Your FIRE number is the total invested wealth required to fund the rest of your life entirely from withdrawals. The textbook answer is the 25× rule: take your annual spending, multiply by 25, and that's your number. £35,000 a year × 25 = £875,000.
The 25× shorthand exists because of the 4% rule: a Trinity Study finding that a 4% withdrawal rate sustained a 30-year retirement across virtually all historical US 30-year periods. 1 ÷ 4% = 25. They're the same thing.
That formula is fine as a starting point, and it's the one nearly every FIRE calculator on the internet stops at. But for a UK investor, it's missing three things, all of which materially change your number.
1. UK pensions are locked until 57 — you need a bridge fund
UK SIPPs and workplace pensions cannot be accessed until age 55, rising to 57 from April 2028, with longer-term plans to track 10 years before State Pension Age. If you want to retire before 57, you cannot draw a pound from your SIPP — every penny of spending between your retirement age and 57 has to come from ISAs, GIAs, or other accessible accounts. That's the bridge fund.
For a 50-year-old retiring on £35,000 a year, the bridge fund is roughly £35,000 × 7 years ≈ £245,000 in accessible accounts, on top of whatever pension pot will fund the post-57 years. Generic 25× rules ignore this entirely. The calculator above models it explicitly and tells you the ISA target separately from the SIPP target.
2. The State Pension is a hidden boost most calculators leave out
The full new State Pension is £12,548 a year for 2026/27, payable from age 67 (rising to 68 between 2044 and 2046 under current legislation). That's not nothing — it's roughly a third of a £35,000/year lifestyle, for life, inflation-linked.
In SWR terms, £12,548 of guaranteed lifetime income is equivalent to a pot of about £358,000 at a 3.5% withdrawal rate. Including it accurately can reduce your FIRE number by £200k–£300k. The standard objection is "what if it's cut?" — fair, but the answer to uncertainty is to model both scenarios, not to ignore one of them. The calculator lets you toggle between Full / Partial / None.
3. SIPP withdrawals are taxed — your gross number is bigger than your net spending
When you take money out of a SIPP after age 57, 25% is tax-free (the Pension Commencement Lump Sum, or PCLS). The remaining 75% is added to your taxable income for the year — so it stacks on top of any State Pension, rental income, dividends, or other taxable income, and is taxed at the standard UK Income Tax bands.
For 2026/27, that means:
- £0 – £12,570: tax-free (Personal Allowance)
- £12,571 – £50,270: 20% (Basic rate)
- £50,271 – £125,140: 40% (Higher rate)
- £125,141+: 45% (Additional rate)
For a SIPP retiree wanting £35,000 net per year before State Pension kicks in, the gross drawdown is roughly £38,200, not £35,000. That difference of £3,200 a year — multiplied across decades and grossed up by the SWR — translates to something like £90,000 of extra SIPP pot required just to cover tax. Calculators that ignore this give you a number that will leave you short.
Putting the three together: the real UK FIRE number
The real UK FIRE number is built from three layers, each calculated on its own terms:
The bridge fund is sized to fund you from your retirement age to 57. The SIPP pot is sized to fund you from 57 onwards, gross of tax, but reduced by the State Pension once it kicks in. The headline number is the sum of both, expressed at retirement age.
That's what the calculator above does. Two retirees with identical £35,000/year spending, both at 5% real return and 3.5% SWR, will get materially different FIRE numbers depending on whether they're retiring at 50 or 60. The 60-year-old needs no bridge, gets State Pension within 7 years, and lands at around £700k. The 50-year-old needs a 7-year bridge, longer until State Pension, and lands closer to £950k+. Same lifestyle, different geometry.
Why 4% is probably too high for UK investors
The 4% rule was built on US data — 1926–1995, mostly US large-cap equities and US Treasury bonds, 30-year retirements. Three reasons UK investors should be more cautious:
- UK equity returns have been lower than US over comparable periods.
- Sequence-of-returns risk is higher over 35–50 year retirements (typical for early FIRE) than the 30-year Trinity horizon.
- Currency exposure on globally diversified portfolios adds variance.
The current consensus among UK FIRE planners sits in the 3.25%–3.5% range for high confidence. A 3.5% rate needs a pot 14% larger than the 4% rule does, and in exchange leaves more margin against a poor run of returns. The sensitivity table above shows you all five rates side by side so you don't have to commit to one.
William Bengen, who first published the 4% rule in 1994, has revised it upward to 4.7% with international diversification and small-cap value tilts, but only for 30-year US retirements. For a UK investor planning a 35–45 year retirement, his original conservatism still applies. 3.5% is a fair default; 4% is the upper bound, not the centre.
What moves your FIRE date
Once you have your number, these change how fast the gap closes:
- Tax wrappers. Growth inside ISAs and pensions is not taxed, so more of it compounds. The allowances are £20,000 a year for ISAs and £60,000 for pensions.
- Employer pension contributions. A matched contribution adds 50–100% to the amount you pay in, and in many schemes stops if yours does.
- Platform and fund fees. A 1% fee gap compounds to roughly 25% of your final pot over 30 years.
- When contributions go in. Money paid in earlier has longer to compound. Early on, growth is small relative to contributions; the balance shifts as the pot grows.
- Track it monthly. Knowing exactly where you are accelerates decisions — which is largely why WealthR exists.