What is a Coast Number? A Coast Number is the invested-asset balance that, left entirely alone and compounding, will grow to your full FI Number by a chosen target retirement age. It is the minimum portfolio size needed to stop adding new savings and still reach FIRE on schedule.
Formula: Coast Number = FI Number ÷ (1 + r)^n, where r is your expected real return and n is years remaining. Worked example: a 28-year-old with a $1,250,000 FI target at age 60 and a 7% real return needs a Coast Number of $1,250,000 ÷ (1.07)^32 = $144,090 today. Every dollar above that is gravy; every dollar short extends the timeline.
The Coast Number drops fast with age because the compounding window shrinks. The same $1,250,000 target at age 40 (20 years of compounding) requires a Coast Number of $323,000, more than double. That is why hitting Coast FIRE in your 20s is disproportionately valuable: $145K saved by 28 equals $323K saved by 40 for the same outcome.
Many FIRE practitioners track their Coast Number monthly alongside their full FI Number, it is the first and most motivating milestone of the entire journey.