PayoutMath
Threshold

FIRE Calculator UK (Financial Independence, Retire Early)

FIRE = saving aggressively to reach a portfolio that supports your annual spending forever via a 4% safe withdrawal rate. £40,000/year spending → £1,000,000 FIRE number. Calculator below shows years to FIRE based on current savings, savings rate, and expected returns.

Last verified: 25 April 2026 Source: GOV.UK — ISAs (the standard UK FIRE wrapper) Next review: 25 October 2026
Inputs
Your expected yearly outgoings once retired. Lower = lower FIRE number.
Total invested across ISAs, pensions, GIAs.
Amount you invest each year. Higher = faster FIRE.
Real (above inflation). UK historical equities: ~5%. Conservative: 4%. Aggressive: 6-7%.
Standard 4% rule. Some FIRE planners use 3.5% for safety, 4.5% for leaner plans.
FIRE number —
Years to FIRE —
Monthly savings —
Detail —
Lean FIRE — £30k spending
£30k spend · £50k portfolio · £15k/year savings · 5% real return · 4% withdrawal

FIRE number: £750,000 (= £30k ÷ 4%). At £15k/year savings + 5% real return + £50k starting, FIRE in ~22.5 years. Lean FIRE is achievable for above-average savers with modest spending targets.

Standard FIRE — £40k spending
£40k spend · £100k portfolio · £25k/year savings · 5% return · 4% withdrawal

FIRE number: £1,000,000. Strong 25k/year savings rate (likely high earner with frugal lifestyle) reaches FIRE in ~18.8 years. The 4% withdrawal rule supports £40k/year spending forever from £1M.

Fat FIRE — £60k spending
£60k spend · £200k portfolio · £40k/year savings · 5% · 4%

FIRE number: £1,500,000. Higher target spending requires either higher income (£40k saving from a £60k+ post-tax income) or longer timeframe. Fat FIRE typically takes 15-25 years from a strong starting point.

FIRE — Financial Independence, Retire Early — is the practice of saving aggressively until your portfolio is large enough that a 4% annual withdrawal covers your living expenses forever. The calculator above tells you your FIRE number and how many years it takes to get there.

The 4% rule

FIRE number = annual spending ÷ 4% = annual spending × 25

Annual spending FIRE number (4%) FIRE number (3.5%)
£20k (Lean FIRE) £500,000 £571,000
£30k £750,000 £857,000
£40k (Standard) £1,000,000 £1,143,000
£60k (Fat) £1,500,000 £1,714,000
£100k (Fat-Fat) £2,500,000 £2,857,000

The maths behind years to FIRE

With starting portfolio P, annual savings S, and real return r, years to reach FIRE number F:

years = ln((S/r + F) / (S/r + P)) / ln(1+r)

The calculator handles this — useful intuition: - Higher savings rate = lower years (linear effect) - Higher returns = lower years (compound effect — more impactful long-term) - Lower spending target = double benefit (smaller F + ability to save more)

UK-specific FIRE considerations

ISAs are UK FIRE’s superpower. £20k/year contribution limit, tax-free growth, tax-free withdrawals at any age. Stocks & Shares ISAs are the standard FIRE wrapper. Build £200k+ in S&S ISA before your 40s for tax-efficient early withdrawal.

Pensions add tax-relief boost. Workplace + SIPPs get 20-45% tax relief on contributions. Locked until 57+ (rising with state pension age). Best for late-FIRE bridge — can’t access early but get massive contribution boost.

State pension provides ground floor. ~£11,500/year from age 67-68 (35 qualifying years NI). For lean FIRE, state pension can cover ~50% of expenses post-67 — meaning your FIRE number for the 30-67 age range can be smaller.

No US-style 401(k) early withdrawal penalty system. UK pensions have hard age locks (57+ from 2028). ISAs have no age restrictions. Different optimisation than US-FIRE blogs typically discuss.

Real return assumptions

UK historical equity real returns: - FTSE 100 1900-2024: ~5% real (after inflation) - Global equities (60% UK / 40% international): ~5.5% real - 60/40 stocks/bonds: ~3.5% real - All bonds: ~1-2% real

Use 5% real return as a reasonable equity-heavy assumption. 3-4% if you hold significant bond allocation. Add 1-1.5pp for nominal-rate planning (i.e. 6-7% nominal).

What this calculator doesn’t model

  • Tax wrappers (ISA vs pension vs taxable)
  • Pension access age (57+, rising)
  • State pension (typically £11.5k/year from 67-68)
  • Sequence-of-returns risk (bad early years more harmful than bad late years)
  • Healthcare/care costs (variable, increasing with age)
  • Property (most FIRE planners exclude main home from portfolio)
  • Inheritance / windfalls

For income tax planning during the accumulation phase, use the take home pay calculator. For CGT on taxable accounts above ISAs, see capital gains tax calculator.

Common mistakes
  • Using nominal returns instead of real returns. UK historical equity returns are ~7-9% nominal but ~5% real (above inflation). Use real returns in FIRE calculations because retirement spending grows with inflation.
  • Trusting the 4% rule blindly. The 4% rule is based on US data 1926-1995. Some UK FIRE planners use 3.5% for safety, especially given UK historic returns being slightly lower. The calculator allows you to adjust withdrawal rate.
  • Forgetting tax wrappers. UK FIRE typically uses ISAs (£20k/year, tax-free withdrawals) + pensions (LTA abolished, 25% tax-free at 55+). The calculator doesn’t model wrapper structure — but using ISAs first saves significant tax in early FIRE years.
  • Ignoring sequence-of-returns risk. A bad first 5 years of retirement (2008-style crash early) is much worse than a bad 5 years late in retirement. Some FIRE planners hold 2-3 years of cash buffer to avoid selling equities in downturns.
  • Not adjusting for healthcare/care costs. UK has NHS but private healthcare, dental, and elder care can be significant. Increase spending estimate for older years or hold separate insurance buffer.
  • Treating FIRE as binary (work vs not work). Most FIRE achievers continue some part-time/passion work. ‘Coast FIRE’ (have enough that growth alone gets you there) and ‘Barista FIRE’ (enough for low-stress jobs to bridge) are intermediate stages worth modelling separately.
  • Underestimating discretionary spending. Most people understate retirement expenses. Build in 20-30% buffer above current spending to allow for travel, healthcare, helping family, unexpected expenses.
What this calculator doesn't cover
  • Doesn’t model UK pension access age (currently 57 from 2028, rising with state pension age).
  • Doesn’t differentiate ISA vs pension vs taxable wrapper tax treatment.
  • Doesn’t model sequence-of-returns risk or market crashes.
  • Single-input expected return; doesn’t account for stocks/bonds glide path adjustments.
  • Doesn’t model UK state pension entitlement (currently £221.20/week from age 67-68 with 35 qualifying years).
  • Doesn’t include inheritance, windfalls, or other lump-sum events.

Frequently asked questions

What's a 'FIRE number'?

The portfolio size at which a ‘safe withdrawal rate’ covers your annual spending forever. At 4% SWR: FIRE number = annual spending × 25. £30k/year spending → £750k FIRE number. £40k/year → £1M. £100k/year → £2.5M.

Is the 4% rule still valid?

It’s a US-based historical rule (1926-1995 data, Trinity Study). UK and global data is mixed: 3.5-4% works in most historic periods. Many UK FIRE planners now use 3.5% as a safety margin. Some use 4.5% for leaner FIRE accepting more risk. The calculator lets you adjust.

ISA vs pension for FIRE?

Both have advantages. ISAs (£20k/year limit) give tax-free growth and unlimited tax-free withdrawals at any age — ideal for early-FIRE. Pensions give tax relief on contributions (20-45% boost) but lock funds until 57+ (rising). Most UK FIRE strategies max ISA first, then pension up to ~£60k/year, then taxable accounts. Combined approach typically optimal.

How does state pension affect FIRE?

State pension provides ~£11,500/year from age 67-68 (with 35 qualifying years’ NI). For a £40k/year spender, state pension covers ~29% of needs after age 67. Some FIRE planners build to FIRE number minus state-pension-equivalent, knowing state pension will eventually arrive.

What's Coast FIRE?

When your existing portfolio, with no further savings, will grow to your FIRE number by your target retirement date. £100k at age 30 with 5% real return reaches £700k by 60 — ‘coasting’ to FIRE without saving more. Useful intermediate goal: stop saving aggressively but keep working to cover current spending.

Should I include my main home?

Most FIRE planners exclude primary residence from the FIRE number — you live in it, can’t easily extract cash. Some include rental properties or planned downsizing equity. Be conservative with property values; equity is real but illiquid.

What if I want to retire abroad?

Lower-cost-of-living countries can dramatically reduce FIRE number. £40k/year UK lifestyle might cost £20k in Spain or £15k in Portugal. Geographic arbitrage is the highest-leverage FIRE accelerator. Be aware of UK tax obligations on overseas income, and EU residency rules post-Brexit.

About this calculator. Maintained by Jordan — a UK seller and creator who actually uses these platforms (Etsy, eBay, Vinted, DistroKid). Built to be accurate, mobile-fast, and honest about its limits. Spot a bug? Email hello@payoutmath.co.uk.