Lean FIRE calculator
Lean FIRE is financial independence on a deliberately small budget: strip spending to what genuinely matters and the pot that frees you shrinks dramatically. Build the monthly budget below and see the number it implies, the age you'd reach it, and what each £100 of monthly spending is really costing you in pot and in working years.
The lean budget, per month
You and your saving
From budget to number
What £100 a month is worth
The leverage in a lean budget
Lean FIRE number = the lean budget × 12 ÷ withdrawal rate. What makes the lean route quick has less to do with heroic saving than with the multiplier hiding in that division: at 3.5%, every £1 a month of permanent spending needs £343 of pot behind it. Cut £100 a month and the target drops by £34,286 while the freed £100 flows into investments, attacking the gap from both ends at once. That twin effect is why the table on this page prices spending cuts in years rather than pounds, and why the FIRE community obsesses over recurring costs rather than one-off treats: a £5 daily habit is a £52,000 line item in pot terms.
The state quietly subsidises frugality
A pleasing UK quirk: draw a lean budget from a pension and the tax system barely notices. A quarter of each withdrawal is tax-free, and on a £16,800-a-year budget the taxable remainder of £12,600 sits almost entirely inside the £12,570 Personal Allowance, so the income tax bill rounds to nothing. Frugal retirement is close to tax-free retirement, which fattens the effective withdrawal rate compared with bigger lifestyles that lose a slice of every draw. And from 67 the State Pension covers most of a genuinely lean budget on its own, meaning the pot only has to carry the full load until then.
What the lean path costs
Honesty matters more here than on any other FIRE flavour, because the budget has no fat to absorb error. Nothing above covers a new boiler, a failing car or the year council tax jumps, and being £150 a month optimistic moves the target by over £50,000. There is also a decades-long commitment being made: the budget you retire on at 45 is the budget you live on at 75, and re-entering work after ten years away is harder than the plans assume. Treat the number as the floor, add margin deliberately, and note this page treats your pot as fully accessible: if most of it is locked in pensions until 57, the FIRE calculator tests that bridge, the Barista FIRE calculator prices a part-time cushion, and the retirement age calculator runs the whole plan with the State Pension included.
Common questions
What is Lean FIRE and how much do I need?
Financial independence on a deliberately minimal budget, typically £20,000 a year or less in the UK. The maths is the standard formula on a small input: £16,800 a year at a 3.5% withdrawal rate needs £480,000, roughly half the target of a £30,000 lifestyle. The trade is a permanently tight budget for a dramatically earlier date.
How much does cutting £100 a month change the target?
Twice over. At a 3.5% withdrawal rate, £100 a month of permanent spending needs £34,286 of pot behind it, so the target falls by that much — and the freed £100 becomes saving, closing the gap faster. On typical numbers a £100 monthly cut brings the date forward by around three years.
Is a lean retirement really almost tax-free?
Close to it, if drawn from a pension. A quarter of each withdrawal is tax-free, and 75% of a £16,800 annual budget is £12,600 — almost entirely inside the £12,570 Personal Allowance, leaving a few pounds of income tax a year. Larger budgets lose a growing slice of every withdrawal, which is one reason lean targets are reached so much sooner than the raw numbers suggest.
These results are estimates for general information only and are not financial advice. A lean budget leaves little room for error — build in margin and check your plan with a qualified adviser. Read the full disclaimer.