FIRE calculator UK: your FIRE number, and when you can actually retire early.

What is a FIRE number?

FIRE stands for financial independence, retire early. Your FIRE number is the size of the invested pot that would let you stop working and live off it indefinitely, without a salary.

The standard method is the 25× rule: take the annual spending you want in retirement and multiply by 25. Want £40,000 a year? Your FIRE number is £1,000,000. The multiplier is just the 4% rule turned upside down — if you can safely withdraw 4% of a pot each year, you need 25 times your spending to generate it.

It is a genuinely useful first sketch. Two minutes of arithmetic tells you whether you are looking at a decade of saving or three. But it rests on assumptions that do not survive contact with the UK tax and pension system, and the gap between the sketch and the real answer is usually measured in hundreds of thousands of pounds.

The 4% rule, and why it needs adjusting in the UK

The 4% rule comes from work by the American financial adviser William Bengen, published in 1994, and the Trinity Study that followed in 1998. Both asked the same question: across historical market data, what withdrawal rate would have survived a 30-year retirement without the money running out? The answer, roughly, was 4% of the starting pot, rising with inflation each year.

It is a solid piece of research. It is also built on US market history, US inflation, and a 30-year retirement. If you are retiring early in the UK, at least three of those assumptions are doing work they were not designed for.

Your retirement is longer than 30 years

Retire at 50 and you may need the money to last 40 or 45 years, not 30. A withdrawal rate that survived almost every 30-year window historically has a noticeably worse record over 45. This is why many people planning early retirement work with something closer to 3% to 3.5% — which raises the multiplier from 25× to somewhere between 28× and 33×. On a £40,000 spending target, that is the difference between a £1m pot and a £1.2m to £1.3m pot.

The rule describes gross withdrawals, not money in your pocket

This is the one that catches most people. If you withdraw £40,000 from a pension, you do not receive £40,000. Pension withdrawals are taxable income. ISA withdrawals are not. So "4% of the pot" and "what I can actually spend" are different figures, and the difference depends entirely on which pot the money comes out of.

Sequence of returns matters more when you retire early

A poor run of markets in the first few years of retirement does far more damage than the same run twenty years in, because you are selling units to live on while prices are down. The longer your retirement, the more first-decade exposure you have. A single flat percentage cannot express that risk, which is why modelling year by year tells you more than any multiplier.

Tax quietly makes your FIRE number bigger

Here is the arithmetic the 25× shortcut skips. Suppose you want £40,000 a year to live on, and it is all coming from a pension.

In 2026/27 the personal allowance is £12,570, and income above that is taxed at 20% up to £50,270. To end up with £40,000 in your hand, you need to withdraw roughly £46,900 gross: the first £12,570 arrives untaxed, and the remaining £27,430 you need has to be grossed up at the basic rate.

Run 25× on the real figure and your FIRE number is about £1,172,000, not £1,000,000. The shortcut just quietly understated your target by around £172,000 — several years of saving for most households.

Two people are cheaper than one

If you are planning as a couple, each of you has your own personal allowance and your own basic-rate band. Splitting that same £40,000 as £20,000 each needs about £43,700 gross between you, against £46,900 for one person taking the lot. Over a full retirement that difference compounds into a materially smaller pot. Any calculator that models one anonymous individual cannot show you this.

The 60% band on the way in

While you are still accumulating, there is a trap between £100,000 and £125,140 of income where the personal allowance is withdrawn at £1 for every £2 earned. The effective marginal rate in that band is about 60%. For higher earners chasing FIRE, pension contributions or salary sacrifice across that band are unusually efficient — which is a tax fact, not a recommendation, and worth putting to an adviser.

The bridge: the years your pension cannot pay for

This is the single biggest thing a generic FIRE calculator misses, and for early retirement it is often decisive.

You cannot touch a private or workplace pension whenever you like. The normal minimum pension age is currently 55, and rises to 57 on 6 April 2028. Some people have a protected lower age, and members of the firefighters', police and armed forces schemes are not affected, but for most savers 57 is the number to plan around.

So if you want to stop working at 50, your pension — frequently the largest thing you own — is unavailable for seven years. Those seven years have to be funded entirely from ISAs, general investment accounts, cash, or other income. Treating your wealth as one big pot hides this completely.

Planning the bridge usually means holding more outside a pension than pure tax efficiency would suggest. The ISA allowance is £20,000 per person per tax year, and it is use-it-or-lose-it — you cannot go back and fill in earlier years. For anyone targeting retirement well before 57, ISA capacity in the years beforehand is the constraint that quietly sets the earliest possible date.

Where the Lifetime ISA fits

A Lifetime ISA takes up to £4,000 a year (inside the £20,000 overall ISA allowance) and the government adds a 25% bonus, up to £1,000 a year. You have to make your first payment before you turn 40, and you can keep paying in until 50.

The catch for FIRE planning is the withdrawal rule. Outside of buying a first home or terminal illness, you cannot take the money out without a charge until age 60, and withdrawing early costs a 25% withdrawal charge — which, because it applies to the whole amount rather than just the bonus, can leave you with less than you put in. A LISA is a fine retirement account and an awkward bridge account.

The State Pension changes the shape of the problem

Your pot does not have to cover your spending forever at the same rate. At State Pension age, a meaningful inflation-linked income switches on and the amount you need from your own savings drops.

The full new State Pension is £241.30 a week — around £12,500 a year — and you generally need 35 qualifying years of National Insurance to get the full rate. State Pension age is rising from 66 to 67 between 2026 and 2028; anyone born from 6 March 1961 onwards reaches it at 67. A further rise to 68 is legislated for 2044 to 2046, and the age is reviewed at least every five years.

For a couple both receiving the full amount, that is roughly £25,000 a year of income that your portfolio no longer has to produce. Against a £40,000 target, the State Pension is doing well over half the work from that date onwards.

This is why a flat 4% across a whole retirement is a blunt instrument. A realistic early-retirement plan usually has three distinct phases with three different withdrawal patterns:

• The bridge — retirement to 57. Everything comes from ISAs and other accessible savings. This is the heaviest-drawdown phase.
• The pension phase — 57 to State Pension age. The pension unlocks, tax-free cash becomes available, and withdrawals become a tax-planning exercise.
• The State Pension phase — onwards. A guaranteed inflation-linked floor arrives and the pressure on the portfolio eases.

Model those three phases separately and the honest answer is often that you can retire earlier than a flat-rate calculator suggests — provided the bridge holds.

Coast FIRE, Barista FIRE and the other variants

Not everyone is aiming at a full stop. Most of the named variants are about which part of the problem you are solving.

Coast FIRE is worth a moment on its own, because it is the one most often miscalculated in the UK. The usual sum — will this grow to my number by 60? — ignores the fact that a large share of most people's coasting pot is inside a pension they cannot reach until 57. You can be comfortably Coast FIRE on paper and still be unable to stop working at 52, because the accessible portion is not there. Coasting is about contributions; the bridge is about access. They are separate problems and it is worth checking both.

Your savings rate sets the timeline

One number moves your FIRE date more than investment returns do, especially early on: the percentage of your take-home pay you save. It works from both ends — a higher savings rate builds the pot faster and lowers the spending the pot has to sustain, which cuts the target itself. That double effect is why the FIRE community talks about savings rate rather than salary. The catch is that the meaningful figure is savings as a share of what you actually keep, after tax, National Insurance and pension contributions — which is exactly the number most people cannot state accurately off the top of their head.

Working out your FIRE number in this planner

This planner was built around the parts of the problem the shortcuts skip: real cumulative PAYE, National Insurance, the personal allowance taper, pension access ages, State Pension timing, and two people rather than one. It runs entirely on your own computer — nothing you type is uploaded anywhere.

Two things are worth doing deliberately once your figures are in.

Test the drawdown order. You can choose to take Cash then ISA then Pension, Pension first, or draw from all pots equally. Because ISA withdrawals are tax-free and pension withdrawals are not, the order changes your tax bill every year and can change how long the money lasts — without you saving another penny.

Look for the bridge in the chart. Watch the years between your planned retirement and 57. If the accessible pots are draining fast in that window, the constraint is bridge funding, not your total wealth, and the fix is different: more in ISAs, or a slightly later date, rather than simply more saving.

For Coast FIRE, set contributions to zero from a chosen year and see whether the projection still holds. For Barista FIRE, model reduced working days or a lower salary for a period rather than a hard stop. Neither has a dedicated preset button — you build them from the levers and scenarios, which also means you can model the messy in-between versions that real life tends to produce.

Common questions

Is 25 times my spending really enough to retire in the UK?

It is a starting sketch rather than an answer. For a 30-year retirement it has reasonable historical support; for someone stopping at 50 with 45 years ahead, many planners work closer to 28 to 33 times. It also describes gross withdrawals, so if the money is coming from a pension you will need more than the shortcut suggests to end up with the same amount to spend.

What age can I actually access my pension?

The normal minimum pension age is 55, rising to 57 on 6 April 2028. Some people hold a protected lower age, and the firefighters', police and armed forces schemes are unaffected. The State Pension is separate and starts later — 67 for anyone born from 6 March 1961 onwards.

Should I save into a pension or an ISA for early retirement?

That depends on your tax position and, critically, on when you want to stop. Pensions are usually more tax-efficient going in but are locked until 57; ISAs are less generous on the way in but are accessible at any age, which is what funds the bridge years. Most early-retirement plans need both, and the balance between them is a genuine advice question rather than a calculation.

Does the State Pension mean I need a smaller pot?

Usually yes, for the years after it starts. The full new State Pension is around £12,500 a year per person, so a couple both receiving it have roughly £25,000 a year that the portfolio no longer needs to produce. It does nothing for the earlier years, which is where early retirement is won or lost.

Can I model Coast FIRE or Barista FIRE here?

Yes, though not through a labelled button. Coast FIRE is contributions set to zero from a chosen point; Barista FIRE is reduced working days or a lower salary for a period. Both are built from the scenario levers.

Why not just use a free online FIRE calculator?

Use one for the first sketch — they are quick and the number is directionally useful. The gap opens up when the details start to matter: cumulative PAYE, the personal allowance taper, two people with two allowances, pension access ages, State Pension timing, and which pot you draw from first. Those are the details that decide whether you can retire at 52 or 56.

What a calculator cannot decide for you

A model can show you the shape of the numbers. It cannot tell you how much investment risk is appropriate for you, whether a particular pension transfer is suitable, whether your investment mix is right, or whether retiring early fits your health, family and working life. Long-term projections are estimates, and every one of them is only as good as the assumptions underneath it.

Financial independence decisions tend to be significant, tax-sensitive and hard to reverse. Before acting on major pension, tax, investment or estate decisions, check that whoever is advising you is appropriately authorised — the Financial Conduct Authority register is the place to do that.

Walking into that conversation already knowing your earliest realistic date, where your bridge years are funded from, and which lever moves your date most is worth considerably more than walking in with a question.