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.
Where a figure is not immediately to hand — a precise interest rate, an exact balance — a reasonable estimate is a perfectly good starting point. Because every result updates instantly, refining a rough guess into the real figure once you have it takes a moment, and nothing about the calculation depends on getting it exactly right on the first attempt.
Why fat FIRE matters
FIRE stands for Financial Independence, Retire Early — reaching a portfolio large enough that investment income covers your living costs, so paid work becomes a choice rather than a necessity. It is not a specific account or product, just the point at which the figures below cross over.
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind fat FIRE is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
In practice, most people arrive at a page like this one having already tried a version of the calculation by hand or in a spreadsheet, and use the calculator here to confirm it rather than replace it. That is a reasonable way to use it — the two should agree to the last decimal place if the same inputs and the same formula are used, and if they do not, the formula shown above is the one to check your own working against first.
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
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
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.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
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.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.