- A portfolio funds pre-tax withdrawals, so a fat target has to be grossed up before the withdrawal rate is applied.
- At a 25% effective rate, ignoring tax understates the target by a third of it — over $1.9M on a $200,000 lifestyle.
- Fat FIRE usually means an early and therefore long retirement, which argues for 3–3.5% rather than 4%.
- Capital scales linearly with lifestyle, but the working years to build it do not — the last increment is the most expensive.
How the Fat FIRE calculator works
Fat FIRE is financial independence without cutting the lifestyle back. There is no threshold that makes a plan "fat" — the label just means the target is built from a comfortable budget rather than a minimal one, and that pushes the portfolio into a range where two assumptions stop being safe to ignore.
The first is tax. Every other FIRE calculator divides spending by a withdrawal rate and stops. That works at $40,000 of spending, where tax on withdrawals is small; at $200,000 it is a large and entirely predictable cost. This page grosses the withdrawal up first, so the target funds what you actually want to spend after tax.
The second is the withdrawal rate itself. A fat retirement is usually an early one, and an early one is a long one. That is why this calculator offers 3%, 3.25% and 3.5% rather than 4% — the difference between 3.5% and 4% on a large portfolio is worth more than a year of most people’s income.
The math
Pre-tax withdrawal = after-tax lifestyle ÷ (1 − effective tax rate). Fat FIRE number = pre-tax withdrawal ÷ withdrawal rate. Both steps matter: skipping the first understates the target by exactly the tax fraction, compounded by the withdrawal-rate divisor.
The effective tax rate is a single blended figure you supply, not a bracket calculation. That is deliberate — a real drawdown plan mixes pre-tax, Roth and taxable accounts, and long-term capital gains and qualified dividends are taxed on their own schedule. Use the /tools/income-tax calculator on the pre-tax withdrawal figure if you want a sharper estimate, then come back and enter the resulting average rate.
The 4% rule you may be comparing against comes from the Trinity study, which tested historical US market data over 30-year retirements. It is a rule of thumb rather than a guarantee, and it was never tested on the 40- or 50-year horizons a Fat FIRE retirement often implies — which is the reason this page’s rates stop at 3.5%.
Worked example
A $200,000 after-tax lifestyle at a 25% effective rate needs a pre-tax withdrawal of about $266,667. At a 3.5% withdrawal rate that is a Fat FIRE number of roughly $7,619,000.
Ignoring tax entirely would have produced $5,714,000 — the omission understates the target by about $1,905,000, which is more than most complete FIRE plans. Starting from $750,000 invested and adding $100,000 a year at a 5% real return, the corrected target takes about 25.7 years, so getting the tax wrong here is not a rounding error; it is roughly a decade of working life either way.
Key terms
- Fat FIRE
- Financial independence funded at a comfortable or high level of spending, rather than a minimal one. No official threshold defines it.
- Effective tax rate
- Total tax paid divided by total income — the blended average across all brackets, which is always lower than your marginal rate.
- Gross-up
- Converting an after-tax amount into the pre-tax amount that produces it: after-tax ÷ (1 − tax rate).
- Safe withdrawal rate
- The share of a portfolio drawn each year, inflation-adjusted, that historically survived a retirement of a given length. Longer horizons imply lower rates.