How to Calculate Your FIRE Number in the UK

What Is a FIRE Number?

Your FIRE number is the size of investment portfolio you need to retire permanently and never run out of money. Once your portfolio reaches this number, your investments generate enough income to cover your living costs indefinitely — without ever needing to earn a salary again.

The concept comes from the 4% rule, which originated with financial planner William Bengen in 1994 and was later confirmed by the Trinity Study. The research found that a diversified portfolio of shares and bonds could sustain annual withdrawals of 4% of its starting value — adjusted for inflation each year — for at least 30 years across virtually all historical market conditions.

From that single insight, the formula follows directly:

FIRE Number = Annual Retirement Spending ÷ 0.04

Or equivalently, because dividing by 0.04 is the same as multiplying by 25:

FIRE Number = Annual Retirement Spending × 25

If you want to spend £30,000 per year in retirement, your FIRE number is £750,000. If your target is £40,000 per year, it is £1,000,000. Simple — but there are important UK adjustments to make before you treat that as your final target.

Step 1 — Work Out Your Annual Retirement Spending

Before you can calculate your FIRE number, you need a realistic estimate of what you will spend each year in retirement. This is not necessarily your current spending — retirement spending is typically different in several ways:

• No mortgage payments — most FIRE pursuers aim to be mortgage-free before or at retirement
• No commuting costs — work-related expenses vanish
• No pension/ISA contributions — you stop saving once you retire
• More leisure spending — travel, hobbies, and socialising tend to increase, especially in the early retirement years
• No National Insurance contributions — self-employed NI disappears in retirement

A useful benchmark is the PLSA Retirement Living Standards, which research real UK retirement costs annually:

Source: Pensions and Lifetime Savings Association (PLSA), 2024 figures

Most UK FIRE pursuers target somewhere between Moderate and Comfortable — enough for annual holidays, eating out regularly, and a comfortable home, without needing to be extravagant. Once you own your home outright, even the Moderate standard represents a genuinely comfortable life.

Step 2 — Choose Your Safe Withdrawal Rate

The 4% rule was designed for a 30-year retirement horizon — that is, someone retiring at 65 and living to 95. If you plan to retire earlier, your money needs to last longer, which means withdrawing a smaller percentage each year.

Here are the recommended UK safe withdrawal rates by retirement age:

For a 45-year-old retiring at 50, the multiplier of 28.5 applies. For £30,000 annual spending, the basic FIRE number is £30,000 × 28.5 = £855,000 — noticeably higher than the £750,000 you would get using the simpler 4% rule.

Step 3 — Apply the UK State Pension Adjustment

This is the step that almost all US FIRE guides miss, and it significantly reduces your UK FIRE number. The full new State Pension is £11,502 per year (2025/26 rates), paid from age 67, requiring 35 qualifying National Insurance years. You can check your State Pension forecast at gov.uk/check-state-pension.

Because £11,502/year of guaranteed income is equivalent — in portfolio terms — to having an extra £287,550 invested at a 4% withdrawal rate, this is the amount you can subtract from your FIRE number once the State Pension begins:

State Pension capital equivalent = £11,502 ÷ 0.04 = £287,550

Here is how this adjustment affects FIRE numbers across different spending levels:

SP-adjusted target assumes full State Pension (£11,502/year) from age 67. The £287,550 saving is constant regardless of spending level. For a couple, each partner's SP reduces the target — potentially saving £575,100 combined.

The important caveat: if you retire well before 67, your portfolio must cover full expenses during the bridge period. Someone retiring at 50 needs to sustain full spending for 17 years before the State Pension arrives. Most UK FIRE planners either hold a slightly larger portfolio to account for this, use a more conservative withdrawal rate in the early years, or plan to draw ISA funds first (which are never taxed, regardless of amount) before shifting to SIPP income from 57.

Worked Example — A Complete UK FIRE Calculation

Let us work through a real example. Alex is 34, earns a good salary, and wants to retire at 52. Here is the calculation:

1. Annual retirement spending

Alex wants to spend £32,000/year. The mortgage will be paid off by retirement, and this covers everything: holidays, dining out, hobbies, utilities, and a new car every decade.

2. Choose a withdrawal rate

Retiring at 52 means a 40+ year retirement horizon. Alex uses 3.5% (multiplier of 28.5).

Basic FIRE number = £32,000 × 28.5 = £912,000

3. Apply State Pension adjustment

Alex expects a full State Pension from age 67, having started NI contributions at 22. After 67, the portfolio only needs to generate £32,000 − £11,502 = £20,498/year.

However, Alex needs to fund 15 years (age 52 to 67) at full cost from the portfolio before SP begins. To keep the maths practical, Alex uses a slightly larger buffer than the pure SP-adjusted figure:

Working FIRE target: £912,000 (no SP reduction applied — conservative approach for 15-year bridge)

4. How long to get there?

Alex already has £95,000 invested across a Stocks and Shares ISA and a workplace pension. With 18 years until retirement at 52, and saving £2,000/month into index funds, assuming 6% real annual returns:

Projected portfolio at 52: approximately £960,000 — on track.

Use the UK FIRE Number Calculator to run your own version of this calculation with State Pension toggle included.

How ISA and SIPP Factor Into Your FIRE Number

Your FIRE number is a total portfolio target — it includes every investment account you hold: ISA, SIPP, GIA, and any other invested assets. But how you draw from these accounts matters enormously for tax efficiency in early retirement.

ISA — the early retirement workhorse

Stocks and Shares ISA withdrawals are completely tax-free, at any age, at any amount. There are no age restrictions and no reporting requirements. This makes the ISA the ideal vehicle for funding retirement between leaving work and age 57 (when your SIPP becomes accessible). Maximise the £20,000 annual ISA allowance every year you can — it is the single best tool UK early retirees have.

SIPP — tax-efficient accumulation, later decumulation

Your SIPP (Self-Invested Personal Pension) provides tax relief on contributions at your marginal rate — effectively a 25% to 67% uplift on money you invest, depending on your tax band. The tradeoff is accessibility: SIPP funds are locked until age 57 (rising from 55 in April 2028). Once accessible, you can take 25% as a tax-free lump sum (capped at £268,275). The remainder is withdrawn as taxable income — but you can draw up to the Personal Allowance (£12,570 in 2025/26) tax-free each year.

A common UK FIRE drawdown sequence:

1. Draw ISA funds from retirement to age 57 (zero tax)
2. From 57, take SIPP tax-free lump sum and begin SIPP drawdown up to Personal Allowance
3. From 67, State Pension supplements the SIPP drawdown

This sequencing minimises lifetime tax paid and is the reason many UK FIRE investors split accumulation equally between ISA and SIPP rather than putting everything in one account. Use our ISA vs SIPP Calculator to compare after-tax outcomes for your specific tax rate.

What Affects Your FIRE Number?

Several factors can raise or lower the number you need to target:

• Home ownership: Owning outright removes the largest single household expense — rent or mortgage. Someone who owns their home needs significantly less annual income than a renter.
• Mortgage-free date: If you plan to retire while still paying a mortgage, that payment must be factored into your annual spending figure — even if the mortgage will eventually end.
• Children: If your children will be financially dependent during any part of your retirement, factor in those costs explicitly. Many FIRE pursuers time their target around children leaving home.
• Spending flexibility: A FIRE plan with room to cut spending by 20–30% in a bad market year is far more robust than one with fixed, inflexible costs. Variable spending strategies (like guardrails or a floor-and-ceiling approach) can allow a slightly higher initial withdrawal rate.
• Inflation: The 4% rule assumes you increase withdrawals with inflation every year. UK CPI has averaged around 2–2.5% historically, though recent years have been higher. A global index fund portfolio has historically kept well ahead of inflation over 20+ year periods.
• Part-time income: Barista FIRE — retiring from your main career but covering some expenses with flexible work — can dramatically reduce your required portfolio. Even £8,000/year of part-time income reduces a £30,000 FIRE number by £200,000 (at 4%).

Frequently Asked Questions

Use Our Calculators to Work Out Your Numbers

• UK FIRE Number Calculator — calculate your personalised target with State Pension toggle and retirement age
• Safe Withdrawal Rate Calculator UK — model different withdrawal rates and portfolio survival probability
• UK Savings Rate Calculator — see how your savings rate translates to years until financial independence
• ISA vs SIPP Calculator UK — compare after-tax outcomes for your tax band and timeline
• All UK FIRE Calculators

This guide is for educational purposes only and does not constitute financial advice. Tax rules, pension access ages, and State Pension rates can change. All figures based on 2025/26 tax year. Safe withdrawal rates are based on historical data and do not guarantee future results. For advice tailored to your circumstances, consult a qualified financial adviser regulated by the FCA.