Coast FIRE calculator
Coast FIRE is the point where your invested balance is large enough to reach your retirement target on compounding alone, with no further contributions. You still need an income to live on — you just no longer need to save for retirement out of it.
Monthly contribution needed
Reaching Coast FIRE earlier costs more per month, but you can stop entirely once you get there. The last card is the alternative: contributing steadily all the way to retirement.
How this is calculated
Your coast number at a given age is simply the target discounted back at your expected return: reach that figure by that age, and growth alone carries you the rest of the way. Monthly contributions use the standard future-value-of-an-annuity formula, with the annual return converted to the monthly rate that actually compounds to it over twelve months rather than divided by twelve.
One thing worth understanding: your coast number grows at exactly the same rate your investments do. If you are behind it today, waiting does not help — the target moves away from you at the same speed you are moving toward it. Only contributions close that gap, which is why the figures above never suggest simply waiting.
Returns are assumed smooth. Real markets are not, and a long run of poor years early on changes the picture considerably. Treat the result as a direction, not a schedule.
See also the dividend reinvestment calculator.