Hundred SummersFIRE calculator: your FI number, and the age you reach it
Lean, Regular and Fat FI on the community's 25× convention, your Coast FI figure, and the year each milestone arrives: projected in today's money at real (after-inflation) growth. The same milestone definitions as the Hundred Summers app's FIRE card, so this page and your full plan can never disagree. Your figures never leave your browser.
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Worked example
Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.
| Inputs | |
|---|---|
| Age | 32 |
| Invested | £120,000 |
| Saving a year | £20,000 |
| Spending a year | £28,000 |
| Growth / inflation | 5.0% / 2.5% |
| Coast FI age | 60 |
| Results | |
|---|---|
| Lean FI number | £490,000; reached at 46 |
| Regular FI number | £700,000; reached at 52 |
| Fat FI number | £1,050,000; reached at 61 |
| Coast FI number today | £356,505 (not yet met) |
| Savings rate | 42% |
The next question
How would I fund the years before my pension? →ISA bridge calculator
Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.
Read the guide
- FIRE in the UK: what the American playbook misses →The 25× rule and savings-rate maths transfer; the mechanics don't. The pension access age bridge, ISA-vs-pension sequencing, UK tax on the way out, and where Coast and Barista FI fit: the honest UK version.
The 25× convention, and what it hides in the UK
The FIRE community's arithmetic is the inverse of the 4% rule: if you can live on 4% of your portfolio, you need 25× your annual spending. It's a fine compass. It is not a UK answer, because three things it ignores all cut against it: income tax on pension withdrawals beyond the tax-free elements; the pension access age (55 now, 57 from 2028), which splits your wealth into money you can touch early and money you cannot; and on the other side, the State Pension from your late 60s, which shrinks what the portfolio must fund in later life.
The bridge problem
Retire at 45 and your pension (usually the biggest, most tax-advantaged pot) is locked for a decade. UK FIRE is therefore a two-pot problem: enough ISA/taxable wealth to bridge to pension access, then the pension takes over. A single 25× number cannot see whether your split is right; a year-by-year projection can, and it's the most common thing that breaks otherwise-sound UK FIRE plans.
The full Hundred Summers plan tests your real FI date with all of it modelled (tax bands, both pots, the State Pension, your actual spending phases), then stress-tests it through thousands of simulated markets and every historical sequence since 1928. The 4% rule guide covers why the rule's US assumptions travel badly.