The “4% rule” comes up in every retirement conversation, usually stated with more confidence than it deserves. It’s a genuinely useful starting point — as long as you know what it actually says and where it stops working.
What it actually says
The rule comes from research into how much someone could withdraw from a retirement portfolio each year without running out of money over a long retirement. The finding, simplified: withdraw 4% of the pot in year one, then increase that amount with inflation each year after.
The critical detail almost everyone drops: you only calculate the percentage once. After year one you’re not taking 4% of whatever the pot is worth — you’re taking your original amount, adjusted for inflation. That’s what makes the income predictable, and it’s the part most summaries get wrong.
A £400,000 pot gives £16,000 in year one. If inflation runs at 3%, year two is £16,480 — regardless of what the market did.
Where the “multiply by 25” shortcut comes from
4% is one twenty-fifth. So working backwards from the income you need, multiply by 25 to get the pot. Need £20,000 a year from savings? That’s £500,000. It’s the same arithmetic viewed from the other end, which is why the two rules of thumb always appear together.
What the rule assumes
The original research rested on assumptions that may or may not match you:
- A roughly 30-year retirement. Retire at 55 and plan for 40 years and the maths changes.
- A portfolio with substantial exposure to shares. Held entirely in cash, 4% doesn’t survive inflation.
- Historical returns from a specific market and period. Largely twentieth-century US data — a strong run by global standards.
- Rigid withdrawals. You keep taking the same inflation-adjusted amount even through a crash.
That last assumption is the least realistic, and interestingly it makes the rule pessimistic. Real people cut back in bad years. Someone who trims spending slightly after a poor run is meaningfully safer than the model suggests.
Sequence risk: why timing beats averages
Two retirees can experience identical average returns over twenty years and get completely different outcomes, purely because of the order those returns arrived.
The reason is that withdrawals lock in losses. If markets fall sharply in your first few years, you’re selling assets at depressed prices to fund living costs, and there’s less left to recover when things turn. The same crash fifteen years in, after years of growth, does far less damage.
This is called sequence-of-returns risk, and it’s the strongest argument for flexibility in the early years specifically. The first five years carry disproportionate weight.
How people actually use it
Treat 4% as a reference point, not an instruction:
- Planning a target? Multiply by 25. It gives a sensible, slightly conservative goal.
- Retiring unusually early? Something nearer 3% to 3.5% is the common adjustment for a longer horizon.
- Already retired? Flexibility beats precision. Willingness to trim in bad years buys more safety than any starting percentage.
Try it against your own numbers
The rule is easiest to understand when you see it applied to figures you recognise. Our free Retirement Countdown & Savings Calculator projects what your savings could become and what that translates to monthly. Comparing that against your real spending is the moment this stops being theory.
And if the gap looks discouraging, read how the target is built first — it’s remarkably sensitive to small changes in spending and timing.
This article is general information, not financial advice. For decisions about your own money, speak to a qualified financial adviser regulated in your country.