Fat FIRE calculator: lean, chubby and fat FIRE
Lean, as-is, chubby and fat FIRE side by side: four spending targets from the same savings, and the age each is reached.
Lean, as-is, chubby, or fat?
Four numbers, four retirements. Nothing is saved. Assumes a 6% annual return, compounded monthly, and a withdrawal rate that drops the earlier you retire.
| Retirement | FI number | Reached at |
|---|---|---|
| Lean $2,800/mo — 70% of today | $974,400 | age 49 |
| As you live now $4,000/mo | $1,392,000 | age 54 |
| Chubby $5,000/mo — 125% of today | $1,620,000 | age 56 |
| Fat $6,000/mo — 150% of today | $1,944,000 | age 58 |
What lean, chubby and fat FIRE are
Every FIRE number is a multiple of spending, so the fastest way to change it is to change the spending it is built on. Lean FIRE plans a retirement that costs less than your life does today. Fat FIRE plans one that costs more. Chubby FIRE sits between your current life and fat FIRE. This Fat FIRE calculator puts all four side by side: lean at 70% of today's spending, as you live now at 100%, chubby at 125% and fat at 150%.
How to use this calculator
Enter your age, what a month costs today, what you have invested and what you add each month. The calculator runs the same projection four times, changing only the spending the retirement has to pay for, at 6% a year. Each row shows that retirement's FIRE number and the age your savings reach it. It runs in your browser and saves nothing.
A worked example
Say you are 34, spend $4,000 a month, have $150,000 invested and add $2,000 a month.
| Retirement | Monthly spending | FIRE number | Reached at |
|---|---|---|---|
| Lean (70%) | $2,800 | $974,400 | age 49 |
| As you live now (100%) | $4,000 | $1,392,000 | age 54 |
| Chubby (125%) | $5,000 | $1,620,000 | age 56 |
| Fat (150%) | $6,000 | $1,944,000 | age 58 |
How to read the result
Compare the ages, not just the numbers. In the example, going from lean to fat adds 9 years, while the FIRE number goes from $974,400 to $1,944,000. The FIRE number at each age already includes the longer-retirement adjustment: an earlier retirement uses a bigger multiple of its spending, so lean gets part of its advantage back and fat gets a small discount.
Common mistakes
- Guessing today's spending. Every row scales from it, so an error in that one input moves all four.
- Treating the percentages as standards. They are this calculator's choices; the labels mean different things to different people.
- Forgetting that lean assumes you can live on less. It plans a retirement budget; it does not cut today's.
- Comparing FIRE numbers at different ages as if the multiple were the same. It is not, and the table accounts for that.
- Leaving taxes and healthcare out of spending. The calculator only uses the figure you enter.
Frequently asked questions
- What is chubby FIRE?
- Chubby FIRE is a retirement that costs more than your life does today but less than fat FIRE. This calculator sets it at 125% of today's spending. At $4,000 a month that is $5,000 a month and a FIRE number of $1,620,000.
- What is fat FIRE?
- Fat FIRE is retiring on a generous budget, with room for travel or a second home. This calculator sets it at 150% of today's spending: $6,000 a month at $4,000 today, which needs $1,944,000.
- What is lean FIRE?
- Lean FIRE is retiring on less than you spend now. This calculator sets it at 70% of today's spending, $2,800 a month in the example, which needs $974,400.
- How much do you need for fat FIRE?
- A fat FIRE number is a multiple of the generous spending you choose. If you spend $4,000 a month now and plan on $6,000, the number is $1,944,000, reached at age 58 in the example.
- Why are the dates not evenly spaced?
- Two things. Your balance grows on its own growth as well as your contributions, so it builds faster the larger it gets. And the multiple steps down with age: in the example the as-you-live-now plan lands at 54 on 29× a year of spending and chubby lands at 56 on 27×, as described under How to read the result. The ages are lean 49, as you live now 54, chubby 56, fat 58.
The calculator starts from a typed spending figure. Reign can start from your real accounts. See this with your real numbers
The idea behind it: Lean FIRE vs fat FIRE: same savings, different dates