Lean FIRE

It originates from William Bengen's 1994 research paper, which tested every rolling 30-year period of historical US stock and bond returns going back to 1926 and found that a 4% initial withdrawal rate, increased for inflation each subsequent year, never ran out of money in any of those historical periods for a balanced portfolio. The 1998 Trinity Study, from professors at Trinity University, used a similar approach and further popularized what's now commonly called the "4% rule."

Bengen's original research was built around a roughly 30-year retirement horizon starting at a traditional retirement age. Lean FIRE retirees often plan to stop working decades earlier, which means their portfolio needs to survive 40, 50, or more years -- a longer horizon gives sequence-of-returns risk (a market downturn early in retirement) more time to compound against a fixed withdrawal schedule, so a more conservative rate like 3.5% or lower is commonly recommended for very long retirements.

Because the withdrawal rate sits in the denominator of the target formula (annual spending divided by the rate), and it's already a small percentage. Dropping from a 4% to a 3.5% withdrawal rate, for example, increases the required portfolio by roughly 14%, even though the percentage-point difference looks small -- the smaller the rate, the more sensitive the resulting target becomes to further small changes in it.

No. It produces a single static portfolio target -- annual spending divided by the chosen withdrawal rate -- based on today's dollars and a fixed assumption. It does not simulate year-by-year investment growth, market volatility, inflation adjustments to spending, taxes, or other income sources like Social Security. Historical backtesting research like the Trinity Study, not this simple division, is what originally established that a given withdrawal rate tends to hold up over time.