How should I account for taxes when calculating how much I need for FIRE?

The 25x and 4% guidelines describe gross withdrawals, but what you can spend is after tax, and where your money sits changes the bill. Withdrawals from a traditional 401(k) or IRA are taxed as ordinary income; qualified Roth withdrawals are tax-free; and taxable brokerage sales are taxed only on gains, often at favorable long-term capital-gains rates.

A practical approach is to estimate your retirement tax rate and inflate your spending target to cover it, or to build your number around after-tax spending and add a tax buffer. Early retirees can often keep taxes very low by managing income to stay in low brackets, harvesting gains at the 0% capital-gains rate, and doing Roth conversions in lean years. The IRS updates brackets annually, so treat any specific figures as moving targets and plan around ranges.