FIRE calculator, built for the UK
You know your savings rate and you have heard the 4% rule. This goes further: your real pots, real UK tax and pension rules, and every candidate FIRE age stress-tested against 86 years of market history. Free, no sign-up, nothing stored.
How this calculator works
The 4% rule is one assumption about one market's past. Instead of assuming a safe rate, this calculator replays your whole plan through every market window since 1940 and reports the share of history it survived.
Two identical pots retire at different ages if one is an ISA and one is a SIPP. Withdrawals are ordered tax-aware across ISAs, taxable accounts and pensions, with pension access at 57 and the State Pension from your cohort's age.
Averages hide the worst starts. Because the backtest replays actual sequences of returns and inflation, retiring into a 1970s-style decade counts against your plan exactly as hard as history says it should.
Assumptions
Every calculator makes assumptions — ours are on the table:
- Investments grow at 7% a year before a 0.25% platform/fund fee (6.75% net) and cash at 3% (nominal) in the point-estimate projection; the backtest replays actual historical returns and inflation, with the same fee applied.
- Inflation of 2.5% a year — every figure is shown in today's money.
- Pensions unlock at 57 with 25% tax-free. The State Pension defaults to the 2026/27 full new rate (£12,548/year) from your State Pension age (66–68 by birth year) — both adjustable — inflation-linked and taxable.
- Income tax uses 2026/27 rest-of-UK bands. Shares outside an ISA (GIA) pay capital gains tax at 18%/24% above the £3,000 annual exemption — we assume no gains are embedded in today's balance. Withdrawals are ordered to minimise tax: allowances and tax-free pots first.
- Your plan runs to age 95. Contributions rise with inflation until retirement; pensions are one pot with your and your employer's contributions, while ISAs, GIA and cash are modelled separately.
Frequently asked questions
Why does this calculator solve for an age, not a FIRE number?
A pot multiple treats every pound as equal, but in the UK it isn't: £500,000 in a SIPP you can't touch until 57 funds a very different early retirement from £500,000 in ISAs. Solving for the earliest sustainable retirement age forces the calculator to deal with tax, wrapper access and the pension bridge, which is where FIRE plans actually succeed or fail.
Does my savings rate really determine when I can retire?
It's the biggest lever you control, and the classic savings-rate table is a good intuition pump. But that table assumes constant real returns, zero tax and one account. Your wrapper split, the years before pension access, and the market sequence you retire into can move the same savings rate's answer by several years. Enter your monthly saving and this calculator models all of that.
What safe withdrawal rate does this calculator use?
None. A fixed SWR is one assumption about one market's past. Instead, your whole plan is replayed through every market window since 1940, with actual returns and actual inflation in sequence, and a retirement age only counts if the plan survives at least 85% of history. You see the share of history your plan survived, not a rule of thumb.
Should I prioritise my ISA or my pension for FIRE?
It's a genuine trade-off: pensions get tax relief and usually an employer match, but ISAs are accessible before 57, which is what funds the bridge years of an early retirement. The right split depends on your numbers. This calculator taxes withdrawals the way HMRC would either way, so you can run both versions of your plan and compare the ages.
What about Coast FIRE or Barista FIRE?
Both are modelled as their own calculators. Coast FIRE asks whether your existing pots could fund retirement with no further saving. Barista FIRE asks when part-time work plus your savings could replace the full-time job. They share this page's engine, tax rules and backtest.
Can I model FIRE as a couple?
Not in this free calculator, which models one person. The paid tracker models couples properly: two owners, each with their own accounts, pension ages and tax position (including Scottish bands), on one shared plan. The live demo includes a couple if you want to see it.
Is this FIRE calculator really free?
Yes. No sign-up, no email, and nothing you enter is stored or sent anywhere. The paid product is different: it tracks your real balances month by month against your plan and shows your FIRE age moving. The projection you get here is free and complete.
Is this financial advice?
No. It's a projection based on your inputs and stated assumptions, for information only. Past market performance doesn't guarantee future returns. For advice on your specific situation, speak to a regulated financial adviser.