Coast FIRE means having enough invested so that, even without further contributions, your portfolio grows to your FIRE number by retirement.
The key formula: Coast FIRE Number = FIRE Target / (1 + r)^years to retirement.
Your FIRE target is typically 25× your annual expenses, based on the 4% safe withdrawal rule.
Coast FIRE means your current savings will grow to your retirement goal without additional contributions.
Regular FIRE requires accumulating the full amount. Coast FIRE only requires hitting a lower threshold today.