How to Calculate Your Coast FIRE Number: The Formula, the Math, and What It Actually Means
Of all the FIRE milestones, Coast FIRE may be the most psychologically powerful. It's the point at which you've already won — you just have to wait for the win to arrive.
Once you reach your Coast FIRE number, your existing investments will grow to your full FIRE number by traditional retirement age without any additional contributions. You no longer need to save aggressively. You just need to cover your current living expenses — whether from a job, part-time work, a side business, or any other source — and let compound interest do the rest.
Understanding exactly how to calculate this number, and what it means in practice, can radically change how you think about your financial situation today.
The Coast FIRE Formula
The formula for your Coast FIRE number has two components: your target FIRE number, and the compound growth that will occur between now and your intended retirement age.
Coast FIRE Number = Full FIRE Number ÷ (1 + r)^n
Where:
- Full FIRE Number = your target retirement portfolio (annual expenses × 25 using the 4% rule, or adjusted for your specific withdrawal rate)
- r = expected real (inflation-adjusted) annual return on your investments
- n = number of years between now and your intended retirement age
This formula is simply the present value of your FIRE number — what that future amount is worth today, given expected investment growth.
A Worked Example
Let's say your full FIRE number is $1,500,000 (targeting $60,000/year at a 4% withdrawal rate). You're 35 years old and plan to be fully financially independent by age 60 — 25 years from now. You expect a 7% real return on your investments.
Coast FIRE Number = $1,500,000 ÷ (1.07)^25
= $1,500,000 ÷ 5.427
= $276,508
If you currently have $276,508 invested, and you never contribute another dollar, that money at 7% real return will grow to approximately $1.5 million by age 60. You have already done the savings work required for retirement.
You still need income to cover your current living expenses between now and 60 — but you don't need to save any of it.
How Assumptions Change the Number
The Coast FIRE number is highly sensitive to your expected return rate and your time horizon. Small changes in these inputs produce large changes in the target.
Using the same $1,500,000 FIRE target, here's how the Coast FIRE number varies:
- Age 35 with 25 years to retirement at 7% real return: $276,508
- Age 40 with 20 years to retirement at 7%: $387,319
- Age 45 with 15 years to retirement at 7%: $542,936
- Age 35 with 25 years to retirement at 5% real return: $443,845
- Age 35 with 25 years to retirement at 6% real return: $351,481
The most important takeaway: every year you delay reaching Coast FIRE increases your Coast FIRE number, because you have fewer years of compounding available to you. The earlier you reach it, the less you need to accumulate.
What Real Return Rate to Use
For U.S. stock market investors, the historical long-run real (inflation-adjusted) annual return for a diversified equity portfolio is approximately 6.5–7%. For a mixed portfolio (e.g., 80/20 stocks/bonds), a real return assumption of 5–6% is more conservative and appropriate for longer planning horizons.
Many conservative planners use 5% real return to account for the possibility of below-historical returns in the coming decades. Using 7% is reasonable for historical calibration but does embed some optimism about future equity performance.
What to Do Once You Hit Your Coast FIRE Number
Coast FIRE doesn't automatically change anything. You still need income to live. What changes is the psychological weight of saving aggressively — you no longer need to maintain a high savings rate or maximize retirement contributions beyond what you want to do.
Options once you hit Coast FIRE:
- Reduce working hours or switch to a lower-stress job — you only need to cover living expenses, not maintain a high savings rate
- Pursue passion work with lower pay — the financial pressure to maximize income is gone
- Take more time off or an extended sabbatical — a gap year or extended travel becomes financially viable
- Transition to Barista FIRE — part-time work covering current expenses, letting investments compound to full FIRE
Coast FIRE is often called the "work optional" milestone — not because you stop working, but because you stop being financially dependent on the specific work you're doing now.
Calculate Your Exact Coast FIRE Number
The formula above is straightforward, but running it with your specific numbers — actual expenses, current portfolio, target retirement age, and realistic return assumptions — gives you an accurate target instead of a ballpark estimate.
Our Coast FIRE Calculator does the calculation instantly, shows you how the number changes across different return assumptions and retirement ages, and tells you how far you are from hitting it today.
→ Calculate Your Coast FIRE Number
Tracking your portfolio balance against your Coast FIRE number requires knowing your actual invested assets in real time. Empower's free investment tracker aggregates all your accounts in one place so you can see exactly where you stand.
Disclaimer: This article is for educational purposes only and does not constitute financial advice. Investment returns are not guaranteed. Consult a qualified financial professional before making investment decisions.