How to use the Fat FIRE calculator
- Enter what you want to spend each year in retirement. The calculator starts at £80,000; change it to your own figure.
- Add what you’ve already got invested (ISAs, pensions, other investments, not cash savings) and how much you invest each month.
- Set your growth and withdrawal rate. I start at 5% and 4%. Use growth after inflation to keep the answer in today’s money.
- Enter your age to see your Fat FIRE number, how long it could take and the age you’d reach it.
What is Fat FIRE?
Fat FIRE is retiring early without cutting back. Rather than covering the basics, your pot pays for the essentials plus the good stuff: regular travel, hobbies, eating out and helping family.
There’s no official threshold. It just means a generous budget, which takes more saving and smart investing to reach. For the full breakdown, read my Fat FIRE guide.
How the maths works
Fat FIRE number = yearly spending ÷ withdrawal rate. At 4%, £80,000 a year needs £2,000,000. The calculator then grows your current pot month by month, adds your monthly investment, and counts how long until you hit the number.
Worked example
You’re 30, want to spend £80,000 a year, have £100,000 invested and add £2,500 a month, with 5% growth and a 4% withdrawal rate.
| Result | |
|---|---|
| Fat FIRE number | £2,000,000 |
| Time to get there | 26 years and 4 months |
| Age you’d reach it | 56 |
A bigger pot needs to last longer and survive more bad years, so plenty of people plan with 3.5%. That pushes the number to £2,285,714 and the timeline to 28 years and 5 months (age 58).
Growth isn’t guaranteed. Your pot will rise and fall along the way.
When you invest, your capital is at risk.
Fat FIRE and UK tax
Big pots bring bigger tax questions, so it’s worth planning where your money sits.
- Tax-free cash is capped. You can usually take up to 25% of a pension tax free, but the total is capped by the lump sum allowance of £268,275.
- Pension income is taxed. Withdrawals above your £12,570 Personal Allowance are taxed at 20%, then 40% above £50,270 in England, Wales and Northern Ireland. Go over £100,000 and you start losing your Personal Allowance.
- ISAs help. ISA withdrawals are tax free, so mixing ISA and pension income can keep more of your money in lower tax bands.
- Pension age. You usually can’t touch a pension until 55, rising to 57 on 6 April 2028. Retiring before then means living off ISAs and other investments.
What else can affect Fat FIRE?
- Market drops. A downturn early in retirement can dent a big pot and the lifestyle it pays for.
- Interest rates. Changes affect returns, especially if you hold bonds.
- Living longer. A generous lifestyle over more years needs a bigger cushion.
- Healthcare costs. Private treatment can eat into your budget for treats.
- Tax rules change. Plan for flexibility, not today’s rules forever.
Track your journey to Fat FIRE
Building a bigger pot takes better tracking. The Gains App, which I built, brings your accounts together in one place while you work towards your number.
What to do next
Most of that pot will sit in a pension and an ISA, so fees matter. I’ve compared the best pension providers and the best stocks and shares ISAs.
Fat FIRE FAQs
How much do I need for Fat FIRE in the UK?
At a 4% withdrawal rate, multiply your yearly spending by 25. £60,000 a year needs £1,500,000, £80,000 needs £2,000,000 and £100,000 needs £2,500,000.
What’s the difference between Fat FIRE and Lean FIRE?
The budget. Lean FIRE covers a simple, low cost life so you can stop sooner. Fat FIRE pays for a generous one, so it takes a much bigger pot.
Is 4% safe for Fat FIRE?
It’s a rule of thumb, not a guarantee. With a big pot and a long retirement, 3.5% gives you more room for bad years.
Should my Fat FIRE money be in a pension or an ISA?
Usually both. Pensions give you tax relief going in, while ISAs give you tax-free money coming out and can be used before pension age.
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.