The FIRE movement — Financial Independence, Retire Early — isn't about never working again. It's about having enough invested that work becomes optional. The core math is simple: save 25× your annual expenses, then withdraw 4% per year, adjusted for inflation. Let's break down exactly what that means for your numbers.
The FIRE Formula
The 4% rule comes from the Trinity Study (1998, updated 2018): historically, withdrawing 4% of a balanced portfolio (60% stocks, 40% bonds) per year, adjusted for inflation, has survived 30+ years in 95% of historical scenarios. For a 40-50 year retirement (early retirees), 3.5% might be safer.
Which Flavor of FIRE Fits You?
| FIRE Type | Annual Expenses | FIRE Number (25×) | Monthly Withdrawal (4%) |
|---|---|---|---|
| Lean FIRE | $30,000 | $750,000 | $2,500/month |
| Regular FIRE | $50,000 | $1,250,000 | $4,167/month |
| Fat FIRE | $100,000 | $2,500,000 | $8,333/month |
Lean FIRE means frugal living — paid-off house, minimal expenses. It's achievable for many middle-income earners. Regular FIRE is the mainstream target. Fat FIRE means business-class flights and nice restaurants. Most people land somewhere between Lean and Regular.
How Much to Save Monthly to FIRE by 45
Assuming you start from zero, invest at 8% annual return, and want to reach your FIRE number by age 45:
| Start Age | Years to 45 | Monthly for Lean ($750K) | Monthly for Regular ($1.25M) | Monthly for Fat ($2.5M) |
|---|---|---|---|---|
| 25 | 20 years | $1,270/month | $2,120/month | $4,240/month |
| 30 | 15 years | $2,170/month | $3,620/month | $7,240/month |
| 35 | 10 years | $4,100/month | $6,830/month | $13,670/month |
The power of starting at 25: you need roughly half the monthly savings of starting at 30, and one-third of starting at 35. Every decade of delay roughly doubles the required monthly contribution.
Coast FIRE: The Middle Path
Coast FIRE means you've already saved enough that, without contributing another dollar, your investments will compound to your FIRE number by retirement age. You still work to cover living expenses, but you stop saving for retirement entirely.
Coast FIRE at 35 for Lean FIRE ($750K by 65)
At 8% return over 30 years, you need about $75,000 invested today. That $75K compounds to $750K with zero additional contributions.
For Regular FIRE ($1.25M): need ~$125K by 35.
For Fat FIRE ($2.5M): need ~$250K by 35.
Coast FIRE is the bridge. Hit these numbers early, then you can take a lower-paying job you love, start a business, or travel — as long as you cover your expenses, your retirement is handled.
The 4% Rule in Detail
The rule: withdraw 4% of your portfolio in year 1. Each subsequent year, adjust that dollar amount for inflation (not re-calculate 4%). Example:
Year 1: $1,250,000 × 4% = $50,000 withdrawal
Year 2: $50,000 + 3% inflation = $51,500 (even if portfolio drops to $1.1M)
Year 3: $51,500 + 3% inflation = $53,045
Critically, you don't recalculate 4% of the current balance each year. You lock in the year-1 dollar amount and inflation-adjust it. This prevents spending spirals when markets are up — and forces belt-tightening when markets are down (but only to the extent of inflation, not market drops).
Sequence of Returns Risk
The biggest threat to early retirement: a market crash in the first 5 years. If you retire with $1.25M and the market drops 30% in year 1, your portfolio is $875K — and you're still withdrawing $50K/year.
| Scenario | Portfolio at Year 1 | Portfolio at Year 10 | Success? |
|---|---|---|---|
| Normal returns (8% avg) | $1,250,000 | $1,420,000 | ✅ Growing |
| −30% crash in Year 1, then 8% | $875,000 | $610,000 | ⚠️ Dangerous |
Protection strategies: Keep 2-3 years of expenses in cash/bonds when you retire. In a crash year, withdraw from cash instead of selling stocks at the bottom. This "bucket strategy" dramatically reduces sequence risk.
Key Takeaways
- FIRE number = annual expenses × 25 — $50K/yr expenses means $1.25M target
- Start at 25: $2,120/month reaches Regular FIRE by 45. Start at 35: $6,830/month needed
- Coast FIRE: $125K by 35 compounds to $1.25M by 65 with zero more contributions
- The 4% rule is inflation-adjusted from year 1's dollar amount, not recalculated annually
- Keep 2-3 years of cash for the first years of retirement — protects against sequence of returns risk