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Financial Independence, Retire Early (FIRE) is built on a single equation: annual expenses × 25 = FIRE number. This comes from the 4% rule (Trinity Study): historically, withdrawing 4% of a stock/bond portfolio annually has sustained 30+ years of withdrawals in 95% of historical periods.
Example: $50,000/year expenses × 25 = $1,250,000. Reach $1.25M in investments and you can, statistically, sustain $50K/year withdrawals indefinitely. $80,000/year × 25 = $2,000,000. The math is surprisingly achievable — the variable that matters most isn't income or investment returns. It's savings rate.
Savings rate dominates everything: at 50% savings rate with 7% returns, you reach FIRE in approximately 17 years regardless of income. At 20% savings rate: 37 years. At 75% savings rate: 7 years. The savings rate determines the timeline because it simultaneously: increases the rate of wealth accumulation AND decreases the target (lower expenses = lower FIRE number).
FIRE variants: Traditional FIRE (full retirement), Coast FIRE (invest enough early that compounding reaches your target at traditional retirement age — then you only need to cover current expenses from work), Barista FIRE (partially retired, working a low-stress part-time job for benefits and supplemental income), and Fat FIRE ($100K+ annual spending — requires $2.5M+).
Criticism and nuance: the 4% rule was calibrated on US market data and assumes a specific stock/bond allocation. Some researchers argue 3.5% is safer. Healthcare before 65 (Medicare eligibility) is the biggest expense gap in early retirement. Sequence-of-returns risk (poor returns in early retirement years) can deplete portfolios faster than the average would suggest. FIRE planning should build in margin.
Tip
The most powerful FIRE insight: reducing expenses by $1 has the same effect as increasing income by $1 for wealth accumulation — but reducing expenses ALSO lowers your FIRE target (you need less to sustain lower expenses). A $100/month expense reduction both adds $1,200/year to savings AND reduces your FIRE number by $30,000 (at 25x). Every expense cut has a double benefit.
FIRE number = annual expenses × 25 (from the 4% rule). Savings rate is the dominant variable — 50% saves rate reaches FIRE in ~17 years regardless of income. Reducing expenses has a double effect: faster accumulation + lower target. Variants: Coast FIRE, Barista FIRE, Fat FIRE. Build margin: consider 3.5% withdrawal rate and healthcare costs before Medicare eligibility.
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