# The FIRE Number: How Much Do You Actually Need?
Your FIRE number is the portfolio size at which passive investment income can sustain your spending indefinitely. It is the clearest goal in personal finance: a single dollar figure that separates working-because-you-must from working-because-you-choose.
The math is simple. The assumptions behind the math are where most people get into trouble.
The formula
**FIRE Number = Annual Spending ÷ Safe Withdrawal Rate**
At a 4% withdrawal rate, the formula simplifies to: Annual Spending × 25.
Spend $40,000/year → FIRE number is $1,000,000. Spend $60,000/year → FIRE number is $1,500,000. Spend $80,000/year → FIRE number is $2,000,000.
The multiplier changes with the withdrawal rate: - 3.5% rate → 28.6x multiplier - 4% rate → 25x multiplier - 4.5% rate → 22.2x multiplier - 5% rate → 20x multiplier
What people get wrong about their spending number
Most people underestimate annual spending because they think in terms of their current monthly budget — not the full picture. Common omissions:
**Irregular large expenses.** Car replacement every 8–12 years, home maintenance and repair, medical costs that rise with age, children's education or weddings, travel aspirations. These average out to significant annual figures.
**Healthcare before Medicare.** If you retire before 65, you are responsible for your own health insurance. A couple in their 50s buying marketplace insurance can easily spend $15,000–$25,000/year before subsidies, depending on income. This expense alone can add $375,000–$625,000 to your FIRE number.
**Sequence-of-returns buffer.** If you retire into a bear market, a spending number that worked historically may not work in your sequence. Many FIRE practitioners add a 10–20% buffer to their spending estimate.
**Lifestyle inflation.** Retired life often costs more, not less, in the early years — travel, hobbies, home improvement projects, helping family. The "spending drops in retirement" assumption is less reliable for early retirees than for traditional retirees in their 70s.
Interactive Model
FIRE Number Calculator
Find your target, track your progress, and see how long until you get there.
Your FIRE number
$1,500,000
$60,000/yr gap ÷ 4% SWR
Current progress
10.0%
$150,000 saved
Time to FIRE under different contribution levels
Portfolio trajectory to FIRE
FIRE number = annual portfolio gap ÷ withdrawal rate. Does not model taxes, sequence risk, or spending changes. Use as a planning target, not a guarantee.
The years-to-FIRE calculation
Given your current savings rate and portfolio, the years to FIRE is a compounding problem: how long until your portfolio grows to your target while you contribute monthly? The calculation is sensitive to your savings rate — small changes in how much you save each month compress or extend the timeline dramatically.
What guaranteed income changes
Social Security, a pension, or rental income reduces how much your portfolio needs to cover. A FIRE practitioner with $18,000/year in rental income and $60,000 in annual spending only needs their portfolio to cover $42,000 — a FIRE number of $1,050,000 instead of $1,500,000.
This is why diversifying income sources is one of the highest-leverage moves in FIRE planning.
The honest caveat
The FIRE number is a target, not a finish line. Markets move. Spending changes. Healthcare surprises. Tax law shifts. The number gives you a goal and a direction. The margin of safety comes from being somewhat below the withdrawal rate you plan to use, having flexibility to cut spending in bad market years, and ideally having some income source — part-time work, a side business, rental income — that can absorb a few bad sequence years without forcing large portfolio withdrawals.
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*Related: [The 4% rule](./real-math-behind-4-percent-rule) explains the research behind the withdrawal rate. [FIRE by age: monthly savings needed](./fire-monthly-savings-by-age) shows how to close the gap.*