What Is the 4% Rule?
The 4% rule is the foundational principle of the FIRE movement. It says: if you withdraw 4% of your invested portfolio in your first year of retirement, then increase that amount with inflation each year, your money should last at least 30 years across virtually all historical market scenarios.
The rule comes from the Trinity Study (Cooley, Hubbard, and Walz, 1998), which examined US market data from 1926 to 1997. They found that a 4% initial withdrawal rate with a 50–75% equity portfolio succeeded in 95%+ of 30-year periods.
The Maths
At a 4% withdrawal rate, your FIRE number is 25× your annual spending:
FIRE number = Annual spending ÷ 0.04 = Annual spending × 25
| Annual spending | FIRE Number (4% rule) |
|---|---|
| £15,000 | £375,000 |
| £20,000 | £500,000 |
| £25,000 | £625,000 |
| £30,000 | £750,000 |
| £40,000 | £1,000,000 |
| £50,000 | £1,250,000 |
Does It Work for UK Investors?
The original research used US data. UK investors face some differences:
Home bias considerations: UK equity markets have underperformed US markets over the last 15 years. But globally diversified portfolios (which most FIRE investors hold) largely track global returns, not just UK.
State pension: The UK state pension (£11,502/yr) provides a meaningful floor that US Social Security doesn’t match proportionally for high earners. This actually makes the 4% rule more safe for UK investors once state pension starts — your effective withdrawal drops significantly.
Sequence-of-returns risk: The biggest threat to any withdrawal strategy. A major market crash in years 1–5 of retirement is far more damaging than one later on. UK investors are exposed to the same global market dynamics as US investors.
Verdict: The 4% rule is a reasonable starting point for UK FIRE planning. Use 3.5% for retirements of 35+ years or if you want extra margin of safety.
When to Use a Lower Withdrawal Rate
| Scenario | Suggested SWR |
|---|---|
| 30-year retirement (retiring at 65) | 4% |
| 35-year retirement (retiring at 55–60) | 3.5%–4% |
| 40+ year retirement (retiring at 45–55) | 3.5% |
| Retiring before 45 | 3%–3.5% |
| Before state pension with no other income | 3.5% |
The Guard Rails Strategy
Many FIRE retirees don’t use a fixed withdrawal rate — they use guard rails. This means:
- Set a target withdrawal rate (e.g., 4%)
- If portfolio rises significantly, you can spend a bit more (upper guard rail)
- If portfolio drops significantly, reduce spending (lower guard rail)
This flexibility dramatically improves success rates. Someone who can cut spending by 10–20% in a market crash can use a higher initial withdrawal rate safely.
Sequence-of-Returns Risk: The Hidden Danger
The sequence in which returns occur matters enormously. Two portfolios with identical average returns over 30 years can produce wildly different outcomes depending on whether the bad years come early or late.
A portfolio with bad early returns can be permanently depleted. This is why:
- Keeping 1–2 years of expenses in cash/short-term bonds protects you from being forced to sell equities at the bottom
- Part-time or flexible income in early retirement is incredibly valuable
- The “one more year” syndrome is rational — the first years are highest risk
Practical Application
- Calculate your FIRE number at 4% SWR first (25× spending)
- Stress test it at 3.5% (28.5× spending) — can you still reach that?
- Model the state pension as a future floor that reduces your required withdrawal
- Plan your cash buffer for the first 2 years of retirement