- Lean FIRE is the same formula on a smaller budget, so the entire exercise is deciding which expenses genuinely survive.
- Every recurring expense is an asset: at 4%, $100 a month of anything is $30,000 of portfolio you have to own.
- Housing is usually half the capital requirement, which makes it the only line worth optimising hard.
- A lean portfolio has the least room to absorb a bad sequence of returns — that is an argument for a lower withdrawal rate, not a higher one.
How the Lean FIRE calculator works
Lean FIRE is financial independence on a deliberately small budget. The target is not smaller because the maths is different — it is smaller because the spending is, and the whole exercise is deciding what spending you would genuinely keep.
So this calculator does not ask for one spending figure. It asks for five: housing, food, transport, insurance and health, and everything else. Adding them up gives an annual essential figure, and dividing by your withdrawal rate gives the Lean FIRE number.
The useful output is the per-line one. Every recurring expense is really an asset you have to own: at a 4% withdrawal rate, $1,200 a month of housing is $360,000 of portfolio. Seeing housing priced at $360,000 next to insurance at $60,000 tells you where the leverage is far more clearly than a monthly budget does.
The math
Annual essential spending = the five monthly inputs summed and multiplied by 12. Lean FIRE number = that annual figure ÷ the withdrawal rate. Each row’s capital requirement is that row’s annual cost ÷ the same rate, so the rows sum exactly to the total.
The comparison against your current spending uses the identical formula on a bigger number, which is why the two timelines differ so much: the target scales linearly with spending, but the years to reach it do not, because compounding is doing more of the work the longer you go.
The 4% withdrawal rate comes from the Trinity study, which tested historical US market data over 30-year retirements. It is a rule of thumb, not a guarantee — and it deserves more scepticism here than anywhere else on the site, because a lean budget has the least room to absorb a bad decade. There is nothing left to cut in a portfolio sized for essentials only, which is the honest argument for choosing 3% or 3.5% on this page.
Worked example
A bare-bones budget of $1,200 housing, $500 food, $300 transport, $200 insurance and $300 for everything else comes to $2,500 a month, or $30,000 a year. At a 4% withdrawal rate that is a Lean FIRE number of $750,000 — and housing alone accounts for $360,000 of it, 48% of the total.
Starting from $150,000 invested and adding $24,000 a year at a 5% real return, that target arrives in about 13.7 years. The same household spending $60,000 a year needs $1,500,000 and gets there in about 23.5 years. Living on half costs roughly 9.8 fewer working years — but it also means arriving with no margin, which is the trade the number does not show.
Key terms
- Lean FIRE
- Financial independence funded by a bare-bones budget of essential spending only, rather than by your current lifestyle.
- Essential spending
- The costs you would still have after cutting everything discretionary — housing, food, transport, insurance and a modest allowance for the rest.
- Capital required
- The portfolio slice that funds one line item forever: that line’s annual cost divided by the withdrawal rate.
- Sequence-of-returns risk
- The risk that poor returns arrive early in retirement, when withdrawals are eating into a portfolio that has not yet recovered. A lean budget has no discretionary spending to pause, so it feels this most.