TL;DR
Fixed-dollar withdrawals look safer in nominal terms but devastate purchasing power. Over the 1966 stagflation cohort, a fixed $40k retiree had only $9k of 1966 purchasing power by 1996.
Two different decisions
When you draw £40,000 in year one of retirement, what should you draw in year two?
- Inflation-adjusted: £40,000 × (1 + last year's inflation). If inflation was 3%, year two is £41,200. Your real spending stays constant.
- Fixed nominal: still £40,000. Your real spending falls by the inflation rate each year.
The 4% rule, Trinity Study, and essentially every modern academic SWR paper assumes inflation-adjusted withdrawals. The implicit reasoning: retirees need constant real lifestyle, so withdrawals must keep pace with the cost of living.
But a non-trivial number of real retirees use fixed-nominal withdrawals — often inadvertently, by setting up a fixed standing order and forgetting about it. The implications are dramatic.
What 155 years of data shows
Run a £40,000 starting withdrawal against the Shiller dataset for both strategies, starting in the worst-case 1966 cohort:
- Inflation-adjusted: portfolio fails at year 24 (1990) because withdrawals grew from £40k to £153k of nominal spending by 1990, while the portfolio couldn't keep up.
- Fixed nominal: portfolio survives the full 30 years easily. But by 1996, the £40k nominal withdrawal is worth only £9,000 of 1966 purchasing power. The retiree is technically alive but spending one-quarter of their original real lifestyle.
The fixed-nominal strategy doesn't fail in the SWR sense (it doesn't run out of money). It fails in the lifestyle sense — the retiree's standard of living collapses through inflation rather than through portfolio depletion. For most people, this is worse than running out, because you experience the decline gradually rather than abruptly.
When fixed-nominal actually makes sense
Three narrow situations:
- You'll have inflation-protected income kicking in later. A 50-year-old retiring with a state pension at 67 might accept declining real withdrawals during the bridge years, knowing the pension's triple-lock will partly catch up the spending once it starts.
- Your essential expenses don't track CPI. This is rare in practice. UK essential spending — food, energy, housing, healthcare — generally tracks or exceeds CPI inflation.
- You want to maximise terminal wealth at the cost of lifestyle decline. Some heirs-focused retirees deliberately under-withdraw to preserve bequest value. Fixed nominal is a way to do this implicitly.
For everyone else — i.e., almost everyone — inflation-adjusted is the right choice. The whole point of a retirement plan is maintaining lifestyle, not preserving the nominal portfolio.
The hybrid: ceiling and floor
A pragmatic middle ground: inflation-adjust your withdrawal annually, but cap the nominal increase at, say, 5% in any single year. If inflation surges to 11% (as the UK saw in 2022), your withdrawal rises by 5% rather than 11%. The retiree absorbs some of the inflation pain in real terms, but the portfolio gets a break.
This works because most inflation surprises are short-lived. UK inflation peaked at 11% in October 2022 and was back below 4% within two years. A cap-and-floor strategy avoids over-reacting to transient spikes while still keeping pace with sustained inflation.
For deeper UK-specific context on how inflation history affects FIRE planning, see our UK inflation history article.
Which CPI measure to use
UK retirees should generally use CPI, not RPI. RPI has been deprecated by the ONS and tends to run roughly 1% higher than CPI due to methodology differences. Pension benefits in the UK have largely shifted to CPI uprating; planning around RPI overstates the inflation challenge.
US retirees should use CPI-U as the standard measure. The "chained CPI" (C-CPI-U) is a more accurate measure of cost-of-living but understates inflation slightly — using it makes your real spending decline subtly.
What we use in the simulator
FIRE Wealth OS uses inflation-adjusted withdrawals as the default, applying realised CPI from each cohort's actual sequence. So a 1966 cohort experiences 1966–1996 inflation; a 1929 cohort experiences 1929–1959 inflation. The math works in real terms, with realistic inflation drag built in. See the 4% rule article for how this calibration matches Bengen's original methodology.
Putting it together
For nearly all FIRE planners:
- Use inflation-adjusted withdrawals
- Use CPI (UK) or CPI-U (US), not RPI or chained CPI
- Consider a 5% nominal cap if you want a behavioural smoother
- Don't accidentally drift to fixed-nominal by leaving an old standing order alone
Run your own plan against both strategies in our withdrawal survival tool. The portfolio-survival numbers and the real-spending numbers will tell you why almost everyone should pick inflation-adjusted.
Frequently asked questions
- Are inflation-adjusted withdrawals always best?
- For maintaining lifestyle, yes. For maximising terminal wealth at the cost of declining real spending, fixed-dollar works better.
- Should I use CPI or some other inflation measure?
- CPI is the standard. RPI (UK) tends to overstate inflation; chained CPI (US) tends to understate it. For planning, use the standard CPI for your country.
- What if inflation surges to 10%+?
- A cap-and-floor approach — adjusting for inflation but capping annual increases at 5% — handles transient spikes without forcing your portfolio to keep up with momentary extremes.
Stress-test your own FIRE plan
FIRE Wealth OS runs your savings rate and expenses against every historical market starting point since 1871. Free to use, no card required.