There is a particular thrill in the Coast FIRE question, because the answer is always smaller than people expect. You do not need a million dollars at 40. You need enough that compounding does the rest while you get on with your life. Let me show you the actual numbers.
The formula
Your Coast FIRE number is your full FIRE number discounted back to today:
Coast FIRE number = FIRE number / (1 + r)^n
Where the FIRE number is your annual retirement spending divided by your safe withdrawal rate (spending / 0.04, the familiar 25x rule), r is your expected annual real return, and n is the years until your target retirement age. The standard assumptions in the FIRE community are a 4% withdrawal rate, 7% returns, and retirement at 65.
The age-40 answer
Take someone who spends $50,000 a year. Their FIRE number is $50,000 / 0.04 = $1,250,000. At age 40 with 25 years to 65 and 7% annual returns, the discount factor is 1.07^25, which is about 5.43. Divide it out:
$1,250,000 / 5.43 = $230,314
That is the number: about $230,000 invested at age 40, left completely alone, grows to $1.25 million by 65 at 7% a year. No more contributions required. You still have to work and cover your bills, but the retirement saving is done.
If you spend $60,000 a year instead, the FIRE number is $1.5 million and the Coast number at 40 is about $276,000. At $40,000 of spending, it is $1 million and $184,000.
The full table
Here is the Coast FIRE number by current age, assuming a $1.25 million FIRE target ($50k annual spending), 7% returns, and retirement at 65:
| Current age | Years to 65 | Coast FIRE number |
|---|---|---|
| 30 | 35 | $117,077 |
| 35 | 30 | $164,210 |
| 40 | 25 | $230,314 |
| 45 | 20 | $323,023 |
| 50 | 15 | $453,071 |
Notice how punishing delay is: every five years you wait, the number grows roughly 40%. Time is the entire engine of Coast FIRE. A 30-year-old needs barely half what a 40-year-old needs, which is why the FIRE community is obsessed with starting early. It is not motivational poster talk; it is the exponent.
What the number assumes (read this part)
I like this formula, but I do not trust it blindly, and neither should you. Three assumptions deserve scrutiny:
- The 7% return. That is roughly the long-run real return of a stock-heavy portfolio, but "long-run average" and "your specific 25 years" are different things. Run your number at 5% too: at 5%, the age-40 Coast number for the same $1.25 million target jumps to about $369,000. If the gap between your 7% number and your 5% number makes you nervous, that nervousness is information.
- Your spending stays flat in real terms. The formula assumes the $50,000-a-year life you project at 40 is the life you still want at 65. Kids, health costs, housing moves, and lifestyle drift all push spending around. Coast FIRE works best when your spending estimate has a margin in it.
- You never touch the money. The whole edifice rests on leaving the portfolio alone for decades. Raiding it for a house down payment or a business resets the clock. Mentally, I treat the Coast portfolio as belonging to my 65-year-old self, not to me.
What changes once you hit it
This is the part the formula cannot show you. Hitting your Coast number does not mean quitting; it means the pressure to maximize income evaporates. You can take the lower-paying job you actually like, go part-time, start the business, or simply stop agonizing over every raise. Your paycheck only has to cover today. Your 65-year-old self is already funded.
That psychological shift is, in my opinion, the real product. The spreadsheet is just the receipt.
What if you want to retire before 65?
Sixty-five is the default assumption, but many Coast FIRE pursuers are really aiming to stop full-time work earlier, with the portfolio covering a traditional retirement age they might still reach at 65. If instead you want the portfolio to fully fund an earlier retirement, the number rises because compounding gets fewer years. For our $50,000 spender at age 40:
| Retirement age | Years of growth | Coast FIRE number |
|---|---|---|
| 60 | 20 | $322,985 |
| 65 | 25 | $230,314 |
| 70 | 30 | $164,205 |
Five fewer years of compounding costs you nearly $93,000 more today. That is the price of time, stated plainly. It is also why Coast FIRE pairs so naturally with "work longer at something you like": every year you push the retirement age out, the number drops meaningfully.
Closing the gap
If you run your numbers and find you are $40,000 short of your Coast number at 40, the gap is more closable than it looks. The practical move is a focused sprint: redirecting $800 a month for three years at 7% accumulates roughly $32,000, and letting that sit for the remaining 22 years roughly quadruples it. Short, intense saving bursts in your late 30s and early 40s are disproportionately powerful because every dollar still gets two decades of compounding. That is the Coast FIRE superpower most people miss: you do not need a high savings rate forever, just a high one now.
Frequently asked questions
How much do I need to Coast FIRE at 40?
With $50,000 of annual spending, a 4% withdrawal rate, 7% returns, and retirement at 65, the Coast FIRE number at 40 is about $230,000. At $60,000 of spending it is about $276,000. Your number scales with your spending and the return you assume.
What is the Coast FIRE formula?
Coast FIRE number = FIRE number / (1 + r)^n, where the FIRE number is annual retirement spending divided by your safe withdrawal rate (usually 4%, hence 25x spending), r is your expected annual return, and n is years until retirement.
Is 7% return realistic for Coast FIRE calculations?
Seven percent is roughly the long-run real return of a stock-heavy portfolio and is the standard assumption, but your actual 25-year window will differ. Run the number at 5% as a stress test; at 5% the age-40 Coast number for a $1.25M target rises from about $230,000 to about $369,000.