Lean FIRE means retiring early on a deliberately small budget. You swap a bigger pot for an earlier finish line: the less you need to live on, the sooner you can walk away from work.
How to use the Lean FIRE calculator
- Enter your yearly spending in Lean FIRE. If you think in months, multiply by 12: £1,500 a month is £18,000 a year. This one figure drives everything else.
- Add what you’ve already got invested and how much you invest each month.
- Set growth (5% to start) and withdrawal rate (4% to start). Use growth after inflation to keep the answer in today’s money.
- Enter your age to see your Lean FIRE number, how long it could take and the age you’d get there.
What counts as Lean FIRE in the UK?
There’s no official line, but most UK Lean FIRE budgets sit under about £25,000 a year, often £15,000 to £20,000 for a single person keeping housing costs low.
It’s the leanest branch of the FIRE movement: a smaller target and earlier freedom, but less room for shocks. A tight budget leaves less buffer for a bad year in the markets, a big bill or a change in circumstances, so go in with your eyes open.
How the maths works
Lean FIRE number = yearly spending ÷ withdrawal rate. At 4%, that’s 25 times your spending, so £18,000 a year means £450,000. The calculator then grows your pot month by month and counts how long until you get there.
Worked example
You’re 30, want to spend £20,000 a year, have £10,000 invested and add £500 a month, with 5% growth and a 4% withdrawal rate.
| Result | |
|---|---|
| Lean FIRE number | £500,000 |
| Time to get there | 31 years and 4 months |
| Age you’d reach it | 61 |
Trim your target to £18,000 a year and you’d get there in 29 years and 8 months, at 59. Every £1,000 less you need each year cuts £25,000 off your number.
Growth isn’t guaranteed, and markets don’t rise in a straight line.
When you invest, your capital is at risk.
Is Lean FIRE right for you?
- Spending discipline. Your number only works if your actual spending stays lean after you stop earning, not just on paper now.
- Housing costs sorted. A paid-off mortgage or genuinely low rent does more for your Lean FIRE number than almost anything else.
- Your National Insurance record. You need 35 qualifying years for the full new State Pension (£241.30 a week in 2026/27). Stopping work early can leave gaps, so check your forecast on gov.uk.
- Room to earn a little. Many Lean FIRE households top up with part-time or seasonal work. If that’s your plan, try the Barista FIRE calculator.
Track your journey to Lean FIRE
Living lean takes sharp tracking. The Gains App, which I built, brings your accounts together in one place so you can keep your plan on course.
What to do next
- Lean FIRE: the UK guide: the full numbers, portfolio strategy and UK-specific things to think about.
- What is the FIRE movement?
- FIRE calculator, Coast FIRE calculator and Fat FIRE calculator
- Best pension providers UK
Lean FIRE FAQs
How much do I need for Lean FIRE?
At a 4% withdrawal rate, 25 times your yearly spending. £15,000 a year needs £375,000, £20,000 needs £500,000 and £25,000 needs £625,000.
Can I really retire on £20,000 a year?
Plenty of people do, especially with low housing costs. The State Pension later on can take pressure off your pot, but you’ll need to cover everything yourself until State Pension age.
What withdrawal rate should I use?
Most people start at 4%. With a lean budget there’s less room to cut back in a bad year, so 3.5% gives you more margin.
What growth rate should I use?
The calculator starts at 5% and won’t go above 8%. Use growth after inflation to keep the answer in today’s money. Growth isn’t guaranteed.