What is a FIRE number? (UK guide)

Your FIRE number is the amount of invested capital you need to cover your annual spending without a salary. It is the simplest possible answer to the question: how much is enough? The most common shortcut is to divide your yearly outgoings by a safe withdrawal rate. A household spending £40,000 a year with a 4% withdrawal rate needs £1,000,000 invested. The same spending at 3.5% needs roughly £1,140,000.

How much do I need to retire early in the UK?

The honest answer is: it depends on how much you spend. The UK version of FIRE is not meaningfully different from the US version at the top level. You still need a portfolio large enough that a small annual withdrawal covers your lifestyle. What changes are the tax wrappers, the currency, the State Pension, and the local cost of housing.

A common rule of thumb for a 30-year retirement is the 4% rule. If you retire earlier than your mid-60s, many UK planners now prefer 3.5% or even 3%. The reason is the same everywhere: a longer drawdown period gives bad sequences of returns more time to hurt you. A lower withdrawal rate means you need a larger pot, but it also sleeps better at night.

Annual spending	3% withdrawal	3.5% withdrawal	4% withdrawal
£25,000	£833,000	£714,000	£625,000
£35,000	£1,167,000	£1,000,000	£875,000
£45,000	£1,500,000	£1,286,000	£1,125,000
£55,000	£1,833,000	£1,571,000	£1,375,000

The table shows the trade-off clearly. A couple spending £35,000 a year who are comfortable with 4% needs £875,000. If they want the extra safety of 3.5%, they need £1,000,000. There is no universal right answer. The right number depends on your age, flexibility, and how much of your spending is truly fixed.

How do you calculate your FIRE number?

The calculation is two steps. First, work out your annual spending after tax. Second, divide it by a withdrawal rate you trust.

FIRE number = annual spending / withdrawal rate

Annual spending should include everything you will still pay once you no longer work. Mortgage or rent, utilities, food, travel, subscriptions, insurance, gifts, and a margin for things that break. It should also include the tax you will still owe on withdrawals from taxable accounts. Money inside an ISA or SIPP is not tax-free on the way out in the same way, so do not double-count the tax benefits.

The withdrawal rate is the percentage of the portfolio you take in year one, then adjust for inflation in later years. The 4% rule comes from a 1994 US study by William Bengen and a later Trinity Study that tested historical US market returns. The UK has a shorter and more volatile equity history, so the rule is often treated as a starting point rather than a promise.

Our calculator runs a seeded simulation so you can see how different spending, savings, and retirement ages change the shape of the answer. It is not a forecast. It is a magnifying glass held over the assumptions.

What is a safe withdrawal rate in the UK?

A safe withdrawal rate is the one that lets your portfolio survive your planned retirement. The famous 4% was designed for a 30-year US retirement with a mixed portfolio of stocks and bonds. The UK is different in three ways.

First, UK real returns have historically been slightly lower than US real returns over long periods. Research by researchers such as Wade Pfau and others has shown that withdrawal rates that look safe in US data can look less safe in international data. A 3.5% starting point is often cited as more prudent for UK retirees.

Second, UK investors tend to hold more home-biased equity. The FTSE 100 pays a higher dividend yield than the S&P 500 but has delivered lower capital growth. A higher yield can help in drawdown, but a lower growth rate can shrink the pot over time.

Third, the pound matters. If you retire abroad, currency risk changes everything. If you stay in the UK, your spending is in pounds and your portfolio should be sensibly hedged to pounds, even if it holds global assets.

The Bank of England and the Office for National Statistics publish the inflation data that underpins any safe withdrawal conversation. The real return you earn is your nominal return minus inflation. If inflation is higher than expected, the real value of your withdrawals rises faster than planned.

What is Coast FIRE and how is it different?

Coast FIRE is the amount you need invested today so that, with no further contributions, the portfolio grows to your full FIRE number by your target retirement age. It is a halfway house that answers a different question: am I already far enough ahead that I can stop saving?

The formula is:

Coast FIRE = FIRE number / (1 + expected return) ^ (target age - current age)

If your FIRE number is £1,000,000, you are 30, and you want to retire at 50, a 6% real return means your Coast FIRE number is roughly £312,000. If you already have £312,000 invested, you could stop adding new money and still hit the target, assuming the return comes through. Most people do not actually stop contributing, but the number is useful because it shows how powerful time already is.

Our calculator shows Coast FIRE alongside your main FIRE number. It is the best single number for knowing whether you are ahead of schedule.

How do UK taxes and wrappers change the number?

The FIRE number itself is a pre-tax number. It tells you how much capital you need. The wrappers tell you how efficiently you can get there.

Wrapper	What it does	Relevance to FIRE
Stocks and Shares ISA	Grows free of income and capital gains tax	Ideal bridge between early retirement and pension age
SIPP	Pension with tax relief on the way in, tax on the way out	Best for funding life after you can access it, currently 55 rising to 57 from 2028
Lifetime ISA	25% government bonus on contributions, limited to £4,000 a year	Useful for first home or a small retirement top-up, but restrictions apply
General investment account	No tax wrapper, subject to tax on dividends and gains	Flexible, but usually the last place to put money after ISA and pension allowances

The sequence of withdrawals matters. A typical UK FIRE plan uses the ISA first, then the SIPP later. If you retire at 50, you might spend down your ISA for seven years before you can touch your pension. That means your ISA needs to be large enough to bridge the gap, even if your total FIRE number looks fine on paper.

HMRC sets the annual allowances and the access ages. The Lifetime Allowance was abolished from April 2024, but the Annual Allowance and the rules on pension access still matter. Always check the latest HMRC guidance before making large decisions.

What about inflation, property, and the State Pension?

The 4% rule already includes inflation adjustments. You withdraw 4% in year one, then increase the pound amount each year by inflation. This is why the portfolio needs real returns, not just nominal ones. The Bank of England inflation target is 2%, but actual inflation can spike, as it did in 2022 and 2023. A flexible withdrawal plan can handle spikes better than a rigid rule.

Property complicates the picture. If your home is paid off, your annual spending is lower. If you still have a mortgage, you need to include it. Some people include home equity in their net worth but not in their FIRE number, because you cannot easily spend a house without selling it or releasing equity. Our calculator focuses on invested, liquid capital for exactly this reason.

The UK State Pension is another adjustment. It starts at the State Pension age, currently 66 for most people and rising. The full new State Pension is worth a meaningful amount of guaranteed income from that age. If your FIRE age is earlier, you can model it as a reduction in spending from pension age onward. Our calculator does not currently model this directly, so treat the number it gives you as a conservative estimate if you expect State Pension income later.

How do I use the calculator?

Start with annual spending. Be honest. Use the last twelve months of bank statements, not a budget you wish were true. Then pick a withdrawal rate. The slider defaults to 4%, but you can move it to 3.5% or lower if you want more safety.

Add your current savings and your monthly contribution. The chart shows the percentile bands from a seeded simulation. The darkening right edge is the Wedge of Death, a visual reminder that your money does not need to last forever, only until you are no longer alive to spend it.

Try the scenario chips. Save a baseline, then save an aggressive version and a conservative version. Switching between them is instant. The URL encodes your state, so you can copy a link and reopen it later. Your inputs never leave your browser.

What are the limits of the FIRE number idea?

The FIRE number is a planning tool, not a guarantee. It assumes constant spending, a fixed withdrawal rate, and average market behaviour. Real life is messier. You might spend more in some years, less in others. You might have children, change country, or face a health event. You might simply decide you enjoy work more than you expected.

The simulation behind the calculator uses fixed return assumptions. It does not predict the future. It tells you how the plan would have behaved under those assumptions. Use it to stress-test your spending and savings, not to decide your life.

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Last updated: 2026-08-02. Reviewed by Glenn Rodgers. This guide is educational and is not financial advice. Please speak to a qualified adviser before making investment or retirement decisions.