The 4% rule, from the Trinity Study, states that a retiree can safely withdraw 4% of their initial portfolio (adjusted annually for inflation) for 30 years with a very high probability of not running out. For FIRE planning, multiply your annual expenses by 25 to find your target portfolio size. Someone spending $40,000/year needs roughly $1 million invested.

The common benchmark is 25× your annual expenses (the 4% rule). For longer retirements (40+ years) or more conservative planning, use 28–33× expenses (a 3–3.5% withdrawal rate). The longer your horizon, the more conservative you should be — a 60-year retirement is very different from a 30-year one.

Savings rate is the single biggest driver of time to FIRE. At a 25% savings rate, FIRE takes roughly 32 years; at 50%, about 17 years; at 75%, roughly 7 years. Cutting expenses has double the effect of raising income — it both increases your savings rate AND lowers the portfolio size you need.

The danger that poor market returns early in retirement permanently damage your portfolio. Two retirees with identical average returns can have wildly different outcomes depending on the order. Mitigations include a cash/bond buffer (2–3 years of expenses), flexible withdrawal strategies, or delaying retirement if you hit a major bear market at the start.

Generally not when you're early in your FIRE journey — it's safer to plan without it and treat it as upside. If you're closer to retirement age, Social Security can meaningfully reduce your required portfolio by providing a baseline income floor.