Coast, Barista, Lean, Fat: the FIRE taxonomy and the math behind each

The FIRE movement started with one idea — save aggressively, retire decades early — and promptly discovered that one size fits almost nobody. The result is a taxonomy: Lean, Fat, Coast, and Barista FIRE each rearrange the same three levers (spending, savings, and work) into genuinely different life plans. The labels matter less than the math, and the math is worth knowing even if you never retire early, because Coast FIRE in particular quietly applies to millions of ordinary savers.

The base math all flavors share

Everything starts from the 25x rule: financial independence is roughly annual spending × 25 (a 4% withdrawal rate), with cautious planners using 28–33x for retirements longer than 30 years. The flavors differ in what number gets multiplied and whether work income keeps flowing.

• Lean FIRE: full retirement on a minimalist budget — typically $25k–$40k/year of spending, so a target of roughly $625k–$1M. Fastest to reach; thinnest margin for error.
• Fat FIRE: full retirement with no lifestyle compromise — $100k+/year of spending, so $2.5M–$5M+. Slowest to reach; most robust.
• Regular FIRE sits between: $40k–$100k spending, $1M–$2.5M target.
• Coast FIRE: you've saved enough that compounding ALONE will fund a normal-age retirement — you stop saving, keep working just to cover current bills.
• Barista FIRE: you retire from your career but work part-time for cash flow and (crucially, in America) health insurance, so your portfolio only fills the gap.

Coast FIRE: the most useful math most people never run

The Coast number asks: how much do I need invested TODAY so that growth alone reaches my retirement target by 65, with zero further contributions? The formula: target ÷ (1 + real return)^years remaining. At a 5% real return, money doubles about every 14 years — so a 35-year-old needs only about a quarter of their final number already banked.

The failure modes, by flavor

• Lean FIRE: no slack. A divorce, a chronic illness, or a decade of bad returns forces you back to work after years out of the workforce — at a steep earnings discount. Health insurance (ACA subsidies) is the load-bearing wall of the whole plan.
• Fat FIRE: lifestyle creep moves the goalposts — every spending increase adds 25x to the target — and the last few million often costs prime years traded for money you'll never spend.
• Coast FIRE: the projection IS the plan. Coasting from 35 to 65 is a 30-year bet on your assumed return; a 4% vs. 6% real return assumption changes the coast number by more than double. Coast on conservative math, and keep getting any employer match anyway.
• Barista FIRE: assumes part-time work with benefits stays available and tolerable for years — a real bet on your health, the job market, and employer policy.

Picking your flavor (or your blend)

1. Compute three numbers tonight: your 25x target, your Coast number for age 65, and your gap-based Barista number. Each takes one line of arithmetic.
2. If work is the problem, test a downshift (Coast/Barista) before full retirement — it's reversible and cheaper.
3. If your job is fine but you want security, aim for Coast first, then decide whether the rest of the climb to full FIRE is worth it.
4. Use conservative assumptions for anything you'll coast on for 20+ years: 4–5% real returns, 30x+ multiples for retirements starting before 50.
5. Reassess at every major life change — the flavors are waypoints on one spectrum, and most people migrate between them.

The flavors at a glance

The coast discount, by age

Because the coast number is just the final target discounted by compounding, age is everything. At a 5% real return, a 25-year-old needs only about 14% of their final number already invested to coast to 65; a 35-year-old needs about 23%; a 45-year-old about 38%; a 55-year-old about 61%. Read those numbers the optimistic way: a 28-year-old with $120,000 saved toward a $1.5 million target is not 8% of the way there — measured in coast terms, they're more than half done, because time will contribute more than they ever did. Read them the sobering way too: starting at 50 means compounding contributes little, and the target must be met almost dollar for dollar with savings. Both readings argue for the same thing — violent front-loading in the first decade of a career, when every dollar is worth four.

However you label the plan, revisit the numbers annually — spending drifts, returns surprise, and the flavor that fit at 32 rarely fits unmodified at 45.

The bottom line

The FIRE flavors are one equation wearing four outfits: spending × 25, adjusted for how much work income sticks around. Lean trades comfort for speed, Fat trades years for robustness, Coast weaponizes early compounding, and Barista splits the difference with a name tag. Run all three numbers — full FIRE, Coast, Barista — and you'll usually discover you're closer to one of them than you thought. That knowledge changes careers even when it never ends one.