Financial independence math is the same withdrawal-rate research used for traditional retirement planning — the "FIRE" (Financial Independence, Retire Early) movement just applies it to a much longer time horizon, which changes the math meaningfully.

The Numbers, Applied to an Early Retirement

The same core research applies: William Bengen's original 4% rule (1994), his 2025 updated 4.7% "Universal Safemax" with a more diversified portfolio, and Morningstar's more conservative 3.9% for 2026. The complication for FIRE specifically: these studies were built around a 30-year retirement horizon — someone retiring at 40 needs their money to last 50+ years, not 30, which most FIRE practitioners address by targeting a more conservative withdrawal rate (often 3-3.5%) than the standard 4% rule.

The detail that matters here: A longer time horizon isn't the only difference — early retirees also don't yet qualify for Medicare (65) or full Social Security, meaning healthcare costs and any income gap before those benefits kick in need separate, explicit planning, not just a bigger version of the standard retirement number.

Someone Targeting Retirement in Their 30s-40s: Use a more conservative withdrawal rate (3-3.5%) than the standard 4% rule, given the much longer horizon your savings need to cover.

Someone Pursuing "Barista FIRE" (partial early retirement with some part-time income): A lower withdrawal-rate requirement applies since part-time income covers part of expenses — recalculate your target number based on the smaller gap your investments actually need to fill.

Calculate Your FIRE Number

  1. Estimate your annual spending in early retirement, including any pre-Medicare healthcare cost estimate.
  2. Apply a conservative 3-3.5% withdrawal rate given your longer horizon (multiply annual spending by ~29-33).
  3. Separately plan for the healthcare and Social Security/Medicare gap years explicitly, not as an afterthought.

See how much money you need to retire for the full withdrawal-rate research this is built on.