Lean FIRE vs Fat FIRE Target
Calculate and compare minimal versus comfortable financial independence numbers side by side in a Google Sheets formula.
=annual_expenses / withdrawal_rate How It Works
Lean FIRE covers only essential expenses - housing, food, insurance, basic utilities. Fat FIRE includes a comfortable lifestyle with travel, dining, hobbies, and a buffer. Both use the same core formula but with different annual expense inputs. Comparing the two shows the range of portfolio sizes that could support financial independence.
Syntax
=annual_expenses / safe_withdrawal_rateUsing the commonly referenced 4% withdrawal rate:
=annual_expenses * 25Multiplying by 25 is equivalent to dividing by 0.04.
Example
| Expense Category | Lean FIRE | Fat FIRE |
|---|---|---|
| Housing | $12,000 | $24,000 |
| Food | $4,800 | $9,600 |
| Insurance | $6,000 | $6,000 |
| Utilities | $2,400 | $3,600 |
| Transport | $3,000 | $7,200 |
| Healthcare | $4,800 | $4,800 |
| Discretionary | $0 | $12,000 |
| Travel | $0 | $8,000 |
| Buffer | $0 | $4,800 |
| Annual Total | $33,000 | $80,000 |
| FIRE Number (4%) | $825,000 | $2,000,000 |
Lean FIRE Formula: =33000/0.04 = $825,000
Fat FIRE Formula: =80000/0.04 = $2,000,000
The gap between the two - $1,175,000 - represents the cost of comfort and flexibility.
Setting Up a Comparison Calculator
| A | B | C |
|---|---|---|
| Category | Lean (Monthly) | Fat (Monthly) |
| Housing | 1000 | 2000 |
| Food | 400 | 800 |
| Insurance | 500 | 500 |
| Utilities | 200 | 300 |
| Transport | 250 | 600 |
| Healthcare | 400 | 400 |
| Discretionary | 0 | 1000 |
| Travel | 0 | 667 |
| Monthly Total | =SUM(B2:B9) | =SUM(C2:C9) |
| Annual Total | =B10*12 | =C10*12 |
| FIRE Number (4%) | =B11/0.04 | =C11/0.04 |
| FIRE Number (3.5%) | =B11/0.035 | =C11/0.035 |
Variations
With Different Withdrawal Rates
A more conservative 3.5% rate increases the target:
=annual_expenses / 0.035| Withdrawal Rate | Lean FIRE | Fat FIRE |
|---|---|---|
| 4.0% | $825,000 | $2,000,000 |
| 3.5% | $943,000 | $2,286,000 |
| 3.0% | $1,100,000 | $2,667,000 |
”Regular FIRE” Middle Ground
Some people target a middle tier:
=lean_annual + (fat_annual - lean_annual) * 0.5This gives a target halfway between lean and fat.
Years to Each Target
Combine with PMT to see how long each takes:
=NPER(return_rate/12, -monthly_savings, -current_portfolio, fire_number)Coast FIRE Angle
Calculate the portfolio needed today so that growth alone reaches each target by a given age:
=fire_number / (1 + annual_return)^years_remainingPro Tips
-
Lean FIRE is a floor, not a ceiling - it’s worth knowing the minimum needed, even if Fat FIRE is the goal
-
Include healthcare - this is often the largest wild card, especially before Medicare eligibility
-
Geographic flexibility - lean FIRE in a low-cost area might feel like fat FIRE in lifestyle terms
-
Inflation adjustment - both targets will rise over time, so consider using real (inflation-adjusted) returns
-
Review annually - actual spending patterns change, so updating both numbers each year keeps them realistic
Common Errors
- Forgetting taxes: Withdrawals from pre-tax accounts (401k, traditional IRA) are taxable income - factor this into expenses
- Using current rent for a 30-year projection: Housing costs change - consider whether the mortgage will be paid off
- Ignoring healthcare: This can be $500-1,500+/month before Medicare age
- Setting lean FIRE too lean: Cutting it too close leaves no room for unexpected expenses