How do I calculate my FIRE number?

Your FIRE number is the portfolio size that lets you live off withdrawals indefinitely, and the quick rule is annual spending × 25. Start with your real annual expenses, not your income — if you spend $60,000 a year, your target is roughly $1.5 million. The 25x multiple is just the inverse of a 4% withdrawal rate, so a more conservative 3.5% rate (28.6x) pushes the same spending to about $1.7 million. Use post-tax spending and add any retirement taxes, healthcare premiums, and lumpy costs (a new roof, a car every decade) that won't show up in a normal month. If a paid-off mortgage is coming, model expenses after that drops off. Tighten the estimate with at least 12 months of actual spending data rather than a guess. Run your own numbers at wealthserene.com/tools/fire-calculator.