Lean FIRE: the smallest number that buys your freedom.
Lean FIRE trims the lifestyle, not the arithmetic. Fewer dollars to fund each year means a smaller portfolio — and the earliest date you could realistically walk away.
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Sample result
PreviewThis household's Lean FIRE number
$840,000
Funds $33,600 a year at a 4% withdrawal rate
Reached at
Age 53
Years away
23 years
Coast number
$221.4K
Coasting by
Age 41
That's $360,000 less than this household's $1,200,000 target number and arrives about 7 years sooner — same formula, leaner lifestyle.
The sample's assumptions
- Age 30, retiring at 65
- $45,000 invested today
- $68,000 take-home pay
- Saving 30% of it ($1,700/mo)
- $48,000/yr target retirement spending
- 4% safe withdrawal rate
- 7% return, 3% inflation
Run it with your own numbers
Create a free account to enter your own spending, edit every tier, and watch all four numbers update — saved across devices.
It isn't a different formula. It's a different lifestyle.
Every tier of FIRE runs the same arithmetic: the money you expect to spend in a year, divided by your safe withdrawal rate. Change the spending and you change the number — that's the whole difference between Lean FIRE and every other flavour.
$33,600 ÷ 4% = $840,000
Lean FIRE spending ÷ safe withdrawal rate = the portfolio that funds it indefinitely.
Lean FIRE
$840K
funds $33,600/yr
Coast number today $221.4K
Your target
$1.2M
funds $48,000/yr
Coast number today $316.3K
Fat FIRE
$2.4M
funds $96,000/yr
Coast number today $632.5K
Every number here comes from the same household above — only the spending figure changes.
What counts as lean?
There's no official line. Lean FIRE usually means a deliberately frugal retirement — low housing costs, one modest car, cooking instead of eating out. ProsperMap starts you at 70% of your target spending and lets you type your own figure, because bare-bones costs wildly different amounts in different places.
Why the lean line comes first
Your FI number is spending divided by your withdrawal rate, so cutting the spending cuts the target in the same proportion. A smaller target gets crossed sooner by the very same savings — which is why lean is always the first line your portfolio touches.
Explore the other flavours
Same calculator, same formula — a different spending figure each time.
Frequently asked questions
See your own lean number.
Create a free account to run every FIRE tier on your real numbers — then learn the system behind them in Build Real Wealth.