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.

As with any other one-step financial calculation, the FIRE number math is based on several assumptions and will vary based on your financial situation. Here’s what retirement experts say you need to know to figure out how much money you’ll need to retire.