How the Coast FIRE number is worked out
It takes two steps. First, your full FIRE number: yearly spending divided by your withdrawal rate. Spending $50,000 a year at a 4% withdrawal rate means $1,250,000. Second, that target is discounted back to today by the return you expect between now and retirement:
Coast number = FIRE number ÷ (1 + return)years to retirement
Using a return after inflation keeps everything in today's dollars, so $50,000 means what $50,000 buys now.
Worked example
You are 30, want to retire at 65, and expect 5% a year after inflation. Your FIRE number is $1,250,000. Thirty-five years of 5% growth multiplies money by about 5.52, so you need $226,613 invested today to coast. With $60,000 saved you are $166,613 short.
Investing $1,000 a month gets you there in about 23 years, at age 53. From then on the balance grows to $1,250,000 by 65 with no new money. At $750 a month you would never quite catch up, because the target rises 5% a year too.
Why the number rises with age
The table shows what you would need at each age if you stopped saving then. Later means fewer years of growth, so the amount climbs until it equals the full FIRE number at retirement. That is the whole appeal of reaching coast early: money invested at 30 does far more work than money invested at 55.
What this leaves out
Returns are not steady. A bad decade right after you stop saving can push your date back by years, which is why many people keep a margin above the number. Taxes on withdrawals, Social Security or a pension, and changes in spending are not modelled. If you expect a pension, lower the spending figure by what it will pay. This is an estimate, not financial advice.
To see when you could stop working entirely, try the FIRE calculator. If you have an employer plan, check you are getting the full 401(k) match first.