Guardrails (Guyton-Klinger): flexible spending rules, with a worked example
The 4% rule asks you to spend a fixed, inflation-adjusted amount no matter what markets do. That's safe, but it means choosing a low starting number to survive the worst case. Guardrails flip the deal: start higher, and agree in advance to trim spending when your portfolio gets stressed. Runway's withdrawal guardrails toggle implements a version of this, and this post shows exactly what it buys and what it costs.
What are the Guyton-Klinger guardrails?
Jonathan Guyton and William Klinger introduced a set of decision rules in "Decision Rules and Maximum Initial Withdrawal Rates" (Journal of Financial Planning, March 2006). The two best known act as "financial guardrails" when market conditions cause the initial withdrawal rate to rise or fall significantly:
- Capital preservation rule: if the withdrawal rate has risen 20% above the starting rate, "the current year's withdrawal is reduced by 10 percent."
- Prosperity rule: if it has fallen 20% below the starting rate, spending is raised by 10%.
The authors report that initial withdrawal rates of 5.2–5.6% are sustainable at their 99% confidence standard for portfolios with at least 65% in stocks, and 4.6% with 50% in stocks. They are candid about the cost: the capital preservation rule's drawback is "chiefly the lost purchasing power each time the rule is triggered."
What does Runway implement?
Runway implements those two rules, with the published 20% bands and 10% adjustments, on top of its inflation-adjusted baseline. It does not model the paper's other rules (a freeze on inflation increases, a sell-the-winners rule, and a longevity exception late in life) because Runway already works in inflation-adjusted dollars and already chooses which account to sell from. Turn it on with the guardrails toggle in Explore Your Plan or What-If Scenarios; when a rule fires, the year shows up in Today's Actions.
What do guardrails do to the odds?
Same household every row — the sample couple (Pat 65 and Alex 64, $1,460,000 saved), Social Security at 69 and 68, 2,000 simulated market paths — changing only the yearly spending and whether guardrails are on:
| Yearly spending | Chance money lasts, off | Chance money lasts, on | Median ending balance, off | Median ending balance, on |
|---|---|---|---|---|
| $95,000 | 99.5% | 100.0% | $1,574,254 | $1,618,614 |
| $110,000 | 87.0% | 100.0% | $1,201,345 | $1,444,941 |
| $125,000 | 55.0% | 100.0% | $147,326 | $1,228,972 |
The pattern is dramatic — and easy to misread. A "100%" success rate with guardrails doesn't mean spending is guaranteed; it means the money never ran out because spending was cut when needed. The table doesn't show how much was cut. That's the next section.
What do the cuts look like?
Take the $110,000 household and add a bad start: markets fall 20% a year for two years beginning at Pat's age 66. Without guardrails the plan keeps spending $110,000 a year and the money is gone before the end (ending balance $0). With them:
| Pat's age | Yearly spending target | What happened |
|---|---|---|
| 66 | $110,000 | Crash begins |
| 67 | $99,000 | Capital preservation rule: cut 10% |
| 68 | $89,100 | Cut again |
| 69 | $80,190 | Cut again |
| 70 | $72,171 | Cut again |
| 71 onward | $64,954 | Cut again, then stable; ending balance $674,229 |
The rule fires five years in a row, each cut compounding on the last, and spending settles at $64,954 — 41% below where it started — in exchange for a plan that finishes with $674,229 instead of running out. If you could live on $64,954 in a crisis, guardrails make a $110,000 start safe. If you couldn't, they won't — which is the question to ask before using them.
How do guardrails fit with the 4% rule and other approaches?
The 4% rule (see what the research really says) is the fixed-spending baseline; guardrails let you start above it by accepting variability. They pair naturally with a spending floor you've chosen in advance, and with the idea that early retirement years matter most — see why the first ten years matter. For where your own number lands, see how much you can safely spend.
Test guardrails on your own plan. Flip the toggle, see the spending path and the odds — free to start, with the full What-If tools in Pro.
Try the plannerHow were these numbers computed?
The simulation rows are Runway's Monte Carlo (2,000 paths, fixed seed 2026) on the sample household with default market assumptions, plan through age 95, with the long-term-care cost left out, changing only spending and the guardrails flag. The spending-path table is a single deterministic projection with a −20%-a-year crash for two years beginning at Pat's age 66 and $110,000 of yearly spending; the dollar amounts are the engine's alerts, in today's dollars. Guardrail bands (20%, 10%) are fixed in the engine and not adjustable.