Fat FIRE

A lower withdrawal rate builds in extra safety margin, and Fat FIRE practitioners tend to want more of it because they're targeting both a longer time horizon (often retiring earlier than a traditional retiree) and a higher, less flexible spending level with fewer discretionary expenses to cut during a market downturn. A leaner budget can absorb a bad market year by cutting non-essentials; a Fat FIRE budget, built around a comfortable lifestyle, has less slack to give up without meaningfully changing how you live.

About a third bigger, and it's a purely mathematical relationship: since the Fat FIRE number is Annual Spend divided by Withdrawal Rate, and 1 divided by 3% (33.3x spending) is roughly 33% larger than 1 divided by 4% (25x spending), lowering your assumed withdrawal rate by just one percentage point meaningfully raises the size of the portfolio you're targeting, even though your actual spending hasn't changed at all.

There's no single official threshold, but Fat FIRE is generally used to describe a retirement spending target well above a bare-bones or "Lean FIRE" budget -- commonly cited as $100,000 or more per year, versus the $40,000-$60,000 range often associated with Lean FIRE. The distinction is about lifestyle choice rather than a strict dollar line: Fat FIRE is about retiring without meaningfully compromising your current standard of living.

No -- Monthly Spend is calculated purely as Annual Spend divided by 12, with no withdrawal rate in the formula at all. It exists as a reference conversion so you can sanity-check whether the annual spending figure you entered actually matches your real monthly budget, not as a projection of anything related to portfolio withdrawals.