How to Calculate Your FIRE Number (And Why It's Probably Lower Than You Think)
Your FIRE number is 25 times your expected annual spending — but that's the ceiling, not the floor. Once you account for Social Security income, a paid-off mortgage, part-time work, or flexible spending, most people's actual number is meaningfully lower than the simple formula suggests.
The 25x Rule: Where It Comes From
The 25x rule is the inverse of the 4% safe withdrawal rate. If you withdraw 4% of a portfolio each year, you need a portfolio 25 times your annual spending. Spend $40,000/year → need $1 million. Spend $60,000/year → need $1.5 million. Spend $100,000/year → need $2.5 million.
This calculation includes all sources of spending: housing, food, transportation, healthcare, travel, taxes on withdrawals, and everything else. Most FIRE calculators default to this rule because it's historically validated for 30-year retirements in the U.S. stock market going back to 1926.
For the standard 25x calculation, use your current annual spending as a baseline — but be honest. Research consistently shows people underestimate their spending by 15–25% in retirement projections. Track actual expenses for 3–6 months before calculating your number.
Real Example: $60,000 Spending Baseline
Suppose you and your spouse spend $60,000 per year in today's dollars. The naive 25x calculation says you need $1.5 million.
But let's add some realistic adjustments:
You expect Social Security of $18,000/year at age 67 (conservative estimate for a two-income household that worked for 15–20 years before early retiring). That reduces your portfolio withdrawal need to $42,000/year → requiring only $1.05 million.
Your mortgage will be paid off in 8 years, saving $14,400/year. If you retire 8 years from now, your spending drops to $45,600 → requiring only $1.14 million.
Combining both: you need $42,000 – $14,400 = $27,600 from your portfolio → requiring just $690,000.
The lesson: the 25x number is right for 100% of your spending coming from the portfolio forever. Adding any sustainable income stream dramatically reduces the required portfolio.
Adjusting for Social Security
Social Security is the most underestimated FIRE adjustment. Even early retirees who stop working at 40 or 45 often have 15–20 years of work history, which translates to a meaningful benefit at 67 or 70.
You can get your Social Security estimate for free at ssa.gov/myaccount. The projection assumes you continue earning your current salary until FRA — so for early retirees, the actual benefit will be lower than the projection. A rough rule of thumb: if you've worked 15–20 years and your income was average to above-average, expect $800–$1,500/month (individual) at FRA.
Even $12,000/year in Social Security reduces your required portfolio by $300,000 (at 4%) to $360,000 (at 3.3%). That's not money to leave out of the calculation.
Note: if you retire early and stop working, your Social Security benefit will be smaller than if you worked until 62 or 67, because the benefit is calculated on your 35 highest-earning years. Zero-income years drag down the average. But "smaller" does not mean "negligible."
Coast FIRE: The Intermediate Milestone
Coast FIRE is the portfolio balance at which you no longer need to make contributions — your existing investments will grow to your full FIRE number by traditional retirement age without any additional savings, purely through compounding returns.
The formula: Coast FIRE Number = Full FIRE Number ÷ (1 + r)^years_to_retirement
Example: You need $1.5 million by age 65 and you're currently 35. At 7% real return over 30 years: $1,500,000 ÷ (1.07)^30 = approximately $197,000.
If you have $197,000 invested today at age 35 and never add another dollar, you'll hit $1.5 million by 65 (assuming 7% average real returns). This means you've "coasted" — you can take a lower-paying job you love, work part-time, or simply stop saving aggressively, because the compounding takes care of the rest.
Coast FIRE is a powerful intermediate milestone because it's achievable much earlier than full FIRE and represents a genuine form of financial freedom — you can't walk away from work entirely, but you can walk away from maximizing savings.
LeanFIRE vs. FatFIRE: Sizing the Spectrum
The FIRE community broadly uses these terms to describe different spending levels in retirement:
LeanFIRE: Annual spending under $40,000 (individual or couple). Portfolio target roughly $1 million at 4% or $1.2 million at 3.3%. Requires intentional frugality and often geographic arbitrage (lower cost-of-living area, slow travel, etc.).
Regular FIRE: Annual spending $40,000–$80,000. Portfolio target $1–2 million. The most commonly modeled scenario.
FatFIRE: Annual spending above $100,000. Portfolio target $2.5 million+. Preserves a lifestyle similar to a high-earning career, with travel, dining, premium healthcare, and estate-planning considerations.
BaristaFIRE is a variant where you partially retire — working part-time (often for health insurance, hence the coffee shop reference) and relying on the portfolio for the remainder. It dramatically reduces the required portfolio because even $15,000–$20,000 in earned income reduces portfolio withdrawals by $375,000–$500,000 at a 4% rate.
There is no right answer — the spectrum reflects personal values about time, spending, and risk tolerance, not moral hierarchy.
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Frequently Asked Questions
How do I calculate my FIRE number?
Multiply your expected annual spending in retirement by 25. This gives you the portfolio target for a 4% withdrawal rate over 30 years. For longer retirements (40+ years), multiply by 28–30 instead. Then subtract the present value of any expected income (Social Security, pension, rental income) from the annual spending before multiplying.
Should I use current spending or retirement spending to calculate my FIRE number?
Use projected retirement spending, which may differ from current spending. Common adjustments: remove mortgage payment if it will be paid off, remove retirement savings contributions (you won't be saving once retired), add healthcare costs (likely higher in early retirement without employer coverage), and add any travel or lifestyle spending you currently postpone.
Does the FIRE number account for inflation?
Yes — the 4% rule (and thus the 25x rule) is inflation-adjusted. You withdraw 4% of your initial portfolio in year one, then increase that dollar amount by inflation each subsequent year. Your portfolio's growth in equities historically outpaces inflation over long periods, which is why this strategy has worked.
What is Coast FIRE?
Coast FIRE is the portfolio size at which you can stop contributing and let compounding carry you to your full FIRE number by traditional retirement age. It's calculated by dividing your full FIRE number by (1 + return rate)^years_to_retirement. At 7% returns, money doubles roughly every 10 years, so you need much less invested today to reach a large number decades from now.
Is a $1 million portfolio enough to retire?
At a 4% withdrawal rate, $1 million supports $40,000 per year in spending. Whether that's enough depends entirely on your lifestyle and location. For a couple with low housing costs and Social Security income covering $20,000–$24,000 of expenses, $1 million is achievable FIRE. For someone in a high-cost city with no other income, it's likely not sufficient.
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