What Is Coast FIRE? How to Calculate Your Coast FIRE Number
The point where you can stop saving for retirement and let compound growth finish the job on its own — here's the math behind it.
Coast FIRE is the point where you've saved enough toward retirement that, even if you never contribute another dollar, compound growth alone will carry your current balance to your full retirement target by the time you reach your target retirement age. Once you hit that number, you're technically done saving for retirement specifically — you can "coast" on growth alone. You still need income to cover your day-to-day living expenses, but the pressure to keep funding a retirement account disappears, which is exactly why the concept has become popular with people who want more flexibility in their career without fully retiring.
1. Coast FIRE vs. regular FIRE: a different kind of freedom
Traditional FIRE (Financial Independence, Retire Early) means saving enough that your investments alone can cover your entire living expenses, starting now — the goal is to stop working completely. Coast FIRE is a narrower, often more achievable milestone: you've saved enough for retirement itself, but you still need to earn income to cover current living costs. The freedom Coast FIRE unlocks isn't "stop working" — it's "stop worrying about retirement savings," which for many people opens the door to a lower-stress job, a career change, part-time work, or simply not feeling obligated to chase every raise or promotion.
2. The formula
Your Coast FIRE number is the amount you'd need invested today so that, left alone to grow, it reaches your full retirement target by your target retirement age:
For example, if your retirement target is $1,500,000, you're 30 years from retiring, and you assume a 7% average annual return, your Coast FIRE number works out to roughly $197,000. Once your invested balance hits that figure, growth alone — without another dollar contributed — mathematically gets you to $1,500,000 by the time you retire, assuming that return holds.
3. A worked example
Say you're 30 years old, want to retire at 65 with $1,500,000, and assume a 7% average annual real return. That's 35 years of growth. Dividing $1,500,000 by 1.07 raised to the 35th power gives a Coast FIRE number of roughly $138,000. If you already have $140,000 invested at age 30, you've technically hit Coast FIRE — every dollar you save toward retirement from this point on is optional, not required, assuming the return holds over that time.
Notice how sensitive this is to your timeline: the same $1,500,000 target at age 45 with only 20 years left would require a Coast FIRE number of roughly $387,000 — more than double — because there's far less time for growth to do the work. This is why Coast FIRE is generally an easier milestone to hit the younger you start.
4. Where the return rate assumption really matters
The return rate you plug into the formula has an outsized effect on the result, precisely because it's raised to a power over many years. A 5% assumption produces a noticeably larger, more conservative Coast FIRE number than a 7% assumption, and the gap widens the longer your time horizon is. There's no single correct rate to use — it depends on your portfolio's actual asset allocation and how conservative you want to be. Using a lower rate builds in a safety margin; using a higher one gets you to "I've hit my number" faster, at the cost of more risk if actual returns fall short.
5. What Coast FIRE doesn't solve
Hitting your Coast FIRE number says nothing about your current living expenses, your emergency fund, or debt you're still carrying. It's specifically a retirement-savings milestone, not a full financial independence milestone. Someone can hit Coast FIRE while still needing a steady paycheck to cover rent, and that's normal — the whole point of the concept is separating "am I on track for retirement" from "can I afford my life today," which are two different questions with two different answers.
6. How to find your own number without a dedicated calculator
You don't need a purpose-built Coast FIRE tool to run this math — our retirement savings calculator can do it with one adjustment. Set your retirement target and target age as usual, but set your ongoing monthly contribution to zero. Then adjust your current savings figure up or down until the projected balance at retirement matches your target. Whatever current-savings number gets you there is your Coast FIRE number, using the same growth assumptions you'd already trust for the rest of your retirement planning.
A caveat worth taking seriously
This entire framework assumes uninterrupted growth at your chosen rate for decades, with zero further contributions — a real market doesn't move in a smooth line, and a bad stretch of returns early on can push your actual balance meaningfully off track even if you technically "hit" your Coast FIRE number on paper. Treat the number as a green light to ease off, not as proof the math is now guaranteed. Checking back in periodically, rather than assuming you're set once and done, is worth the occasional five minutes.
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- → Lean FIRE vs Fat FIRE: which early retirement path fits you?
- → How to bridge healthcare before Medicare when you retire early
Frequently Asked Questions
Coast FIRE is the point at which you have saved enough for retirement that, left untouched, compound growth alone will carry your investments to your full retirement goal by your target age, even if you never contribute another dollar.
Traditional FIRE means saving enough to stop working entirely, right now. Coast FIRE only means you can stop saving for retirement specifically. You still need to cover your current living expenses through work, just without the pressure of also funding retirement.
Coast FIRE number = Retirement target ÷ (1 + expected annual return)^years until retirement. This tells you how much you need invested today for growth alone to reach your retirement target by your target age.
Not entirely. It means you're free from needing to save more for retirement, but you still need income to cover today's living costs. Many people use Coast FIRE to justify a lower-paying but more enjoyable job, or to work part-time, rather than quitting work altogether.
Most people use a long-term average real return somewhere between 5% and 7%, depending on how conservative they want to be. A lower assumed return produces a larger, more conservative Coast FIRE number.
Yes. Enter your retirement target and target age, set your current contributions to zero, and adjust your current savings until the projected balance at retirement matches your goal. Whatever savings figure gets you there is your Coast FIRE number.
Find your FIRE number directly, or compare it against a Coast FIRE approach where growth alone gets you there.