Calculate Fire Number

---
name: calculate-fire-number
description: Use when determining the portfolio size needed to retire early and sustain spending indefinitely
source: Trinity Study (Cooley, Hubbard, Walz 1998, updated 2011); William Bernstein "The Four Pillars of Investing"; ERN Safe Withdrawal Rate series
tags: [finance, personal-finance, fire, retirement, safe-withdrawal-rate, financial-independence]
verified: true
---

# Calculate FIRE Number

Determine the portfolio size at which passive investment income can sustain your lifestyle indefinitely.

## Why This Is Best Practice

**Adopted by:** FIRE community (Mr. Money Mustache, Mad Fientist); foundational to fee-only financial planning practice
**Impact:** Trinity Study analyzed 30-year retirement periods from 1926–1995 and found a 4% withdrawal rate succeeds in ~95% of historical scenarios with a 50%+ equity portfolio.

**Why best:** The 4% rule provides a defensible starting point that is grounded in 70+ years of market data across multiple cycles including the Great Depression. Adjusting the withdrawal rate for longer retirement horizons (40–60 years for early retirees) and current valuations improves the reliability of the estimate beyond the basic rule.

## Steps

1. **Calculate annual spending** — Track actual 12-month spending or use the 50/30/20 budget; include taxes in retirement, healthcare, and irregular expenses (car replacement, home maintenance ~1–2% of home value/year).
2. **Adjust for retirement-specific costs** — Add healthcare premiums pre-Medicare (often $500–$1,500/month), remove commuting/work costs, adjust for lifestyle inflation or deflation.
3. **Apply safe withdrawal rate** — Use 4% for 30-year retirement; 3.5% for 40-year; 3.25% for 50+ years or conservative market valuations. FIRE number = Annual Spending / SWR.
4. **Account for income sources** — Subtract present value of Social Security, pension, or rental income from the required portfolio; recalculate FIRE number on net spending only.
5. **Model sequence-of-returns risk** — Early retirement faces the highest sequence risk; plan for a 2-year cash buffer or flexible withdrawal strategy (reduce spending 10% in down years).
6. **Validate with Monte Carlo** — Run 10,000+ simulations using your asset allocation; target >95% success rate; free tools include Portfolio Visualizer and FIRECalc.
7. **Build in buffer** — Add 10–15% margin to the calculated FIRE number to account for model uncertainty, unexpected expenses, and lifestyle creep.
8. **Set milestone tracking** — Calculate your current FI% = (Portfolio / FIRE Number) × 100; track quarterly.

## Rules

- Never use a withdrawal rate above 4% for retirements longer than 30 years without a documented flexible spending plan.
- Always include healthcare costs in spending projections; this is the most commonly underestimated expense.
- Account for taxes on withdrawals from tax-deferred accounts (401k, IRA distributions are ordinary income).
- Maintain 1–2 years of expenses in cash/short-term bonds to avoid selling equities in a down market.
- Revisit the FIRE number annually; spending and return expectations change.

## Examples

**Scenario:** Annual spending = $60,000. FIRE horizon = 45 years (retiring at 40). SWR = 3.5%. FIRE number = $60,000 / 0.035 = $1,714,286. Expected Social Security at 67 = $18,000/year (PV at 3% discount = ~$360,000 equivalent). Adjusted FIRE number = ($60,000 − $18,000) / 0.035 = $1,200,000. Add 10% buffer → target $1,320,000.

## Common Mistakes

- **Ignoring taxes** — Withdrawals from pre-tax accounts are taxable income; effective tax rate in retirement can be 15–22%.
- **Using nominal not inflation-adjusted returns** — The 4% rule assumes real (inflation-adjusted) withdrawals; do not mix nominal returns with nominal withdrawal amounts without adjustment.
- **Treating FIRE number as permanent** — Revisit after major life changes; a divorce, health issue, or location change can shift the number significantly.

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> **Finance disclaimer:** This skill encodes professional best practices for educational purposes. It is not financial advice. Consult a licensed financial advisor before making investment decisions.