Your Pension Is Not a US 401(k): The Drawdown Maths That Actually Fits Britain

The 4% rule has become the default setting for British pension drawdown. It is not a British rule. It was never meant to be.

In 1994, US financial adviser Bill Bengen published research suggesting a retiree could withdraw about 4% of a balanced portfolio each year, adjust for inflation, and stand a decent chance of not running out over 30 years. American data. American markets. American assumptions. Somehow it crossed the Atlantic and became a comfort blanket for UK retirees — a number repeated in forums, at dinner parties and, worse, in drawdown plans.

That is not a reason to ignore it. It is a reason to stress-test it. Because the Britain you retire into in 2026 is not the America of Bengen’s study.

A rule built for someone else’s retirement

The US case had several advantages. A huge domestic equity market. A long history of relatively stable inflation after the 1980s. Social Security as a backstop. And, crucially, a fee landscape that — while not perfect — is often lower than the all-in costs many UK investors pay.

Britain is different. Your SIPP is not a 401(k). Your state pension is not Social Security. Your inflation basket is not the US CPI. And your portfolio is probably stuffed with global equities priced in dollars, which adds currency swings on top of market swings.

None of that makes a 4% withdrawal impossible. It makes it more fragile than the headline number suggests — especially if you take it as a fixed rule rather than a starting point.

The Britain problem in three numbers

First, fees. A 4% withdrawal is not 4% if you are paying 1.5% a year in platform charges, fund costs and advice. That money comes out before your income does. A gross 4% can behave more like a net 2.5% to 3% once the machinery has been fed. Over 30 years, that gap is not a rounding error. It is a different retirement.

Second, inflation. The UK has had periods of nasty inflation within living memory, and 2022 was a sharp reminder that CPI can spike while your portfolio is falling. The 4% rule adjusts withdrawals for inflation. That is exactly what makes it dangerous when inflation is high and markets are weak at the same time. You end up selling more units at lower prices to fund a rising income.

Third, sequence risk. This is the wonkish term for a simple nightmare: a bad market in the first few years of drawdown. If you retire into a 30% fall and keep withdrawing, you lock in losses. The portfolio has less time to recover. A plan that looked fine on average can fail in the specific order that actually happens.

What actually works here

There is no single magic number. Anyone who gives you one without asking about your state pension, spending flexibility, tax position and health is selling certainty, not analysis.

But for a UK retiree with a balanced portfolio, a 30-year horizon and total costs of around 1% to 1.5%, a more realistic starting range is 3.0% to 3.5%. Not 4%. And that range assumes you are willing to adjust spending when markets wobble.

If you retire early — say mid-50s with a 40-year horizon — the range should be lower: closer to 2.8% to 3.2%. If your state pension and any defined-benefit pension cover your essential bills, you can afford to take more risk with the rest. If they do not, you cannot.

The 4% rule is not useless. It is a useful warning shot. It tells you that a portfolio cannot fund an unlimited lifestyle. It does not tell you what your lifestyle should cost.

The stress test to run before you touch the SIPP

Before you set a withdrawal rate, run this simple four-part test. You can do it on a spreadsheet or the back of an envelope. The point is not precision. The point is to see what breaks.

1. Split your spending into essential and discretionary. Essential means mortgage or rent, energy, food, insurance, transport and council tax. Discretionary means holidays, gifts and the nice-to-haves. Subtract your state pension and any guaranteed income from the essential figure. That gap is the number your portfolio actually has to cover.

2. Model a bad start. Take your portfolio value and knock 30% off it in the first two years. Keep withdrawing the same amount. Does your essential spending still work? If not, your starting rate is too high or your cash buffer is too thin.

3. Add the fees. Reduce your expected return by 1.5% a year. Then reduce it again for inflation. If your plan only works with a 7% nominal return and low fees, it is not a plan. It is a hope.

4. Check your flexibility. If the market falls 20%, can you cut discretionary spending by 10% to 15% for a year or two? If the answer is no, you need a lower starting rate or an annuity floor to cover the essentials.

Here is the uncomfortable arithmetic. A £500,000 portfolio at 4% gives £20,000 a year. If the portfolio falls 30% to £350,000, that £20,000 is now a 5.7% withdrawal. To get back to 3.5%, you would need to cut income to £12,250. That is a brutal adjustment. Better to start lower and build in room than to discover the maths in a bear market.

The comfort blanket has a dollar sign

The 4% rule became popular because it is simple. Simple is comforting. But retirement is not a US case study, and your pension is not a laboratory. The UK version of the rule needs to account for our inflation, our fees, our state pension and our sequence risk.

A 3.0% to 3.5% starting range is not a guarantee. It is a more honest baseline. And the stress test matters more than the number. If your drawdown plan only works when markets behave, it is not a plan. It is a bet.

Run the test before you touch the pension. The comfort blanket can stay on the sofa.