Lean FIRE vs Fat FIRE Calculator

FIRE Awareness · free calculator · runs in your browser

Lean FIRE, Fat FIRE, and the price tag between them

Same movement, two very different lifestyles. Lean FIRE is retiring early on a deliberately tight budget — the community's rough convention is under about $40,000 a year per household. Fat FIRE is early retirement without the frugality: $100,000 a year or more. Regular FIRE sits in the middle.

The labels are fuzzy; the math isn't. Both targets come from the same formula:

FIRE number = annual expenses ÷ SWR

At a 4% safe withdrawal rate, that's 25× annual spending — which means every extra $1,000 a year of retirement lifestyle costs about $25,000 of extra portfolio. That's the single most useful number in this whole debate. A $500/mo nicer lifestyle in retirement isn't a $6,000/yr question, it's a $150,000 question.

This calculator prices the gap in the currency that actually matters: years of your working life. It simulates both paths month by month — balance grows, savings land, repeat — and shows when each goal falls.

A worked example

Take $100,000 invested and $3,000/mo saved at a 7% real return. A lean budget of $2,500/mo ($30,000/yr) needs $750,000 at 4% SWR and arrives in roughly 10½ years. A fat budget of $8,000/mo ($96,000/yr) needs $2.4 million and takes about 22 years. The upgrade costs ~12 extra working years — a fair trade for some, unthinkable for others. There's no right answer; there's just an honest price tag.

Many people land on a hybrid: retire when the lean number is hit, keep some enjoyable part-time income, and let the portfolio grow toward fat in the background.

Honest caveats

• Inflation. The 7% default is a real (after-inflation) return, so keep both budgets in today's dollars. A nominal return here would flatter both dates.
• Lean budgets have less slack. A rent shock or health event eats a bigger share of a small budget. Lean plans depend on flexibility — earning a little, moving, or cutting in bad years. Margin for error is precisely what Fat FIRE buys.
• Sequence-of-returns risk. A bad market early in retirement hits both plans, but the lean plan has fewer levers left to pull.
• The 4% rule assumes ~30-year retirements. For a 50-year horizon, many planners use 3.25–3.75% — lower the SWR input and both numbers rise.
• Taxes and healthcare aren't modeled — bake them into the expense figures. This is a planning estimate, not financial advice.

FAQ

What counts as lean vs fat? Roughly: lean under ~$40k/yr per household, fat at $100k/yr+. But your numbers are the ones that matter — that's why both budgets are inputs.

How much longer does Fat FIRE take? Whatever the calculator says for your savings rate. In the example above, tripling the budget costs about 12 extra years.

Is Lean FIRE risky? It carries less margin for error, so it leans on flexibility. If you can't imagine cutting spending or earning a little in a bad decade, aim higher than lean.

More free calculators

• FIRE Calculator — your FIRE number and projected retire-early date.
• Coast FIRE Calculator — the amount you need invested today so growth alone finishes the job.