Your Coast FIRE number is the amount you need invested today so that, with no further contributions, it grows to your full FIRE number by the age you want to retire. The formula is:
Coast FIRE number = FIRE number ÷ (1 + real return) ^ years until retirement
A 30-year-old who wants $1.5 million by 60 and assumes a 6% after-inflation return needs about $261,000 invested today. Once you have that, saving for retirement becomes optional. Your income only has to cover what you spend now.
What is Coast FIRE? #
Traditional FIRE means having enough to stop working entirely. Coast FIRE is an earlier milestone: enough that time alone will get you to that number by a normal retirement age. You keep working, but only to pay current bills, which opens up part-time work, a lower-stress career or long breaks.
Once you reach it, the next question is what to do with that freedom; our guide to the Coast FIRE transition covers when to actually stop saving and how to move on to full retirement.
The Coast FIRE formula, explained #
Coast FIRE number = FIRE number ÷ (1 + r) ^ t
- FIRE number: the amount you’ll need at retirement, usually annual spending x 25.
- r: your expected real return, meaning after inflation. Planners often use 5% to 7% for a stock-heavy portfolio.
- t: years from now until you plan to start withdrawing.
Use a real return, not a nominal one. With a real return your FIRE number stays in today’s dollars. Plug in a nominal 10% and your Coast number comes out far too low.
Step by step: calculate your Coast FIRE number #
Step 1: estimate your retirement spending #
Use what you expect to spend each year once retired, in today’s dollars. Costs like commuting and retirement savings disappear; others, like health insurance and travel, may rise. Our example uses $60,000.
Step 2: calculate your FIRE number #
Multiply by 25 for a 4% withdrawal rate:
$60,000 x 25 = $1,500,000
If you expect a pension or want to count Social Security, subtract that income first. That lowers your FIRE number and your Coast number. The details are in how to calculate your FIRE number.
Step 3: count your years of growth #
- Current age: 30
- Retirement age: 60
- Years of growth: 30
Step 4: pick a real return #
| Assumption | Real return |
|---|---|
| Conservative | 5% |
| Moderate | 6% |
| Optimistic | 7% |
The example uses 6%.
Step 5: run the math #
1.06^30 ≈ 5.7435
$1,500,000 ÷ 5.7435 ≈ $261,165
If you’re 30 with about $261,000 in broad index funds, and you never add another dollar, the balance is projected to grow to about $1.5 million in today’s buying power by 60.
Retire Goals has a Coast FIRE calculator that runs this for you: it finds the amount that grows to your FIRE number by retirement age with zero further contributions. It’s quick to rerun at different returns and retirement ages, which is exactly what you should do next.
How your assumptions change the answer #
For a $1.5 million FIRE number, starting at age 30:
| Retire at | 5% real return | 6% real return | 7% real return |
|---|---|---|---|
| 60 (30 years) | $347,066 | $261,165 | $197,051 |
| 65 (35 years) | $271,935 | $195,158 | $140,494 |
Two lessons jump out. The return you assume can move the answer by more than $100,000, so use a conservative one or add a buffer. And every extra five years of growth cuts the number sharply, which is why Coast FIRE works best when you start young.
Does your Coast FIRE number change as you age? #
Yes, it rises every year, because you have one less year of growth. At a 6% real return it rises about 6% a year, the same rate your untouched balance is expected to grow. That’s why crossing the line matters: once your balance reaches your Coast number, average growth keeps it roughly on track without new money.
Real markets don’t grow at the average every year. A bad decade can put you behind the line, so check again once a year.
Common Coast FIRE mistakes #
- Using a nominal return. It makes your number look far smaller than it is.
- Counting money you’ll spend sooner. A house down payment fund or emergency fund isn’t coasting toward retirement.
- Ignoring taxes on pre-tax accounts. A $1.5 million traditional 401(k) buys less than $1.5 million in a Roth. Build expected taxes into your spending.
- Forgetting health insurance. If you downshift to a job without benefits, your current spending rises.
- Assuming you’re done forever. If your retirement spending plans grow, your FIRE number and your Coast number grow with them.
Frequently asked questions #
Is Coast FIRE safe? #
It’s sound math with real risk. If returns disappoint for a long stretch after you stop contributing, you’ll fall short. Using a 5% real return, keeping an emergency fund and rechecking each year makes it much safer.
What’s the difference between Coast FIRE and Barista FIRE? #
With Coast FIRE, your job covers all your current spending and your portfolio isn’t touched. With Barista FIRE, your portfolio already covers part of your spending through withdrawals and a part-time job covers the rest. See Barista FIRE vs traditional retirement.
How does inflation affect my Coast FIRE number? #
If you use a real return, the result is already in today’s dollars. If you’d rather work in future dollars, inflate your FIRE number first and use a nominal return. Our guide to adjusting your FIRE number for inflation shows both methods.
Can I keep contributing after reaching Coast FIRE? #
Yes. Contributions after Coast FIRE either move your full retirement date earlier or build a safety margin. Many people keep contributing at least enough to collect their employer’s 401(k) match.