Coast FIRE: The Point Where Compounding Finishes the Job

The companion calculator computes your Coast FIRE number, the amount that, left alone with no further contributions, will grow through compounding into a full FIRE number by your target retirement age. Reach it, and you can stop saving for retirement and cover only current expenses through a lower-stress job or career break, while compounding finishes the job. The Coast FIRE number is simply the future FIRE goal discounted back to today, a direct application of present value, and it shrinks dramatically the earlier you reach it. Understanding present value, why Coast FIRE works, why time makes it so much smaller when young, and how to use it turns a Coast FIRE calculation into an appreciation of letting compounding do the work. This is informational, not personalized financial advice.

Letting Compounding Finish the Job

Coast FIRE is the milestone where your existing savings, with no further contributions, will compound on their own into a full FIRE number by your target retirement age, so once you reach it, you no longer need to save for retirement, only to cover current living expenses. Reaching the coast point means your invested balance is large enough that, given your expected return and the years until retirement, compounding alone will grow it to the amount you need, so you can "coast": stop retirement contributions and let the existing savings finish growing, freeing you to take a lower-stress or part-time job, a career break, or a passion job that covers only today's expenses, as the calculator's premise describes. This is powerful because it decouples the point at which you can ease off saving from the point of full retirement: you don't need the whole FIRE number now, just enough that it will grow to the FIRE number by retirement, which is a smaller, earlier target. The Coast FIRE number is thus the balance needed today for compounding alone to reach the full goal, as the calculator computes, so understanding it means understanding that time and compounding can complete the accumulation without further contributions. Recognizing that Coast FIRE lets compounding finish the job is the starting point for appreciating the present-value math behind it. Understanding that Coast FIRE lets compounding finish the job is the starting point: reaching a balance that will grow to the full FIRE number by retirement with no more contributions lets you stop saving and coast. The calculator computes the Coast FIRE number; understanding coasting is what reveals what the number means, the balance needed for compounding alone to reach the goal, so the Coast FIRE number the calculator computes is an earlier, smaller target than full independence.

Present Value: Discounting the Goal Back to Today

The Coast FIRE number is the full FIRE number discounted back from retirement to today at your expected return, an application of present value: the amount today that will grow to a future goal is the goal divided by the growth factor over the years.

Present value is the flip side of compounding: just as a sum today grows to a larger future value, a future goal has a present value, the amount today that will compound to it, found by dividing the future goal by the growth factor (one plus the return, raised to the number of years), as the calculator computes the Coast FIRE number as the FIRE number divided by that growth factor. So the Coast FIRE number is the present value of the future FIRE number: it's what you need now so that, growing at your expected return for the years until retirement, it reaches the full FIRE number, as the calculator's formula shows (Coast FIRE number equals FIRE number over one-plus-return to the years). This is the same discounting used throughout finance to value future amounts today, so Coast FIRE is a direct application of the time value of money, viewed from the present-value side rather than the future-value side. The longer the time until retirement, the larger the growth factor, so the smaller the present value (Coast FIRE number) needed, which is the key to why reaching Coast FIRE early requires far less money. Understanding that the Coast FIRE number is a present value, the discounted goal, reveals the math beneath it and connects it to compounding's mirror image. This present-value framing is what makes Coast FIRE a precise, computable milestone. Understanding present value reveals the Coast FIRE math: it is the future FIRE number discounted to today at your return, the amount now that compounds to the goal. The calculator discounts the goal to today; understanding present value is what reveals why the Coast FIRE number is what it is, it's the present value of the future need, so the number the calculator computes is the FIRE goal discounted back through the years of expected growth.

Why Reaching It Early Requires Far Less

Because the Coast FIRE number is the goal discounted over the years until retirement, the more years of compounding remain, the smaller the number, so reaching Coast FIRE young requires a much smaller balance than reaching it later, dramatically rewarding early saving. Since discounting divides by a growth factor that increases with the number of years, a longer horizon (younger age) means a larger divisor and thus a smaller present value, so the Coast FIRE number is much smaller for a 25-year-old than a 45-year-old aiming at the same retirement goal, because the younger person's savings have more time to compound, as the calculator's table shows the Coast FIRE number rising steeply with current age. This means a relatively modest sum saved early can, through decades of compounding, grow into a full FIRE number without any further contributions, so early aggressive saving can achieve Coast FIRE with a surprisingly small balance, and the required amount roughly doubles for every decade or so less time to compound, as the calculator's note explains. This is the front-loaded power of compounding viewed from the present-value side: time is so valuable that reaching the coast point early takes far less money, which is a strong argument for saving heavily when young. Understanding why reaching Coast FIRE early requires far less reveals the enormous advantage of time and early saving, and why the Coast FIRE number climbs steeply as the horizon shortens, so the calculator's age-based figures illustrate the reward for starting early. This insight motivates front-loading retirement saving to reach the coast point sooner. Understanding why reaching it early requires far less reveals time's power: discounting over more years shrinks the Coast FIRE number, so reaching it young takes far less money, rewarding early saving. The calculator shows the number rising with age; understanding the discounting is what reveals why early saving is so advantageous, more compounding time means a smaller required balance, so the Coast FIRE numbers the calculator computes are far smaller for the young, the reward for front-loaded saving.

Using Coast FIRE as a Milestone

The practical value is that Coast FIRE provides an earlier, motivating milestone and a decision point: once you've coasted, additional income need only cover current expenses, freeing career choices, so computing it, as the calculator does, informs downshifting, sabbaticals, and passion jobs, this is informational, not personalized advice. The calculator computes your Coast FIRE number and whether your current savings have reached it, so you know if you've hit the point where you can stop retirement contributions and let compounding finish, which is a concrete, earlier target than full FIRE, motivating for long-horizon savers, as its context notes for milestone-based motivation. Reaching it enables real choices: downshifting to a lower-stress or lower-paying job, taking a sabbatical, or pursuing a passion role, since additional income only needs to cover today's expenses, not retirement savings too, so Coast FIRE offers flexibility well before full retirement, as the calculator's context describes for downshifting careers and evaluating a sabbatical. It also rewards early aggressive saving by showing how a smaller balance reached young suffices, encouraging front-loading contributions, as the calculator's context notes reaching it early requires far less. Because the growth relies on an assumed return over many years, and real returns vary, the Coast FIRE number is a planning estimate, not a guarantee, so it guides decisions rather than promising outcomes, ideally with professional advice. Used as a milestone, Coast FIRE turns the present-value insight into an actionable, freeing target, letting you know when compounding can take over. Understanding how to use Coast FIRE as a milestone completes the picture: it provides an earlier target and a decision point, enabling career flexibility once reached, informed by the computed number, as the calculator supports. The calculator computes the Coast FIRE number and whether you've reached it; understanding present value and the power of time is what reveals how to use it, reaching the coast point frees you to let compounding finish, so computing it, as the calculator does, gives an earlier milestone that can reshape career choices, as a planning estimate. This is informational, not personalized financial advice.

Understanding Coast FIRE

Use the calculator to compute your Coast FIRE number, and understand the idea: it is the balance that, with no further contributions, will compound into a full FIRE number by retirement, calculated as the future FIRE goal discounted to today (its present value) at your expected return, so the more years of compounding remain, the smaller the number, meaning reaching it young requires far less money. The calculation discounts the goal to today; understanding present value and the power of time is what reveals why Coast FIRE works and why early saving is so rewarding, once reached, you can stop saving and let compounding finish, freeing career choices, making Coast FIRE an earlier, motivating milestone. This is informational, not personalized financial advice.