Guide · 6 min read · figures through 2024
What Fat FIRE costs
How annual spending and the withdrawal rate set the target, and why a larger target does not add years in a straight line.
A Fat FIRE target begins with planned annual spending. The withdrawal rate converts that spending into a portfolio target; current savings and future contributions determine how long historical accumulation periods took to reach it.
The number itself is one division
At a 3.5% initial withdrawal rate, divide annual spending by 3.5% to calculate the starting portfolio. The withdrawal then rises with inflation each year.
| Annual spending | At 3.0% | At 3.5% | At 4.0% |
|---|---|---|---|
| $80,000 | $2,666,667 | $2,285,714 | $2,000,000 |
| $120,000 | $4,000,000 | $3,428,571 | $3,000,000 |
| $160,000 | $5,333,333 | $4,571,429 | $4,000,000 |
| $200,000 | $6,666,667 | $5,714,286 | $5,000,000 |
| $250,000 | $8,333,333 | $7,142,857 | $6,250,000 |
| $300,000 | $10,000,000 | $8,571,429 | $7,500,000 |
Read across a row and you can see the second thing this article is about: the withdrawal rate moves the number as violently as the spending does. Going from 4.0% to 3.0% costs you exactly as much as increasing your spending by a third.
The years are the expensive part
Doubling annual spending doubles the target at a fixed withdrawal rate. It does not double the accumulation time because returns compound on a growing balance. The example below starts with $300,000, adds $100,000 a year and uses a 80/20 allocation. The chart reports the median historical time to each target.
The curve flattens because compounding is doing more of the work at the top end and less at the bottom. Early on, the portfolio grows mostly because you are feeding it; later, it grows mostly because it is large. The extra years buy disproportionate amounts of money, which is the single strongest argument for Fat FIRE over lean FIRE if you can stand the wait: the last five years of accumulation are worth more than the first fifteen.
The final working years can add more to the portfolio than the first years because both the existing balance and new contributions continue to compound.
Separate fixed and flexible spending
A $60,000 budget is mostly rent, food, insurance and transport. A $200,000 budget is those things plus a great deal that is genuinely optional: travel, a second car, private schooling, renovations, generosity. That optional layer is not indulgence in a Fat FIRE plan — it is the risk control. When the market hands you a 1966, a household that can cut $45,000 of spending for three years without anyone's life materially changing is far safer than a lean household that has nothing left to cut.
This is why the flat withdrawal rates in the textbooks are the wrong lens for Fat FIRE in both directions. The number they produce is too conservative for someone with a flexible budget, and too aggressive for someone who has built a fixed one — a big mortgage, tuition contracts, a boat — where the spending genuinely cannot move.
Apply it to a plan
- Split your budget into fixed and flexible before you pick a number. Fund the fixed part at a conservative rate. Fund the flexible part at whatever rate you would be willing to suspend in a bad decade.
- Do not let the withdrawal rate be an afterthought. It moves the target as much as the spending does, and unlike the spending, it is not obvious what the right value is. The next guide is about exactly that.
- Test the plan against the slow decades, not the median. A number you reach in the median year is a number you might reach eight years later. If that still works, compare the resulting date with the latest date you are willing to work.