Lean FIRE optimizes for the smallest possible FIRE number, typically $25–40K annual expenses per person. Default values below reflect a typical Lean FIRE single-person scenario.
FIRE number
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Coast FIRE number
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Years to FIRE
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Age at FIRE
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Year-by-year projection
| Year | Age | Portfolio |
|---|
Why the number shrinks so fast
Lean FIRE is not a different math from regular FIRE. It is the same 4 percent rule applied to a deliberately smaller spending figure. Because your target portfolio equals annual expenses divided by your withdrawal rate, every dollar you trim from yearly spending removes 25 dollars from the finish line at a 4 percent rate. Cut your spending from $50,000 to $30,000 and the target falls from $1.25 million to $750,000. That is a $500,000 difference created by a $20,000 lifestyle decision, which is the single most powerful lever in the whole exercise.
The 4 percent figure traces back to the Trinity Study and to William Bengen's 1994 research on safe withdrawal rates. It assumes a roughly 30 year retirement and a stock-heavy portfolio. A Lean FIRE retirement that starts in your forties can run 45 years or more, so many people in this camp dial the rate down to 3.5 percent for extra cushion, which raises the same $30,000 target from $750,000 to about $857,000. This tool lets you test that tradeoff directly.
Running the default numbers
The calculator opens with a single-person Lean FIRE case: $30,000 of annual spending, a starting portfolio of $60,000, $25,000 saved each year, a 5 percent real return, and a 4 percent withdrawal rate. Here is what those inputs produce, step by step.
| Step | Figure |
|---|---|
| Annual expenses | $30,000 |
| FIRE number ($30,000 divided by 0.04) | $750,000 |
| Starting portfolio | $60,000 |
| Balance after 5 years | $221,625 |
| Balance after 10 years | $427,903 |
| Years to reach $750,000 | 16 |
| Age at Lean FIRE | 48 |
Each year the model adds the $25,000 contribution and then grows the whole balance by 5 percent. The portfolio crosses $750,000 in year 16, landing at about $751,982, which is why the tool reports a Lean FIRE age of 48 rather than the target of 50. This saver is two years ahead of plan.
The healthcare gap nobody budgets for
The most common mistake I see in Lean FIRE plans is leaving health insurance out of the annual expense figure. If you retire at 48, you have 17 years before Medicare eligibility at 65, and a bronze ACA marketplace plan for one person can run several hundred dollars a month before subsidies. The good news is that the Premium Tax Credit is keyed to your modified adjusted gross income, and a Lean FIRE household living on $30,000 of mostly long-term capital gains and Roth withdrawals can show very low taxable income. That often produces large subsidies, sometimes bringing the net premium close to zero. Build the realistic post-subsidy premium into your expenses before you trust the FIRE number, because a surprise of even $400 a month adds nearly $5,000 a year, which at a 4 percent rate means $120,000 more in your target.
Who this calculator fits
Lean FIRE works best for single people or partners with no children who can live happily in a low or moderate cost-of-living area, and for anyone willing to use geographic arbitrage. It is a poor fit for families locked into an expensive metro by jobs or schools. If a $30,000 budget feels impossibly tight, that is useful information: it usually means your honest number is closer to regular FIRE, and you should run the standard tool instead.
What return assumption should I use?
Use a real return, meaning the return after inflation. A diversified stock and bond portfolio has historically delivered roughly 5 to 7 percent real, so the 5 percent default is reasonable and slightly conservative. Entering a nominal number like 8 percent without subtracting inflation will make your timeline look shorter than it really is, because your $30,000 of expenses will keep rising with prices while your projected balance does not.
Can I retire before 59 and a half without penalties?
Yes, but you need a bridge. Tapping a traditional 401(k) or IRA before age 59 and a half normally triggers a 10 percent penalty. Lean FIRE retirees get around this with a Roth conversion ladder or with substantially equal periodic payments under IRS Section 72(t), and by keeping a few years of spending in taxable brokerage accounts and Roth contributions, which you can withdraw at any time. Plan the bridge before you quit, not after.