How To Calculate Your FIRE Number (Step by Step)

Your FIRE number is the amount of money you need invested so that you can live off the returns indefinitely — without ever touching a paycheck again. It sounds complicated, but the core calculation fits on a napkin. Once you know it, every financial decision you make gets simpler.

What makes the FIRE number so useful isn't just that it gives you a target — it's that it reframes every spending decision as a compounding decision. A recurring $200/month expense isn't just $200 a month. Under the 25x rule, it represents $60,000 of required portfolio value ($200 × 12 × 25) that has to exist purely to sustain that one line item forever. Seen this way, a subscription you barely use or a larger apartment than you need isn't a small monthly cost — it's tens of thousands of dollars of capital permanently earmarked to cover it. That reframe is why so many people in the FIRE community describe calculating their number as the single moment their relationship with spending changed.

The Formula: Annual Expenses × 25

The FIRE number formula is:

That's it. If you spend $66,000 a year, your FIRE number is $1,650,000. If you spend $40,000 a year, it's $1,000,000. If you spend $100,000, it's $2,500,000. The number scales perfectly with your lifestyle.

The Two Numbers You Actually Need

Most people stop at one number, but a complete FIRE calculation actually has two components worth separating: your current annual expenses and your projected retirement annual expenses. These are rarely identical. Some costs disappear in retirement (commuting, work wardrobe, retirement account contributions themselves). Some costs appear or grow (healthcare before Medicare eligibility, more travel, more hobby spending in the newly free hours). Calculating your FIRE number off your current spending is a reasonable starting estimate, but it's worth running the calculation a second time off a deliberately projected retirement budget, then using the higher of the two as your real target. Most FIRE planners find the two numbers land within 10-15% of each other, but that gap is still real money — a 10% gap on a $1.65 million number is $165,000.

Why 25x? The 4% Rule Explained

The "25x" multiplier derives from a landmark 1994 academic study by William Bengen, later expanded by researchers at Trinity University, known as the Trinity Study. They analyzed U.S. stock and bond market data going back to 1926 and asked a simple question: what's the highest percentage of a portfolio you can withdraw each year without running out of money over a 30-year retirement?

The answer was 4% — a portfolio split roughly 60% stocks / 40% bonds survived every historical 30-year window they tested. Since 4% = 1/25, you need 25 times your annual spending. The math is clean and the research is solid.

For early retirees with longer time horizons — say, a 40-year or 50-year retirement — many FIRE planners use 3.5% (28.5x) or even 3.3% (30x) to add a safety margin. The full breakdown of the 4% rule explores this in detail.

It's worth being precise about what the Trinity Study actually tested, because the 4% figure gets oversimplified in popular retirement writing. The study modeled a fixed initial withdrawal rate, adjusted annually for inflation thereafter, held constant regardless of how the portfolio performed in any given year. It did not model a retiree cutting back spending during a market downturn, which real retirees often do voluntarily — meaning the historical 4% figure is already somewhat conservative relative to how people actually behave. It also assumed a 30-year retirement horizon, which is exactly right for a 65-year-old retiree and noticeably short for someone retiring at 40 or 45 who might need the portfolio to last 50+ years. That's the entire reason the FIRE community adjusts the multiplier downward (a lower withdrawal rate, a higher multiple) for earlier retirement ages.

Step-by-Step Walkthrough: The $5,500/Month Example

Let's work through a realistic example. Meet Priya: she's 32, earns $95,000/year, and spends about $5,500/month. Here's how she calculates her FIRE number:

Step 1: Add up annual expenses

Priya's $5,500/month breaks down as follows:

• Rent/housing: $1,800/month ($21,600/year)
• Food & groceries: $700/month ($8,400/year)
• Transportation: $450/month ($5,400/year)
• Healthcare & insurance: $400/month ($4,800/year)
• Travel & vacation: $600/month ($7,200/year)
• Entertainment & fun: $300/month ($3,600/year)
• Utilities, subscriptions, misc: $250/month ($3,000/year)

Total: $5,500/month = $66,000/year

Step 2: Apply the formula

$66,000 × 25 = $1,650,000

That's Priya's FIRE number. When her investment portfolio reaches $1.65 million, she can withdraw $66,000/year at a 4% rate and — based on historical data — the money should last forever.

Step 3: Adjust for inflation

The good news: you don't need to manually inflate your expenses. The 4% rule already accounts for inflation. It assumes you increase your annual withdrawal by CPI each year. So if inflation runs at 3%, in year two you'd withdraw $67,980. The 25x calculation handles this automatically.

Step 4: Sanity-check against your actual current spending

Before treating $1,650,000 as final, Priya should pull three months of actual bank and credit card statements and compare them against her $5,500/month estimate. This step catches the single most common FIRE-number error: forgetting an entire category of spending because it doesn't happen every month. Car insurance paid semi-annually, an annual subscription renewal, a biannual dentist visit with an out-of-pocket cost — these show up as $0 in eleven months out of twelve and then as a real expense in the twelfth, and estimates built from memory alone routinely miss them. Priya finds she'd forgotten $1,400/year in car registration, annual software subscriptions, and gift-giving around the holidays — bumping her real annual number from $66,000 to $67,400, and her FIRE number from $1,650,000 to $1,685,000.

What to Include in Your Annual Expenses

Many people undercount their expenses when first calculating their FIRE number. Be honest and thorough. Include:

• Housing: rent or mortgage + property taxes + insurance + maintenance (budget 1% of home value per year for repairs)
• Food: groceries + restaurants + coffee + work lunches
• Transportation: car payment + gas + insurance + registration + occasional Uber/Lyft
• Healthcare: this is critical for early retirees — budget $400–$600/month per person for premiums + out-of-pocket costs before Medicare
• Travel: include annual vacations, not just everyday spending
• Fun & hobbies: gym, streaming, golf, books, concerts
• Gifts & special occasions: holidays, birthdays, weddings
• One-time irregular costs: car replacement every 8–10 years, home renovations — amortize these into annual figures
• Taxes on withdrawals: if most of your portfolio sits in a traditional 401(k) or IRA, withdrawals are taxed as ordinary income — your FIRE number may need to be grossed up to account for this, or you'll need a separate withdrawal strategy that manages your tax bracket
• Pets: food, routine vet care, and a buffer for emergency vet bills, which can run into the thousands for a single incident

The taxes-on-withdrawals point deserves emphasis because it's easy to overlook when your FIRE number is calculated purely off spending. If your annual spending is $66,000 and nearly all of it will come from a traditional 401(k), you may need to withdraw somewhat more than $66,000 gross to net $66,000 after tax, depending on your total taxable income and filing status that year. Retirees who hold a mix of traditional, Roth, and taxable brokerage accounts have more flexibility to manage this — pulling from the account type that keeps their taxable income in a favorable bracket each year. MyFIRE's withdrawal strategy calculator models this account-mix optimization directly.

What to leave out

Just as important as what to include is what not to double-count. Leave out anything that stops the day you retire and won't restart: your 401(k)/IRA contributions themselves, payroll taxes (FICA), commuting costs if you won't be commuting, and work-specific expenses like a work wardrobe or professional association dues. Also leave out your mortgage principal-and-interest payment if you plan to have the home paid off before your target retirement date — but keep property taxes, insurance, and maintenance, since those continue regardless of mortgage status. Conflating "what I spend now, including work costs" with "what I'll spend in retirement" is the second most common FIRE-number error, right behind undercounting irregular costs.

FIRE Numbers at Different Spending Levels

Here's the full picture across common monthly spending levels:

Monthly Spending	Annual Spending	FIRE Number (25x)	FIRE Type
$2,000	$24,000	$600,000	Lean FIRE
$3,000	$36,000	$900,000	Lean FIRE
$4,000	$48,000	$1,200,000	Regular FIRE
$5,000	$60,000	$1,500,000	Regular FIRE
$6,000	$72,000	$1,800,000	Regular FIRE
$7,000	$84,000	$2,100,000	Fat FIRE
$8,000	$96,000	$2,400,000	Fat FIRE
$10,000	$120,000	$3,000,000	Fat FIRE

Notice how a $1,000/month lifestyle change — from $5,000 to $4,000/month — reduces your FIRE number by $300,000. That's why your spending level is the most powerful lever in FIRE math, more impactful than almost any investment decision.

This table also explains why the same underlying strategy — save aggressively, invest in low-cost index funds, hit 25x expenses — produces such different-looking outcomes across the FIRE community online. Someone posting about hitting $600,000 and someone posting about hitting $3,000,000 are running the exact same formula; they've simply made different choices about their target lifestyle. Neither number is more "correct" than the other. The formula doesn't judge your spending level — it just tells you honestly what that spending level costs to sustain indefinitely.

How Social Security Reduces Your FIRE Number

If you're planning to retire at 50 or 55, you'll eventually receive Social Security benefits starting at age 62 (reduced) or 67 (full). This dramatically changes your calculation for the later decades of retirement.

Here's how to factor it in: subtract your expected Social Security income from your annual expenses, then apply the 25x multiplier to the remainder.

Example: Priya expects $18,000/year from Social Security at 67. Her current annual expenses are $66,000. Adjusted for Social Security: $66,000 - $18,000 = $48,000 still needs to come from the portfolio. New FIRE number: $48,000 × 25 = $1,200,000 instead of $1,650,000. That's $450,000 less to save.

The catch: you have to fund the gap years before Social Security kicks in. If Priya retires at 50 and Social Security starts at 67, she needs 17 years of full $66,000 withdrawals from her portfolio, then the load lightens significantly. Tools like MyFIRE's planner model this two-phase retirement automatically.

A note of caution: don't build your entire FIRE number around an assumed Social Security benefit decades before you'll actually claim it. Your real benefit depends on your full earnings history, which for someone in their early 30s is still mostly unwritten. Use the Social Security Administration's own estimator at ssa.gov for a rough baseline, treat it as directional rather than exact, and revisit the number every few years as your earnings record fills in. Some FIRE planners deliberately exclude Social Security from the core FIRE number entirely and treat it purely as a safety margin — a conservative approach that means if the benefit turns out smaller than expected, or the claiming rules change before you reach 62, your plan was never depending on it in the first place.

Couples and Shared FIRE Numbers

For couples, the math is additive but not simply doubled. Two people sharing a household don't pay rent twice, don't need two refrigerators, and share most fixed costs. A single person spending $4,000/month rarely means a couple spends $8,000/month for the same lifestyle — more commonly it's closer to $5,500-$6,500/month, since housing, utilities, and many subscriptions are shared while food, healthcare, and discretionary spending scale more directly per person. When calculating a household FIRE number, build the expense estimate from actual shared-household spending rather than doubling one person's individual budget — doubling typically overstates the true number by 20-30%.

Couples should also decide explicitly whether they're calculating one combined FIRE number for a single retirement date, or two dates — since one partner may want to retire earlier or later than the other, or one may have significantly higher expected Social Security or pension income. MyFIRE's planner supports couple mode with separate income and contribution inputs for exactly this reason.

Adjusting for Retirement Spending Changes

Research consistently shows that retirement spending follows a "smile" pattern: higher in the active early years (travel, hobbies), lower in the middle years, then higher again at the end (healthcare). Some FIRE planners use slightly different expense estimates for each phase:

• Ages 50–65 (go-go years): 100% of estimated expenses (travel, activity)
• Ages 65–75 (slow-go years): 80–85% of peak spending
• Ages 75+ (no-go years): 70% of peak, but healthcare rises sharply

For a simple calculation, using your current spending is perfectly reasonable. Just make sure your healthcare line is realistic — it's the most underestimated expense for early retirees.

Healthcare deserves its own line item precisely because it breaks the "smile" pattern above — for early retirees, it doesn't dip in the middle years. Before Medicare eligibility at 65, a couple buying an ACA marketplace plan without employer subsidy can reasonably expect $1,000-$1,600/month in premiums alone, before deductibles and out-of-pocket costs, depending on state, age, and income-based subsidy eligibility. This is frequently the single largest line item in an early retiree's budget after housing, and it's the one most commonly underestimated because working professionals are used to seeing only their payroll-deducted portion of a much larger true premium. Run your FIRE number with a realistic pre-Medicare healthcare estimate, not your current employer-subsidized cost.

Why Most People Overestimate Their Expenses

Here's the paradox: while people often miss irregular costs, they consistently overestimate the expenses that will disappear in retirement. Think about what goes away when you stop working:

• Work clothing, commuting costs, lunches out
• Childcare and school expenses (if kids are grown)
• Mortgage payments (if paid off by retirement)
• The expensive "stress spending" — the Friday wine, the takeout because you're too tired to cook

Many FIRE retirees find they're genuinely happy spending $10,000–$20,000 less per year than they did while working, because time replaces money as the primary resource for enjoyment. You can cook instead of ordering in. You can travel slowly and cheaply instead of taking expensive rushed vacations.

The practical upshot: run your calculation with your current spending, then mentally test whether that number would still feel right if you had complete time freedom. Many people discover their "actual" FIRE number is 10–20% lower than their working-life spending implies.

How Often to Recalculate Your FIRE Number

Your FIRE number isn't a one-time calculation — it's a figure that should move as your life does. Recalculate at minimum once a year, and immediately after any major life change: a new child (childcare and later education costs shift the number meaningfully), a move to a lower or higher cost-of-living area, a change in health status, a marriage or divorce, or a significant, sustained change in your actual spending habits. Many people set their FIRE number once in their late 20s or early 30s and never revisit it, then feel confused a decade later when the number that once felt motivating no longer matches their actual life. Treat the FIRE number the way you'd treat a budget: a living figure, not a monument.

It's also worth tracking your FIRE number alongside your actual portfolio progress over time, rather than calculating it once and then only checking your portfolio balance in isolation. Watching both numbers move together — your target adjusting slightly as your life changes, your portfolio climbing toward it — gives a much clearer sense of real progress than either number alone. MyFIRE's planner recalculates your FIRE number automatically as you update your inputs, so the target and the progress bar stay in sync.