The Core Math of FIRE
FIRE is built on one formula: accumulate 25 times your annual spending. At that point, the 4% withdrawal rule suggests your investments can sustain your lifestyle indefinitely. This is your “FIRE number.”
Annual spending $40,000 → FIRE number $1,000,000. Annual spending $60,000 → $1,500,000. Annual spending $80,000 → $2,000,000.
The path to your FIRE number is determined almost entirely by your savings rate — what percentage of income you invest each year.
| Savings Rate | Years to FIRE (from $0) |
|---|---|
| 10% | ~43 years |
| 25% | ~32 years |
| 40% | ~22 years |
| 50% | ~17 years |
| 60% | ~12.5 years |
| 75% | ~7 years |
Assumes 7% annual investment returns. Starting from $0 net worth. Time to accumulate 25× annual expenses. Source: Mr Money Mustache framework.
Types of FIRE
Lean FIRE: Retire on a very modest income — typically below $40,000/year. Requires the smallest portfolio but demands frugality indefinitely. Works well in low cost-of-living areas or for people with genuinely simple lifestyles.
Fat FIRE: Retire with a comfortable spending level — $80,000–$150,000+/year. Requires a larger portfolio but allows a more conventional lifestyle.
Barista FIRE / Coast FIRE: Achieve enough invested assets that compound growth alone will reach your retirement target by traditional retirement age — no more contributions needed. Work a low-stress part-time job for current expenses without needing to save further.
What Most FIRE Guides Leave Out
Healthcare before Medicare (65). This is the biggest underestimated expense for early retirees in the US. Before Medicare eligibility, health insurance on the ACA Marketplace is significant — potentially $500–$1,500/month for a couple. Factor this into your spending target. Low-income years in early retirement may qualify for substantial ACA subsidies if income is managed carefully.
Sequence of returns risk. Retiring at 40 with a 50-year horizon is significantly more exposed to sequence risk than a 65-year-old with 30 years. A severe market crash in the first 5 years of retirement is far more damaging than one later. Consider a 3–3.5% withdrawal rate rather than 4% for 40+ year retirements.
Roth conversion ladders. Early retirees often need to access retirement account funds before 59½ without penalty. The Roth conversion ladder — converting Traditional IRA funds to Roth 5 years before you need them — creates penalty-free access to retirement savings.