Guyton-Klinger Guardrails: What They Are and How to Model Them in Scenarios

The Problem It's Solving

The classic 4% rule gives you one number and asks you to stick to it: withdraw 4% of your portfolio in year one, then increase that amount by inflation every year after, regardless of what the market does. It never adapts. It raises your spending in a year your portfolio just fell 20%, and it never lets you spend more after a decade of strong returns, even if your portfolio has nearly doubled.

In 2006, financial planner Jonathan Guyton, later joined by William Klinger, published "Decision Rules and Maximum Initial Withdrawal Rates" in the Journal of Financial Planning. Instead of one fixed number, they proposed a small set of rules that check your withdrawal rate against your portfolio each year and adjust spending accordingly. The result is usually a higher starting withdrawal rate than a rigid rule would allow, for a similar risk of running out, because the strategy responds to trouble instead of ignoring it.

The Three Rules, With Actual Numbers

Say you retire with a £500,000 portfolio and plan to withdraw £20,000 in year one. That's a 4% initial withdrawal rate, this number gets locked in as your baseline and never changes for the rest of the plan.

1. The capital preservation rule. Each year, divide that year's planned spending by the current portfolio value. If that current rate has drifted more than 20% above your 4% baseline, cut spending by 10%.

Say a couple of weak years leave the portfolio at £415,000 while planned spending is still around £20,000. Current rate: £20,000 ÷ £415,000 = 4.82%. That's 20.5% above the 4% baseline, past the threshold, so spending gets cut by 10%, from £20,000 to £18,000. The rule exists to stop a bad sequence from locking in a withdrawal rate the shrunken portfolio can no longer support.

2. The prosperity rule. The mirror image. If the current rate has fallen more than 20% below the 4% baseline, increase spending by 10%.

Say the portfolio grows to £650,000. Current rate: £20,000 ÷ £650,000 = 3.08%, which is 23% below the 4% baseline, past the threshold in the other direction, so spending rises by 10%, from £20,000 to £22,000. Without this rule, a retiree who did everything right would just keep spending the original amount and leave far more unspent than intended.

3. The inflation rule. After a year your portfolio had a positive return, the following year's spending increases with inflation as normal. After a year it had a negative return, the following year's inflation increase is skipped entirely, spending stays flat in nominal terms rather than rising. This is the rule that stops inflation from quietly ratcheting spending up right after a year the portfolio can least afford it.

Guyton and Klinger's original paper also included a fourth rule about which asset classes to sell from after a bad year, that's an implementation detail about how you raise the cash, not about how much you spend, so it isn't covered here.

Why It's Not Just "A Different Percentage"

None of these rules require guessing a better fixed number in advance. They require checking, every year, whether this year's spending is still a sensible fraction of what's actually left, and nudging it if it isn't. That's the entire mechanism, and it's why a Guyton-Klinger plan can typically start higher than a rigid inflation-linked plan for a comparable chance of running out: the strategy is willing to cut spending before a shortfall compounds, so it doesn't need as much margin built in from day one.

How to Model This in Scenarios

Guyton-Klinger isn't one setting, it's two, and both need to be switched on together to get the full system described above. They live on the Withdrawals tab (part of the paid plan), under two separate cards:

1. Under Inflation Rules, select Guyton. This is rule 3, the inflation increase gets skipped after a negative-return year.
2. Under Withdrawal Rules, select Guardrails. This is rules 1 and 2, the capital preservation and prosperity adjustments.

Picking only one of the two gives you a partial version, not the full strategy, so it's worth checking both cards if that's what you're trying to model.

Once Guardrails is selected, four numbers are editable, all with the defaults used in the example above:

• Window: how many years into retirement the guardrails stay active, 15 by default, adjustable from 1 to 40. After this window, spending just follows whatever inflation rule you've selected, with no further guardrail adjustment.
• Prosperity threshold: how far the current rate has to fall below the initial rate to trigger a raise, -20% by default.
• Prosperity boost: the size of that raise, 10% by default.
• Capital preservation threshold and cut: the mirror-image settings for cutting spending, 20% and -10% by default.
• Check frequency: how often the comparison runs, every year by default, adjustable if you'd rather it checked every 2 or 3 years instead.

There's nothing to calculate by hand first. Set your spending target, select both cards, and the simulation applies the same year-by-year comparison described above across each of the thousand simulated futures behind your results. You can build a free plan to model your own numbers, then switch on Guardrails on the paid plan when you're ready to see it applied.

Further Reading

• Guyton, J.T. & Klinger, W.J. (2006). "Decision Rules and Maximum Initial Withdrawal Rates." Journal of Financial Planning, 19(3), 48–58.
• Bengen, W. (1994). "Determining Withdrawal Rates Using Historical Data." Journal of Financial Planning, 7(4), 171–180.