Coast FIRE is the point where your retirement savings can finish the job on their own. Once the invested balance is large enough that compound growth alone will carry it to your target by, say, 65, you can stop contributing and simply cover today's bills. You still work, but every dollar earned is for now, not for later.
The math depends on time. Suppose you want $1,500,000 at 65 and expect 7% average annual growth. At 35, you would need about $197,000 invested to coast there. At 40, about $276,000. At 45, about $387,000. The earlier the balance is in place, the smaller it needs to be.
What coast FIRE buys is flexibility. It makes a lower-paying but more meaningful job affordable, or a few years of part-time work while children are small, without giving up on retirement. It is also a useful midpoint to celebrate on the way to full financial independence.
The risk is that the projection is only as good as its assumptions. Seven percent is a long-run average that includes decades of both boom and bust; a bad ten-year stretch right after you stop contributing can leave the balance short. Many people keep contributing something, or set a coast number using 5% or 6% for a margin of safety.
The common mistake is hitting the coast number and then spending down the balance, or counting unvested stock toward it.