A formula-only safe withdrawal rate spreadsheet from firenum.com replays a retirement through every start year in Shiller's S&P 500 data. At 4% over 30 years, 122 of 125 start years kept the money alive. At Bengen's new 4.7%, 118 did.
-- September 2026 — Only three of 125 retirement start years since 1872 would have broken the 4% rule over 30 years, according to a 4% rule backtest spreadsheet published this week by firenum.com, which also runs a free FIRE calculator and retirement planner. All three began in the Great Depression: 1928, 1929 and 1930. The workbook replays a retirement through every start year in Robert Shiller's S&P 500 data and reports how many ran out of money. The download needs no email address or account, and the workbook uses no macros.
At the defaults of a $1M portfolio, a 4% withdrawal rising 3% a year for inflation and a 30-year retirement, the portfolio survived in 122 of 125 start years, a 97.6% success rate. A retiree who started in 1929 ran out of money in year 15. Lowering the rate to 3.5% raised survival to 98.4%. Raising it to 5% dropped survival to 88.8%, with 14 failures spread from 1873 to 1931.
The release lands in a live debate over the number itself. William Bengen, who published the 4% rule in 1994, argued in his 2025 book "A Richer Retirement" for 4.7%, based on a seven-asset portfolio. Morningstar's 2025 State of Retirement Income report put its base case at 3.9%. Run at 4.7% on a 100% stock portfolio, the workbook shows 118 of 125 start years surviving. Three of the seven failures began in 1881, 1882 and 1883, a decade the 4% debate rarely mentions.
The more common outcome was the opposite problem. The median start year finished with $7.1M in nominal dollars, about $2.9M after the workbook's own 3% inflation assumption. In 77 of 125 start years the portfolio ended at more than double its starting value in real terms. The 1975 start year finished with $33M.
The assumptions are stated on the download page. The portfolio is 100% S&P 500 with dividends reinvested and no fees or taxes, so the page describes the results as a ceiling. Withdrawals rise at a flat rate the user sets, which treats the 1929 retiree more harshly than the deflation of the 1930s did. Shiller's prices are monthly averages, so returns run slightly smoother than close-to-close figures. One withdrawal rate is tested per run.
The returns come from Yale economist Robert Shiller's S&P 500 series, which the site also publishes as a monthly dataset running from February 1871 to September 2026. The workbook compounds the monthly figures into 154 annual returns and runs them through a 154-row by 50-column grid of ordinary spreadsheet formulas. It opens in Google Sheets, Excel and LibreOffice. Every result is a cell the user can trace.
"Most people meet the 4% rule as a sentence in a blog post. It should be a table they can argue with. Change the rate, change the year, watch what breaks," said a spokesperson for the site.
The 4% rule dates to Bengen's 1994 paper and the Trinity Study of 1998. Both used mixed stock and bond portfolios with actual inflation, so the spreadsheet tests a harsher, all-stock version of the rule.
The same release expands the site's FIRE spreadsheet from five tabs to nine. FIRE stands for Financial Independence, Retire Early. New tabs cover Barista FIRE, a smaller target when part-time income covers part of retirement spending, a year-by-year retirement drawdown to a chosen age, a savings-rate ladder showing years to financial independence and a 24-month net worth log that feeds every other tab. The existing Coast FIRE tab matches the site's browser-based Coast FIRE calculator, which finds the portfolio size at which saving can stop. For planners who want multiple accounts, per-expense inflation and stress tests, the site's retirement planner runs the same historical data alongside Monte Carlo simulation, thousands of randomized market scenarios.
About firenum.com
firenum.com is a free retirement planning tool with Monte Carlo simulation, historical backtesting and stress testing. All calculations run in the browser with no data collection or account requirements. Downloadable spreadsheets and the Shiller S&P 500 dataset are available at firenum.com/tools.
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