Retirement planning reduces to a single equation: accumulate enough assets that the income they produce covers your expenses indefinitely. The math is surprisingly simple once you understand the core formulas.
Step 1: Your FIRE Number
FIRE Number = Annual Expenses × 25 This is the inverse of the 4% rule: 1/0.04 = 25. If you spend $50,000/year, you need $1,250,000.
Step 2: The 4% Rule
The 4% rule comes from the Trinity Study (1998, updated multiple times). Researchers tested every 30-year period from 1926-1995 and found that withdrawing 4% of a 60/40 stock/bond portfolio in year one, then adjusting for inflation, succeeded in over 95% of historical periods. The rule provides a practical, research-backed withdrawal rate.
| Withdrawal Rate | Multiplier | 30-Year Success Rate |
|---|---|---|
| 3% | 33× | ~100% |
| 3.5% | 28.6× | ~98% |
| 4% | 25× | ~95% |
| 4.5% | 22.2× | ~82% |
| 5% | 20× | ~72% |
Step 3: Time to Retirement
Your savings rate is the primary determinant of how long it takes to retire. At a 50% savings rate, you can retire in approximately 17 years regardless of income level. At 20%, it takes about 37 years. Model your specific scenario with the FIRE Calculator.
Calculate your FIRE number with Monte Carlo simulation
Try the FIRE Calculator →Key Takeaways
- FIRE Number = Annual Expenses × 25 based on the 4% safe withdrawal rate.
- The 4% rule has a 95%+ success rate over 30-year periods historically.
- Savings rate determines your timeline more than investment returns or income level.
- Consider a 3.5% withdrawal rate for early retirees with 40-50 year horizons.