How does paying off my house change my FIRE number?

A paid-off house lowers your FIRE number because your withdrawal-funded expenses drop by the mortgage payment, and at 25x that reduction is dramatic. If your mortgage is $2,000 a month — $24,000 a year — eliminating it cuts your required portfolio by roughly $600,000. You'll still owe property taxes, insurance, and maintenance (budget around 1–2% of the home's value annually for upkeep), so model expenses after payoff, not zero housing cost. The catch is the cash you put toward the mortgage isn't invested, so there's a real trade-off between a guaranteed reduced expense and potential market growth. Many FIRE planners value the certainty: a paid-off home shrinks both your number and your sequence-of-returns risk, since you need to withdraw less in bad years. Decide based on your mortgage rate versus expected returns and your peace of mind.