How to Calculate Your FIRE Number

Your FIRE number is the single most important figure in your early retirement plan. It's the portfolio balance at which your investment returns can sustain your spending—the point where work becomes optional. Every savings decision, every investment choice, every spending tradeoff you make is ultimately in service of reaching this number.

Calculating it is straightforward in theory but deceptively tricky in practice. The basic formula takes thirty seconds. Getting it right for your situation takes considerably more thought.

The Basic Formula: 25x Your Annual Spending

The most common FIRE number formula is simply your annual spending multiplied by 25. This comes from the 4% rule—if you can safely withdraw 4% of your portfolio each year, then you need a portfolio that is 25 times your annual withdrawal (since 1 / 0.04 = 25).

The math is clean:

This is a useful starting point and a perfectly fine back-of-the-envelope calculation. It gives you a concrete target to work toward, which is psychologically powerful. But it rests on assumptions that may not match your reality, and treating it as gospel can lead you astray in either direction.

Why 25x Might Be Wrong for You

The 4% rule was derived from 30-year retirement periods, which roughly corresponds to someone retiring at 65 and planning to age 95. If your retirement horizon is different, your multiplier should be too.

You're Retiring Early (Age 30-45)

If you're planning a 40, 50, or even 60-year retirement, 25x may not provide enough cushion. Over longer time horizons, the probability of encountering a devastating sequence of poor returns increases. Most FIRE planners targeting very early retirement use a multiplier of 28x to 33x, corresponding to withdrawal rates of 3.0% to 3.5%. A 35-year-old retiree spending $50,000/year might target $1.5M to $1.65M rather than the $1.25M that 25x suggests.

You Have Other Income Sources

If you'll receive Social Security, a pension, rental income, or any other reliable income stream, you don't need your portfolio to cover all of your spending. You only need to cover the gap between your other income and your total spending.

For example, if you spend $70,000/year and expect $25,000/year from Social Security starting at 67, you only need your portfolio to generate $45,000/year. At 25x, that's $1,125,000 instead of $1,750,000—a $625,000 difference that could mean years less of working.

For early retirees, keep in mind that these income sources may not kick in for decades. You may need a larger portfolio to bridge the gap until Social Security or a pension begins. A tool like the FIREwiz retirement simulator can model these phased income streams directly.

You Use Dynamic Withdrawal Strategies

The 4% rule assumes you withdraw a fixed inflation-adjusted dollar amount regardless of market performance. But if you're willing to reduce spending during downturns and increase it during good years, you can sustain a higher initial withdrawal rate—which means a lower FIRE number. Strategies like Guyton-Klinger guardrails or variable percentage withdrawal have historically supported starting rates of 4.5% to 5.5%, which translates to multipliers of 18x to 22x. The tradeoff is income volatility: you need to be genuinely comfortable spending less in some years.

Your Spending Has Large Discretionary Components

If a significant portion of your budget is travel, dining, or hobbies that you could scale back in a pinch, your effective FIRE number is lower than a strict 25x calculation implies. You have a built-in safety valve. Conversely, if nearly all your spending is non-negotiable (housing, healthcare, food), you need more cushion because there's nowhere to cut if markets turn against you.

The Spending Estimate Is Everything

Here's the uncomfortable truth: most people agonize over whether their multiplier should be 25x or 28x or 30x while using a spending estimate that's off by 20%. Whether you multiply by 25 or 30 matters far less than whether the number you're multiplying is accurate.

The only way to get a reliable spending estimate is to track your actual spending for at least six months, ideally a full year. A tool like Monarch Money makes this easy by automatically categorizing your transactions. This captures seasonal variation (holiday spending, annual insurance premiums, property taxes) that a single month's snapshot will miss.

Beyond tracking current spending, you need to account for how your expenses will change in retirement. Some costs go away or shrink:

• Commuting and work-related expenses
• Retirement account contributions (you're no longer saving)
• Payroll taxes (FICA)
• Work clothing, lunches, and professional expenses

But other costs appear or grow, and these are the ones people chronically underestimate:

• Healthcare: This is the big one for early retirees. If you're retiring before 65, you won't have Medicare. ACA marketplace plans for a family can easily run $15,000 to $25,000+ per year, depending on your state and income level.
• Taxes on withdrawals: Money withdrawn from traditional 401(k) and IRA accounts is taxed as ordinary income. A $60,000 withdrawal is not $60,000 of spending money. Plan for an effective tax rate of 10-20% on withdrawals, depending on your total income and state.
• Home maintenance: Budget 1-2% of your home's value annually. A $400,000 home means $4,000 to $8,000 per year in maintenance and repairs, on average.
• Travel and leisure: Many retirees spend more on travel and activities in the first decade of retirement than they did while working.
• Lifestyle inflation: More free time often means more spending. Be honest with yourself about this.

A good exercise: take your tracked annual spending, remove work-related costs, add estimated healthcare and tax costs, then add 10% as a buffer for the things you haven't thought of. Use that number in your FIRE calculation. For a deeper look at total retirement spending, see our guide on how much you actually need to retire.

Lean FIRE vs. Fat FIRE

The FIRE community broadly recognizes a spectrum of retirement lifestyles, and where you fall on it dramatically affects your target number.

Lean FIRE typically means household spending of roughly $40,000/year or less. At 25x, that's a $1M target—achievable for many people within 10-15 years of aggressive saving. The appeal is obvious: a lower target means reaching financial independence faster. The risk is equally obvious: there's almost no margin for error. An unexpected medical bill, a needed car replacement, or a bout of higher inflation can strain a lean budget quickly. Lean FIRE works best for people who genuinely prefer a minimalist lifestyle, not those who are simply trying to escape work as fast as possible.

Fat FIRE generally means $100,000+/year in spending, implying a portfolio of $2.5M or more. This takes longer to reach but provides substantial flexibility. You can absorb market downturns more easily because there's room to cut discretionary spending without affecting your quality of life. Fat FIRE also tends to be more resilient against the spending surprises that derail lean budgets.

Neither approach is inherently better. The right target depends on your actual spending needs, your tolerance for financial risk, and how much flexibility you want. The worst mistake is picking a target based on what sounds good in a Reddit thread rather than what matches your real life. If you're not sure where you land, our guide to Coast FIRE covers a middle-ground approach that can reduce pressure while you figure out your long-term number.

Your FIRE Number Is Not Static

One of the most common mistakes in FIRE planning is calculating your number once and then treating it as fixed. Your FIRE number should evolve as your life does.

Your spending patterns will change. A growing family costs more than a single person. Moving to a lower cost-of-living area reduces your target. Paying off a mortgage drops your annual expenses significantly. Health conditions emerge. Hobbies change. Every major life event is an opportunity to recalculate.

Market conditions matter too. If you're five years from your target and equity valuations are historically elevated, you might want a slightly larger cushion. If bond yields are high and you plan a conservative allocation, you might need less. The point isn't to chase a moving target obsessively—it's to revisit your assumptions annually and make sure your plan still reflects reality.

As you approach your FIRE number, shift from rough estimates to detailed modeling. A Monte Carlo simulation that tests your specific spending, allocation, and withdrawal strategy against thousands of possible market scenarios will give you far more confidence than any single multiplier ever could.

Calculate Your Number

The 25x rule gives you a starting point. Adjusting for your retirement age, income sources, withdrawal flexibility, and actual spending gets you closer to the truth. But to really pressure-test your FIRE number, you need to simulate it.

The FIREwiz accumulation calculator will show you how long it takes to reach your target given your current savings rate and asset allocation. Once you've hit your number, the retirement simulator lets you stress-test your withdrawal plan against historical and Monte Carlo scenarios—so you can retire with confidence, not just hope.