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FIRE number = Annual expenses × 25 (4% safe withdrawal rate)
Conservative FIRE number = Annual expenses × 30 (3.33% SWR, for 40+ year horizon)
Adjusted FIRE number = (Annual expenses − Guaranteed income) × 25
Worked example: Annual expenses 30,000. State pension covers 12,000 from age 67. Net expenses portfolio must fund: 18,000. FIRE number = 18,000 × 25 = 450,000. Current savings 80,000, saving 20,000/year at 5% real return → approximately 13 years to reach FIRE number.