The calculator
Starting at $40,000, a constant 5.0% return with 2.0% inflation produces 3 cuts and 0 raises over 30 years, and ends at $1,102,604.
The four rules
The public form of Guyton–Klinger (2006) is four decision rules, not a formula. Year-one spending is the initial rate times the portfolio. Each later year, spending rises with inflation unless the portfolio fell — that is the inflation rule. If the current withdrawal rate then sits above the upper rail (initial rate × (1 + band)), spending is cut by the adjustment. If it sits below the lower rail, spending is raised. See guardrails.
Constant vs history vs 1,000 paths
A constant return is the honest teaching case and a weak stress test: the rails barely trip. Historical mode runs every rolling window of the requested length through the same function, using calendar-year S&P 500 price returns from 1928 (the same monthly series the product backtest uses). Simulated mode draws 1,000 seeded paths around the return you typed. The full guardrails explainer is the long form.
Assumptions
No fees, no taxes, and the published inflation rule. Historical windows are price returns, not total returns — they understate the dividend, which is conservative for a spending rule. Switch the return model above rather than treating the constant-return readout as the last word.
A rigid year-one rate is the 4% rule calculator. The unlocked rate is the safe withdrawal rate calculator. Smooth alternatives are the Vanguard dynamic spending calculator, the VPW calculator, and the CAPE-based withdrawal calculator. The Kitces/Tharp sibling that trips on remaining odds is the risk-based guardrails calculator.