, once you hold around $230,000 at 35 you could stop retirement saving entirely, cover only your current spending, and still arrive at the full number by 65. The figures are illustrative; your real return and timeline will differ.

Is $1 million enough?

Depends entirely on what you spend.

$1M × 4% = $40,000/year. If that covers your life (housing, food, healthcare, travel, everything) then yes. If you need $80,000 a year to live the way you want to live, $1M gets you halfway there.

“Is $1 million enough?” is the wrong question. “What do I actually spend, and what will I spend in retirement?” is the right one.

The limitations the rule doesn’t advertise

The Trinity Study modeled 30-year retirement periods. If you retire at 40, you might need 50 years of portfolio survival. That changes the math.

At longer horizons, the failure rate at 4% creeps up. Many in the FIRE community use 3.5% (a 28.6× multiplier) or 3% (a 33.3× multiplier) to build in extra margin. Conservative but rational, especially if you’re retiring decades before traditional retirement age.

Sequence of returns risk is the other issue that doesn’t get enough attention. The order market returns arrive in matters enormously. A portfolio with a 7% average return that crashes 30% in years one and two is a very different beast than one that gains steadily for a decade before a dip. You sell shares at the bottom to fund your life. The portfolio never fully recovers.

Bengen’s data includes bad sequences: the Great Depression, 1970s stagflation. The 4% rule survived those historically. But “survived historical worst cases” is different from “guaranteed to work.”

How to estimate your annual expenses

Every number on this page hinges on one input: annual expenses. The quick way to get it wrong is to guess from memory. A better approach is to total twelve months of actual spending, then adjust for how retirement changes the categories. Separate one-time costs (a roof, a car) from recurring ones, so a single expensive year does not inflate the baseline. Model the things that will change: a mortgage that gets paid off drops the number for good, while rent does not; commuting and work costs fall; healthcare and travel often rise. The goal is the figure you will actually live on, not last year’s total, and it is worth revisiting as those categories shift.

Expenses are the lever most people underestimate

There’s no faster way to change your FIRE number than to change what you spend. Cutting $500/month does two things simultaneously: it lowers the amount you need to generate from your portfolio, and it frees up $500/month to invest.

That’s not a rounding error. Shaving $500/month off your life costs you $150,000 less to retire, and your savings rate goes up at the same time.

This is why expenses are the most powerful lever in FIRE math. Income matters, but spending has a multiplied effect in both directions.

The fastest way to reduce expenses is eliminating debt payments. If you’re carrying high-interest debt, choosing between the avalanche and snowball methods is the first decision.

What the number tells you, and what it doesn’t

Knowing your FIRE number tells you the portfolio size you’re aiming for. It doesn’t tell you how long it will take to get there, whether healthcare costs will run higher than planned, how your expenses will shift as you age, or what happens if inflation runs hotter than history suggests.

Use the 4% rule and the 25× multiplier as a starting point. Build in margin. A small amount of income in early retirement (even a few thousand dollars a year) dramatically reduces the stress on your portfolio.

Healthcare and taxes before 65

Two costs do the most damage to an early-retirement plan, and the 25× formula only captures them if your expense figure already does. The first is healthcare. Retire before Medicare eligibility at 65 and you lose employer coverage, which usually means buying a plan on the ACA marketplace. Marketplace premium subsidies are tied to your taxable income, and an early retiree often has the unusual ability to manage that income (living partly from cash or already-taxed savings), which can lower the premium owed. The mechanics are worth understanding before you settle on a number, because what you actually pay can swing widely with how you draw down.

The second is taxes. Withdrawals from tax-deferred accounts are taxable income, so if your expense figure is what you actually spend (take-home), the portfolio has to generate more than that to cover the tax bill on top. A $40,000 spending target is not a $40,000 withdrawal target once taxes apply. The fix is simple to state: build the expense number on gross costs, including the taxes you expect to owe and a realistic healthcare premium, rather than on take-home spending. Specific premium and subsidy figures change yearly and vary by state and household, so confirm current numbers before relying on them.

FAQ

What is a FIRE number?

A FIRE number is the portfolio size at which your investments can fund your living expenses indefinitely without you needing to work. It is calculated by multiplying your annual expenses by 25, which is the inverse of the 4% safe withdrawal rate.

How is the FIRE number calculated?

FIRE number = annual expenses × 25. If you spend $50,000 per year, the formula produces a target of $1,250,000. The multiplier comes directly from the 4% rule: 1 ÷ 0.04 = 25. The variable you most directly control is your annual spending, not the withdrawal rate itself.

Is the 4% rule still considered safe?

The 4% rule was derived from historical U.S. market data covering 30-year retirement periods. For longer retirements (40–50 years, common in early retirement), some researchers suggest a more conservative 3–3.5% withdrawal rate to reduce failure risk. The rule is a useful starting point, not a guarantee; sequences of poor returns early in retirement can materially increase portfolio stress even when the long-run average looks fine.

What are Lean FIRE, Fat FIRE, and Coast FIRE?

Lean FIRE targets a minimal lifestyle with low annual expenses, producing a smaller portfolio target. Fat FIRE targets a more comfortable lifestyle with higher annual spending. Coast FIRE is reached when your existing savings, left to grow without additional contributions, will reach your full FIRE number by traditional retirement age. At that point the math only requires you to cover current expenses, not accelerate saving further.

Should the FIRE number account for taxes and healthcare?

If your annual expense figure already includes taxes and healthcare premiums, the FIRE number calculation reflects those costs automatically. If your estimate covers only take-home spending, you need to gross it up for expected taxes and add healthcare premiums separately. Both can be substantial for early retirees who lose employer-sponsored coverage before Medicare eligibility at 65.