Somewhere on the way to planning your own retirement, you will meet the 4% rule. It is beautifully simple: work out your annual spending, multiply it by 25, and that is the pot you need. Reach it, withdraw 4% in year one, then rise with inflation each year after, and your money should last roughly three decades. It has launched a thousand spreadsheets and just as many daydreams. It is also, for a British investor, a little too comfortable.
Where the 4% rule actually comes from
The number was not handed down on tablets. It comes from a 1994 study by the American financial planner William Bengen, later reinforced by the 1998 "Trinity study" from three professors at Trinity University in Texas. Both asked the same question: across every rolling 30-year period in US market history, what starting withdrawal rate would have survived even the worst stretches, including the crashes of 1929 and the stagflation of the 1970s?
The answer, for a portfolio of roughly half shares and half bonds, was about 4%. Take more and some historical retirees would have run dry before the 30 years were up. Take 4% or less and, in the US data, you always made it.
Two things are easy to miss. First, this is American data, powered by the best-performing large stock market of the twentieth century. Second, the test was a fixed 30 years. Retire at 40 rather than 65 and you are asking the same pot to last far longer.
Why a British portfolio needs a haircut
Transplant the rule to a UK saver and three quiet problems appear.
Higher and stickier inflation. British retirees have lived through inflation spells that were sharper and longer than the US average. Because the rule increases your withdrawal with inflation every year, a burst of high prices early in retirement forces you to sell more units of a falling portfolio, exactly the wrong time. This "sequence of returns" risk is the real enemy, not the average return.
Fees. The Bengen and Trinity tests assumed no charges at all. In the real world a platform fee plus a fund charge can quietly remove a chunk of your return every year, and that comes straight off your safe withdrawal rate. If you have never checked what you are paying, that is the first job.
Longer retirements. The early-retirement crowd may need the pot to stretch 40 or 50 years, not 30. The maths is unforgiving: the longer the horizon, the lower the rate that reliably survives it.
"The 4% rule is not wrong. It is just American, fee-free and 30 years long, and you are probably none of those three."
So what number should you actually use?
There is no single agreed British figure, and anyone who gives you one to two decimal places is overselling their certainty. What you find instead is a range: many UK-focused analyses land somewhere below 4%, with 3.5% a common reference point and lower still for a very long or very early retirement. That sounds like a small change. It is not. At 4% you need 25 times your annual spending. At 3.5% you need about 29 times, and at 3% a full 33 times. On a £40,000-a-year lifestyle, that is the difference between a £1m target and a £1.33m one.
Before that number frightens you off, remember two things work in your favour. You will not actually spend at a rigid, inflation-linked rate for 40 years; real retirees flex their spending, trimming in bad years. And most Britons have a powerful, inflation-proofed backstop the American studies ignore: the State Pension. The full new State Pension is worth around £12,500 a year in 2026/27 (£241.30 a week), rising each year under the triple lock, and it kicks in from State Pension age, currently 66 and heading to 67. That is income your private pot does not have to provide, which meaningfully lowers the pile you need to build yourself.
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Treat 4% as an optimistic ceiling and around 3.5% as a sensible planning number, then build in flexibility rather than chasing false precision. Keep costs low, because every fee you cut is a rise in the rate you can safely take. Hold a cash buffer of a year or two of spending so you are never forced to sell shares into a crash. And count your State Pension honestly, because it shrinks the private target more than most people expect. The rule is a compass, not a contract.
