Financial Independence, Retire Early (FIRE) is a movement built around one core idea: accumulate enough invested assets that your portfolio generates sufficient passive income to cover all your living expenses — indefinitely. The number you need to hit? Your FIRE number.
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Your FIRE number is the total investment portfolio value you need to sustain your lifestyle without ever working again. It's not a salary — it's a lump sum invested in assets (stocks, bonds, real estate) that generate ongoing returns from which you withdraw annually to live.
The mathematical foundation is the 4% safe withdrawal rate (SWR), derived from the 1994 Trinity Study at Trinity University. Researchers found that a portfolio of 50–75% stocks and 25–50% bonds could support a 4% annual withdrawal (inflation-adjusted) for 30 years with 95%+ probability of not running out of money, based on historical market data going back to 1925.
This is the simplest version. For a 4% SWR, your FIRE number is 25× your annual expenses (because 1 ÷ 0.04 = 25). For a more conservative 3.5% SWR, it's 28.6× annual expenses.
This is the most important input — and where most people underestimate. Track your actual spending for 3–6 months before estimating. Categories to include:
Many FIRE planners intentionally add a 10–20% buffer to their estimated expenses to account for lifestyle creep and unexpected costs in early retirement.
The "right" SWR depends on your retirement horizon:
| Retirement Length | Suggested SWR | Multiplier |
|---|---|---|
| 30 years (traditional retirement) | 4% | 25× |
| 40 years (early retirement age 50) | 3.5% | 28.6× |
| 50+ years (retire in 30s) | 3–3.25% | 30.8–33.3× |
| Never touch principal (generational wealth) | 2.5–3% | 33.3–40× |
The FIRE community has developed variants for different lifestyle goals:
Your savings rate is the primary determinant of your FIRE timeline — more so than investment returns or income. This relationship is non-linear:
| Savings Rate | Years to FIRE |
|---|---|
| 10% | ~46 years |
| 20% | ~37 years |
| 30% | ~28 years |
| 40% | ~22 years |
| 50% | ~17 years |
| 60% | ~12 years |
| 70% | ~8.5 years |
| 80% | ~5.5 years |
Assumes 7% annual return, starting from $0.
The FIRE calculator uses a default 7% real return (after inflation), which reflects the historical average real return of the US stock market. Important nuances:
Where you hold your assets matters enormously for the FIRE community:
Is the 4% rule still valid today?
The 4% rule was developed using data through the early 1990s. With today's lower bond yields and higher valuations (higher CAPE ratios), some researchers argue that 3–3.5% is more appropriate for a 40-year retirement. Wade Pfau, a leading retirement researcher, suggests 3.3% for a 40-year horizon with 90% success probability. However, FIRE retirees also have the flexibility to reduce spending in bad market years, making the 4% rule more resilient in practice than in theory.
What about healthcare before Medicare at 65?
Healthcare is the #1 expense wildcard for early retirees. Options include: (1) ACA marketplace plans — subsidies are available if your income (MAGI) is 100–400% of the federal poverty level, which is achievable with tax-efficient FIRE withdrawals; (2) Health sharing ministries (cheaper, more restrictions); (3) Continuing a spouse's employer coverage; (4) Part-time work for benefits (Barista FIRE). Budget $500–$1,500/month per person for healthcare if retiring before 65.
Should I include Social Security in my FIRE number?
If you're retiring before 62 (earliest SS claiming age), don't include SS in your FIRE calculation — it's too uncertain decades out. If you're in your 50s and retiring at 55–60, you can include a conservative SS estimate (maybe 70% of your projected benefit to account for potential future cuts). Use it as a bonus cushion rather than a core assumption.