The 4% rule for UK retirement: does it still work?
The 4% rule is a famous shorthand for "how much you can spend in retirement without running out of money". It is also widely misunderstood. This guide explains where the rule came from, what it actually says, and the adjustments UK savers should consider before relying on it.
Where the rule came from
The rule originates in a 1994 paper by US financial planner William Bengen, and was refined in the late-1990s "Trinity study" by three Texas professors. Both used historical American stock and bond returns to ask the same question: starting with a balanced portfolio, what is the highest fixed real-terms withdrawal a retiree could make in year one — increased by inflation each year afterwards — that would have survived every rolling 30-year period in the data set?
The answer was approximately 4%. A retiree with a $1,000,000 portfolio could withdraw $40,000 in year one, raise that figure with inflation each subsequent year for 30 years, and have run out of money in none of the historical scenarios tested.
What the rule actually says
The full statement is more specific than its popular summary:
- 4% of the starting pot, not the current pot. The number is set on day one and indexed to inflation. It is not "always withdraw 4% of whatever the pot is worth today".
- 30-year horizon. The original work assumed a 30-year retirement.
- Roughly 50/50 stock-bond mix. Less equity meant a lower safe rate; much more equity also reduced survivability because volatility hurts withdrawals.
- US data. The result depends on the twentieth-century returns of US equities and US Treasuries.
- Pre-tax, pre-fees. The original studies ignored both taxes and platform/fund fees.
Why UK savers cannot use it unchanged
Recent academic work, notably by Wade Pfau and Morningstar, has shown that 4% is closer to a US-specific upper bound than a global constant. For UK and most international portfolios, the historically safe rate is somewhat lower — typically estimated in the 3.0%–3.5% range over a 30-year horizon, depending on asset mix and the period studied.
Several UK-specific factors compress the safe rate further:
- Tax on drawdown. Withdrawals from a SIPP above the personal allowance are taxed as income. A "4%" withdrawal in gross terms is meaningfully less in net spending power.
- Investment fees. A combined platform and fund fee of 0.5% per year is a permanent reduction in your effective return. Studies that assume zero fees overstate the safe rate.
- Longer retirements. A UK retiree at 60 today has a meaningful probability of needing 35–40 years of spending, not 30. Lengthening the horizon lowers the safe rate.
- Sequence-of-returns risk. A poor early decade — the 1973–74 oil crisis, 2000–2002, 2008 — is what breaks plans. The 4% figure already factors this into its historical worst case, but recent valuation levels are at the high end of historical bands, which has historically correlated with lower forward returns.
The UK State Pension changes the picture
The 4% rule was designed for retirees with no other income. In the UK, the State Pension provides around £11,500 per year of inflation-linked, government-guaranteed income from State Pension age. That dramatically reduces the strain on the investment portfolio, and means the practical safe withdrawal rate for the investment pot is harder to summarise in a single number — it depends on the gap between your spending and your guaranteed income.
How to think about it instead
The 4% rule is best treated as a sanity-check rather than a plan. Use it to ask: "Does my pot, multiplied by 4%, get close to the spending I want from investments?" If yes, your plan is in the right neighbourhood. If no, either the pot or the spending needs to change.
For an actual retirement plan, three things move you beyond the rule of thumb:
- Use a flexible withdrawal approach. Cut spending in poor years and increase it in good ones. This alone significantly raises the sustainable starting rate.
- Model UK tax explicitly. A net spending target needs a higher gross withdrawal. The planner does this automatically using current bands.
- Use Monte Carlo, not a single rule. A simulation gives you a probability of success rather than a binary answer. A 90%+ success rate over 35 years on a realistic returns assumption is a more useful target than "is my withdrawal under 4%".
Run a Monte Carlo simulation
The Pension AI planner runs Monte Carlo simulations using lognormal, correlated returns for pensions, ISAs and other growth assets. It applies UK income tax to drawdown, factors in the State Pension, and reports the probability of running out of money at the spending rate you choose. That is closer to a modern retirement test than any single percentage rule.