The number and what drives it
At a 4% withdrawal rate, your FIRE number is 25 times annual expenses. Spending 40,000 a year means a target of 1,000,000. At a more conservative 3.5%, the multiple rises to roughly 28.6 times.
What determines how fast you get there is not income but savings rate, the share of income you do not spend. Someone saving 50% of their income reaches independence in roughly 17 years from zero, regardless of whether they earn 50,000 or 500,000, because higher spending raises the target as fast as it raises the saving.
Coast FIRE and the variants
Coast FIRE is the point where your existing portfolio, left completely alone, will grow into your full FIRE number by traditional retirement age. Reaching it means you never have to save another unit for retirement, only cover current expenses, which is a genuine and often overlooked form of freedom.
Other variants include Lean FIRE, targeting a deliberately modest lifestyle to finish sooner, Fat FIRE, targeting a comfortable one, and Barista FIRE, where part-time work covers the gap between a smaller portfolio and full independence.
The risks worth naming
A 40 or 50 year horizon is far longer than the 30 years the 4% rule was tested against, which argues for a lower withdrawal rate. Healthcare costs, particularly where they are not state-provided, are the most commonly underestimated line item. And the plan must survive a major crash in its first five years, which is where holding a cash buffer earns its keep.