The formula
How to calculate lean FIRE
Lean FIRE is financial independence on a deliberately minimal budget — typically under £25,000 a year for a household. The maths is identical to standard FIRE; what changes is the spending figure you feed it.
Because lean budgets have less slack, most people pair them with a lower withdrawal rate. There is no discretionary spending left to cut when markets fall, so the buffer has to be built into the rate instead.
The inputs, one by one:
- Essential monthly spending — housing, food, utilities, transport, insurance — the stripped-back budget
- Withdrawal rate (%) — lean plans usually assume a longer retirement, so a lower rate
- Invested today
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
Worked example
A concrete run-through, using the values already in the fields:
- Essential monthly spending: 1,800
- Withdrawal rate: 3.5 %
- Invested today: 200,000
That gives:
- Lean FIRE number: 617,142.86
- Progress: 32.41 %
- Annual spending covered: 21,600
Reading the result
The advantage is speed: a target of £600,000 instead of £1,000,000 can be a decade earlier. The cost is fragility — a lean plan has no room for a new roof, a health problem or a change of mind about the lifestyle.
Where this goes wrong. Budgeting from a good year. Housing costs rise, cars need replacing and insurance premiums climb, and a lean budget built on the current year with no capital replacement line will not hold for thirty.
Most people using the term mean annual household spending somewhere between £18,000 and £25,000, which at 3.5% implies a portfolio of roughly £510,000 to £715,000. It depends heavily on whether housing is already paid for.
It carries more risk than a full plan, because there is nothing left to cut when something goes wrong. Most people who retire lean keep some earning capacity available, which is effectively a Barista FIRE arrangement.
It returns lean FIRE number. With 1,800 essential monthly spending, 3.5 % withdrawal rate and 200,000 invested today, that comes to 617,142.86. Change any field and the figure moves with it.