The Rule of 25: A Simple Way to Turn Spending Habits Into a Retirement Number

The Formula, in Plain Language

The idea is simple: annual expenses × 25 = your retirement target. If you spend £30,000 a year, the rule suggests you’d want roughly £750,000 invested before stepping away from paid work. Spend £20,000 a year and the target drops to £500,000.

The logic behind the multiplier comes from research into how much a retiree can safely withdraw from an investment portfolio each year without running out of money — commonly known as the 4% rule. If 4% of your savings covers your annual spending, then your savings must equal 25 times that spending (since 100% ÷ 4% = 25). The idea traces back to analysis of historical market returns, most famously the "Trinity Study," which tested how various withdrawal rates would have held up across decades of past market cycles, including downturns.

Why 4% Became the Benchmark

Researchers who ran the numbers found that withdrawal rates above roughly 5% ran into trouble in a meaningful share of historical scenarios, mostly because of what’s called sequence-of-returns risk: if markets fall early in your retirement while you’re also withdrawing money, the combination can drain a portfolio far faster than the averages suggest. A 4% starting rate, adjusted upward each year for inflation, offered a comfortable margin against that risk in the historical data researchers examined.

Importantly, even the researcher most associated with the concept has described it as closer to 4.5% under some assumptions, and stressed that it should adapt to real conditions rather than function as a fixed law: major market downturns early in retirement and periods of high inflation both push the "safe" number down. Other financial planning research has gone further, showing that retirees willing to skip or trim their annual inflation adjustment during weak years can safely start with a meaningfully higher withdrawal rate, while retirees who insist on rigid inflation-linked increases every year need more caution built in from the start. In short: the 4% figure is a well-studied starting point, not a law of physics, and it comes from historical simulations — not a promise about future markets.

What the Numbers Actually Look Like

Because the whole rule hinges on your annual spending, small changes in that number ripple outward dramatically. Here’s how different spending levels translate into different targets using the 25x multiplier:

Annual spending	Rule of 25 target	Annual withdrawal at 4%
£20,000	£500,000	£20,000
£25,000	£625,000	£25,000
£30,000	£750,000	£30,000
£35,000	£875,000	£35,000
£40,000	£1,000,000	£40,000

Notice that the difference between spending £30,000 and £40,000 a year isn’t £10,000 — it’s £250,000 in required savings. That’s the core insight: your retirement number is driven by what leaves your bank account, not what enters it. Two people with very different salaries but the same spending habits end up needing the exact same nest egg.

Spending Is the Lever You Actually Control

Markets will do what markets do — nobody can dependably control average investment returns. But everyone has some influence over two things: how much they save and how much they spend. Of the two, trimming spending is often underestimated. Every £1,200 shaved off annual expenses (£100 a month) removes roughly £30,000 from the eventual target, because that reduction gets multiplied by 25. That’s not a promise that cutting a subscription will get you to retirement faster by some guaranteed amount — the exact effect depends on your own numbers, investment returns, and timeline — but the direction is consistent: lower recurring spending means a smaller mountain to climb.

This is also why the rule is best used as a budgeting exercise rather than a retirement "hack." The real work isn’t the multiplication — it’s honestly tracking what you spend. A budget built on guessed or aspirational numbers is far less useful than one built on your actual bank statements from the last few months.

From Tracking to Target: A Simple Workflow

Turning this idea into something usable follows a repeatable sequence:

flowchart LR
 A[Track real spending] --> B[Estimate annual expenses]
 B --> C[Multiply by 25]
 C --> D[Set as savings target]
 D --> E[Review yearly and adjust]

The last step matters as much as the first. Spending changes — a mortgage gets paid off, a commute disappears, health costs shift with age — so the target isn’t a one-time calculation to file away. It’s a number to revisit at least once a year, especially as life circumstances change.

Where the Rule Runs Into Real Life

The Rule of 25 is a helpful compass, not a GPS with guaranteed accuracy, and a few limits deserve honest attention.

It doesn’t fully account for taxes and fees. The 4% figure describes what comes out of a portfolio; taxes and investment management fees typically come out of that withdrawal, not on top of it, which can effectively shrink the amount available to spend.

Portfolio composition changes the math. The original research assumed a specific mix of stocks and bonds; a more conservative or more aggressive allocation can shift both how much you can safely withdraw and how your ending balance behaves over time.

Retirement length matters. The rule is generally built around a roughly 30-year retirement horizon. Someone planning to stop working in their 40s or 50s may need a longer runway for their money, which is one reason some planners suggest more conservative withdrawal rates — sometimes closer to 3–3.5% — for very early retirement.

Flexibility beats rigidity. Nearly every source on this topic agrees on one point: retirees who are willing to spend a little less in a rough market year, or a little more in a strong one, tend to have far more breathing room than those locked into a fixed formula.

Guaranteed income changes the equation — but only for you, specifically. Some readers have access to a state pension, workplace pension, or other guaranteed income that can reduce how much needs to come from personal investments. In the UK, for example, the full new State Pension is a specific weekly amount set by current government policy, but pension ages and amounts are subject to future change and depend on an individual’s National Insurance record. It’s worth understanding whether you’re entitled to something like this, but it shouldn’t be treated as a fixed, guaranteed offset baked into your plan decades in advance — and readers outside the UK will have entirely different systems to consider.

The Bottom Line

The Rule of 25 won’t tell you the exact age you’ll retire, and it isn’t a substitute for professional financial, tax, or investment advice tailored to your situation. What it does well is something psychological as much as mathematical: it replaces "I should probably save more" with a concrete number you can track. And because that number is built entirely from your spending, the most immediate lever most people have isn’t a stock pick or a market prediction — it’s understanding, and gradually trimming, what actually leaves their wallet each month. Start there, review often, and treat the number as a compass rather than a finish line.