The math behind the FIRE number calculation

The FIRE number calculation is rooted in the so-called “4% rule,” which was popularized in an influential 1998 research report known as the “Trinity study.” Included in the research was an examination of past market performance to determine a safe withdrawal rate in retirement.

The conclusion: In 99% of cases, retirees could withdraw 4% per year, adjusted for inflation, from a portfolio of stocks and bonds without running out of money.

When calculating your FIRE number, remember that the multiple of 25 is really just an easier way of dividing by a 4% withdrawal rate. Returning to to Sabatier’s earlier example, if you intend to spend $40,000 a year in retirement, divide by 0.04 to get to your million dollars.