Why 25× spending, £500k-for-everyone and half-your-age rules all miss, what actually moves the number — tax, the State Pension, your dates — and how a real projection solves it.
Every version of this question gets answered with a multiple — 25× your spending, £500,000, "a million to be safe". All of them skip the three things that actually set the number.
Tax. Pension withdrawals beyond the tax-free elements are income: funding £30,000 of spending needs roughly £33,000–£35,000 of gross drawdown once the bands take their share, every year, compounding across a 30-year retirement. The State Pension. £12,548 a year, inflation-protected, from your late 60s — across a long retirement it quietly does £300,000+ of the job, and any multiple that ignores it overshoots badly for normal-age retirees. Your dates. Retiring at 55 adds expensive unsupported years before the State Pension; planning to 95 rather than 85 adds a decade of funding at the far end. Move either and the target shifts by six figures.
The honest method inverts the projection: pick a candidate pot, run the full year-by-year plan — spending inflated, withdrawals grossed up through the real tax bands, the State Pension arriving on schedule — and check every year funds; then search for the smallest pot that passes. Hundred Summers' how-much-to-retire calculator does exactly this in your browser. The answer routinely lands 20–30% away from 25×, in a direction the rule can't predict.
Then stress-test it: a steady-growth answer survives contact with real markets only if the sequence of returns is kind. The full Hundred Summers plan replays your own figure through thousands of simulated paths and every historical start since 1928 — the sequence-risk guide shows why that matters more than the average return.