Safe Withdrawal Rate — How Much Can You Spend in Retirement?

A safe withdrawal rate is the share of your portfolio you can spend in year one of retirement, with the same dollar amount adjusted for inflation each subsequent year, and still expect the portfolio to last for the entire retirement horizon.

The classic framing: pick the highest withdrawal rate that, applied to historical market data, would have never exhausted a portfolio over a target horizon. The 4% number comes from this exercise applied to 30-year horizons in US data.

Bill Bengen's 1994 paper Determining Withdrawal Rates Using Historical Data tested withdrawal rates against rolling 30-year periods of US stocks and intermediate-term Treasuries from 1926. He found that a 4% inflation-adjusted withdrawal survived every historical 30-year period studied — even retirees who started in 1929 or 1966, the two worst-case starts.

The 1998 Trinity Studyby Cooley, Hubbard, and Walz extended the analysis to multiple stock/bond mixes and confirmed that 4% gave a 95%+ success rate over 30 years. Together these papers built the rule of thumb that became FIRE's operating math.

Three structural changes break the original Trinity assumptions for someone retiring at 35–45:

• The horizon is longer. A 50-year retirement has roughly 20 more years for a bad sequence to compound. Across modern simulations, 4% drops to a ~80% success rate at 50 years vs ~95% at 30 years.
• Starting valuations matter.US equity CAPE ratios sit well above the long-run average. Forward 10–15-year real returns have historically been lower from elevated CAPE starts. Big ERN's research embeds a CAPE adjustment as a first-class input.
• Bond returns are no longer free risk-on.The 2022 bond drawdown was the worst since 1842. The 60/40 portfolio is back to being attractive at higher yields, but the “bonds always cushion” assumption is gone.

These are starting points for capital-preservation plans. If you are willing to accept some chance of running out — or expect to die before 90 with no bequest — you can run hotter.

The static rule throws away the most useful information you have in retirement: what the market actually did. Dynamic strategies adjust withdrawals based on portfolio state and can support higher average spending without higher failure risk.

• Guyton-Klinger guardrails.Set an initial withdrawal rate. If the portfolio falls so much that the current withdrawal becomes > 20% above the original rate, cut the dollar withdrawal by 10%. If it falls below 80% of the original rate, increase by 10%. Historical simulations support starting rates around 5% with this approach.
• Variable percentage withdrawal (VPW). Withdraw a fixedpercentage of the current balance each year, not an inflation-adjusted dollar amount. Eliminates ruin risk by definition; spending becomes volatile.
• CAPE-based withdrawal. Tie the rate to current valuations: lower when CAPE is high (low expected returns), higher when CAPE is low. The Big ERN framework operationalizes this.
• Bucket strategy. Hold 1–2 years of expenses in cash, 3–7 years in bonds, the rest in stocks. Refill from stocks only after up years. Reduces the psychological pressure to sell stocks at lows during a drawdown.

A retiree who can comfortably cut 10–15% of discretionary spending for one or two consecutive years effectively converts variable risk into option value.

For most planners the recipe is: choose a static base rate (3.5% if 40+ years), keep a 1–3 year cash buffer, ignore the temptation to optimize for the highest rate, and revisit annually. That outperforms most exotic strategies on a stress-adjusted basis.

Pair this article with Your FIRE Number to translate the chosen rate into a portfolio target, and Sequence of Returns Risk to understand what the rate is actually defending against. The 4% deep-dive lives at The 4% Rule.