Not 25× anything: a binary search over pot size where every candidate is tested by a full year-by-year UK projection — income tax on withdrawals, the State Pension from your late 60s, real drawdown mechanics. Your figures never leave your browser.
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The popular answers — 25× spending, half your age as a percentage, £500k for everyone — all skip the same three things. Tax: withdrawals beyond the tax-free elements are income, so a gross pot funds less net spending than it looks. The State Pension: ~£12,500 a year, inflation-protected, from your late 60s — over a 30-year retirement it quietly does £300,000+ of the job. Your dates: retiring at 55 versus 68, and planning to 85 versus 100, moves the answer by hundreds of thousands.
It solves for the smallest pot whose full projection funds every year: each candidate pot is run through the same engine as the Hundred Summers app — spending inflated year by year, withdrawals grossed up for the actual tax bands, the State Pension arriving on schedule — and a binary search narrows to the answer. The result usually lands meaningfully away from 25×, in a direction the rule can't predict.
The number is still a steady-growth answer. The full Hundred Summers plan stress-tests your own figure through thousands of simulated markets and every historical sequence since 1928 — and models your real accounts, spending phases, and everything else this one-pot page simplifies away.
Hundred Summers