The formula
How to calculate fat FIRE
Fat FIRE is financial independence without cutting back — a retirement budget at or above your working-life standard of living. The formula is unchanged; the target simply gets much larger, and tax starts to matter.
The second output exists because large numbers far out are misleading. A £2.9 million target in today's money is roughly £4.1 million in fifteen years at 2.5% inflation, and both figures are correct depending on which money you are counting in.
Here is what each field means:
- Desired annual spending
- Withdrawal rate (%)
- Years until retirement (years)
- Inflation (%)
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
Worked example
Work through the defaults on this page:
- Desired annual spending: 100,000
- Withdrawal rate: 3.5 %
- Years until retirement: 15 years
- Inflation: 2.5 %
That gives:
- Target in today's money: 2,857,142.86
- Same target in retirement-year money: 4,137,994.76
- Monthly spending it supports: 8,333.33
Reading the result
At this scale tax drives the plan. Drawing £100,000 a year is well into higher-rate territory, so the mix of pension, ISA and taxable accounts changes the gross portfolio you need by a substantial margin.
Where this goes wrong. Lifestyle creep during accumulation. Every extra £10,000 of annual spending adds £286,000 to the target at 3.5%, so a rising budget can push the goal away faster than a rising portfolio approaches it.
There is no formal threshold, but the term is generally used for household spending above roughly £80,000–£100,000 a year, implying a portfolio somewhere north of £2.5 million at conservative withdrawal rates.
Larger portfolios usually mean earlier retirements and longer horizons, and the 4% rule was tested over 30 years. Fat FIRE plans also carry more of their assets in taxable accounts, where drag on returns is higher.
It returns target in today's money. With 100,000 desired annual spending, 3.5 % withdrawal rate and 15 years years until retirement, that comes to 2,857,142.86. Change any field and the figure moves with it.