Which rails
| Rule | Tripwire | Calculator |
|---|---|---|
| Guyton-Klinger | Current withdrawal rate versus the initial rate ± a 20% band | Open |
| Risk-based guardrails | Remaining probability of success versus a floor and a ceiling | Open |
A 4% illustration
On a $1,000,000 portfolio, 4% start, 5% every year, 2% inflation and a 20% band, year-one spending is $40,000. The upper rail sits at 4.8% and the lower rail at 3.2%. That path is a teaching case — a constant return barely moves the rate. The Guyton-Klinger calculator also has a historical mode and a 1,000-path mode so the rails actually trip.
Open a calculator
The Guyton-Klinger guide is the four rules in prose. The 4% vs guardrails page is the comparison most people mean.