How to Calculate Your FIRE Number: A Step-by-Step Guide

What Is a FIRE Number?

Your FIRE number is the total amount of invested assets you need to retire early and live off your portfolio indefinitely. It is the finish line of the financial independence journey — the point at which paid work becomes optional and your money works for you instead of the other way around.

Calculating your FIRE number is not complicated, but getting it right requires honest accounting of your actual expenses, realistic assumptions about investment returns, and consideration of factors like taxes, healthcare, and inflation.

The Basic Formula: 25 Times Annual Expenses

The most widely used method to calculate a FIRE number comes from the 4% Rule, which emerged from the Trinity Study — a 1998 academic paper that analyzed historical portfolio survival rates across different withdrawal rates and asset allocations.

The finding: a portfolio of 50-75% stocks and 25-50% bonds could sustain a 4% annual withdrawal rate for 30 years with a very high probability of success (historically, 95%+). Flipping this around gives the 25x formula:

FIRE Number = Annual Expenses × 25

Examples:

• Spending $30,000/year → FIRE number of $750,000
• Spending $50,000/year → FIRE number of $1,250,000
• Spending $80,000/year → FIRE number of $2,000,000
• Spending $100,000/year → FIRE number of $2,500,000

Step 1: Calculate Your True Annual Expenses

Your FIRE number is only as accurate as your expense estimate. Most people underestimate what they spend. Go through 12 months of bank and credit card statements and categorize every dollar:

• Housing (mortgage/rent, insurance, taxes, maintenance)
• Food (groceries plus dining out)
• Transportation (car payment, insurance, fuel, maintenance, public transit)
• Healthcare (premiums, out-of-pocket, prescriptions)
• Utilities (electricity, gas, water, internet, phone)
• Entertainment and subscriptions
• Travel and vacation
• Personal care, clothing
• Gifts and charitable giving
• Irregular but predictable expenses (car replacement, home repairs)

Add a buffer of 10-15% for expenses you haven't anticipated. Honesty here is crucial — underestimating by $10,000 per year adds $250,000 to your FIRE number.

Step 2: Adjust for Retirement-Specific Expenses

Your retirement expenses may differ from your current expenses in important ways:

Healthcare

If you retire before 65 (Medicare eligibility), you're responsible for your own health insurance. Marketplace plans under the ACA can cost $500-$1,500+ per month for a family, depending on age, location, and income. Budget this explicitly — it's one of the most common oversights in FIRE planning.

Taxes

Tax treatment of retirement income depends on your account types. Withdrawals from traditional 401(k)s and IRAs are taxed as ordinary income. Roth withdrawals are tax-free. Long-term capital gains in taxable brokerage accounts have preferential rates. Plan your withdrawal strategy to minimize lifetime taxes.

Inflation

A dollar today is worth more than a dollar in 20 years. The 4% rule implicitly accounts for inflation by assuming portfolio growth keeps pace. But unusual inflation spikes (like healthcare inflation, which historically exceeds general CPI) deserve special attention.

Step 3: Choose Your Safe Withdrawal Rate

The 4% rule is a starting point, not a guarantee. Several factors affect how conservative your withdrawal rate should be:

• Retirement length: The Trinity Study modeled 30-year retirements. If you retire at 35 and live to 90, you need 55 years of funding. Many experts recommend a 3-3.5% withdrawal rate for very long retirements.
• Flexibility: If you can reduce spending by 10-20% during market downturns, a higher withdrawal rate is more sustainable.
• Other income sources: Social Security, rental income, or part-time work income reduces your reliance on portfolio withdrawals.
• Equity allocation: Higher stock allocations have historically supported higher withdrawal rates over long periods, though with more volatility.

Adjusted formulas:

• Conservative (3% withdrawal): FIRE Number = Annual Expenses × 33
• Moderate (3.5% withdrawal): FIRE Number = Annual Expenses × 28.5
• Standard (4% withdrawal): FIRE Number = Annual Expenses × 25

Step 4: Account for Social Security and Other Income

If you expect Social Security benefits in retirement, that income reduces the portfolio withdrawals you need. To factor it in, subtract your expected Social Security income from your annual expenses before calculating your FIRE number.

Example: You spend $60,000/year. You expect $15,000/year from Social Security at 67. Your portfolio only needs to cover $45,000/year. FIRE Number = $45,000 × 25 = $1,125,000 (instead of $1,500,000).

Step 5: Determine How Long to Reach Your Number

Once you know your FIRE number, use compound interest math or an online calculator to estimate how long it will take based on your current savings, monthly contributions, and expected return. A common assumption is 7% real annual return (10% nominal minus 3% inflation).

Key insight: your savings rate has a far bigger impact on time to FIRE than your investment return assumption. Doubling your savings rate cuts your timeline far more dramatically than optimizing your portfolio for an extra 1% annual return.

The Bottom Line

Calculating your FIRE number is empowering because it turns a vague aspiration into a concrete target. Run the numbers honestly, build in appropriate buffers, and revisit them annually as your situation changes. The FIRE number is not a ceiling — it's a compass that keeps your financial decisions pointing in the right direction.