Coast FIRE is the amount you need invested today so that, if you never touch it, compound growth alone will carry it to your full retirement number by the time you retire. After hitting it, you only need to earn enough to cover your living expenses. No more saving required.
Coast FIRE is a milestone, not a finish line: once your current investments are large enough to grow to your full FIRE number on their own, you can stop contributing to retirement entirely. From there you just need to cover today's living expenses, which is a much easier bar to clear than saving for your whole future at the same time.
Your Coast FIRE number is your FIRE number discounted back to today at your expected return:
Someone with a $2M FIRE number retiring in 30 years at a 6% real return needs about $348,000 invested today, roughly $2M / (1.06)³⁰, to coast the rest of the way without saving another dollar.
Full FIRE means you never work again. Coast FIRE doesn't get you there immediately, you still need income to cover today's expenses. The difference is you're freed from saving for the future, since it's already funded. That opens the door to lower-paying, lower-stress work, or a slow transition into part-time hours. See Barista FIRE for the version of this where part-time income covers your expenses specifically.
Coast FIRE feels like a safety net more than a destination. Especially with AI reshaping tech jobs, it's genuinely reassuring to know I could get by on something like a low-stress job with health insurance if I had to. The number is already working for you, you just have to not panic and let it.