Coast FIRE vs. Lean FIRE vs. Fat FIRE: Which Path Fits You?

"FIRE" gets treated like a single finish line, but in practice people mean pretty different things by it. Coast, Lean, and Fat FIRE aren't three separate strategies. They're the same 25x-your-expenses math from what is FIRE, just aimed at different lifestyles and different relationships with work.

Coast FIRE: let compounding finish the job

Coast FIRE means you've already saved enough that, left alone to grow, your current investments will reach your full FIRE number by a normal retirement age, without adding another dollar. From that point on you only need to earn enough to cover today's living expenses; retirement savings takes care of itself in the background.

The math is just compounding run forward with no further contributions: at a 7% average return, roughly $93,663 invested at age 30 grows to about $1,000,000 by 65 with zero additional deposits. Reach that number earlier (say by 25 instead of 30) and the principal required today is even smaller, since compounding gets more years to work.

Lean FIRE: a smaller number, reached sooner

Lean FIRE applies the same 25x math to a deliberately modest annual spending figure, often in the neighborhood of $25,000–$40,000 a year, depending on cost of living. A smaller expense number produces a smaller target portfolio, which usually means reaching independence sooner, at the cost of a tighter budget once you get there. It suits people who are already comfortable living frugally and don't expect that to change much in retirement.

Fat FIRE: a bigger number, no compromise on lifestyle

Fat FIRE is the opposite trade: it targets a retirement that maintains (or upgrades) a comfortable, unrestricted lifestyle, which means a bigger annual expense figure and therefore a bigger portfolio target, often well above the standard 25x if it also includes a safety margin. It typically takes a higher income, a higher savings rate, or more years of compounding to reach, but the payoff is not having to budget tightly once work becomes optional.

They're all the same formula

Every version starts from the same two questions: how much do you plan to spend per year, and what withdrawal rate do you trust (see the 4% rule explained)? Multiply those together and you have your number. Coast, Lean, and Fat just plug in different assumptions about spending and how much you keep contributing along the way. The underlying compounding engine never changes.

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