How do I convert a target withdrawal rate into the portfolio size I need?

Divide your desired annual spending by your chosen withdrawal rate, expressed as a decimal. The formula is: portfolio needed = annual expenses / withdrawal rate.

Examples for $50,000 of yearly spending: at 4% you need $1.25 million (50,000 / 0.04); at 3.5% you need about $1.43 million; at 3% you need roughly $1.67 million. Notice how a small drop in the assumed rate meaningfully raises the target, which is the price of extra safety over long horizons. This is just the reciprocal of the multiple-of-expenses rule (1 / 0.04 = 25x). WealthSerene's FIRE Calculator at wealthserene.com/tools/fire-calculator does this math and layers in inflation and expected returns for you.