At some point on your savings journey, you hit a number where compound interest takes over. From that point, even if you never added another dollar to your retirement accounts, your existing investments would grow to fund your retirement entirely on their own. That number is called your Coast FIRE number. Here's how to calculate it, what to do once you hit it, and how it compares to other FIRE variations.
What Is Coast FIRE? (Plain English)
Coast FIRE is like cresting a hill on a bike. Once you've done the hard work of climbing, you can stop pedalling and let momentum carry you the rest of the way. In investing terms, the "hill" is getting enough money invested early enough that compound growth handles the rest. It's often the first major milestone on the path to financial independence because it takes the "retirement" problem off the table early in your career.
Essentially, once you hit your Coast FIRE number you can stop contributing to retirement accounts entirely. You still need to earn enough to cover your current living expenses — but you're no longer racing to build a nest egg. That changes everything about your relationship with work. It provides a level of freedom that regular FIRE takes much longer to achieve.
Instead of grinding for another 20 years to fully fund your retirement, you can decide to take a lower-stress job, go part-time, start something of your own, or simply stop feeling trapped in a high-paying role you don't enjoy. You're effectively "coasting" toward a guaranteed retirement while living the life you want today.
For many in the community, the psychological relief of hitting Coast FIRE is even greater than hitting the final number. It transforms your career from a necessity of survival into a choice. You're no longer working to buy your future; you're working to enjoy your present.
The Coast FIRE Formula — Step by Step
Calculating your coast number is a two-step process. First, you need to know how much you're aiming for in total. Then, you work backwards to see how much that would be in today's money if left alone to grow.
Step 1 — Calculate your FI Number:
FI Number = Annual retirement spending × 25
This comes from the 4% safe withdrawal rule. If you spend $50,000 per year, your FI Number is $1,250,000.
Step 2 — Calculate your Coast FIRE Number:
Coast Number = FI Number ÷ (1 + growth rate) ^ years to retirement
Target: $1,250,000 (FI Number)
Growth Rate: 7% (1.07)
Calculation: $1,250,000 ÷ (1.07)^30
Calculation: $1,250,000 ÷ 7.612
Coast Number: $164,183
The maths here is actually kind of remarkable — $164,000 left alone for 30 years becomes $1.25 million without you ever adding a single cent. That is the real power of starting early and letting compounding do the heavy lifting whilst you live your life.
Coast FIRE Calculator
Using our compound interest calculator is the easiest way to run your own scenarios. To check if you've hit Coast FIRE, enter your FI Number as the future value target, your current savings as the starting principal, the years until you retire as time, and 7% as the annual rate. If the calculator shows that your current savings already grow to your FI Number target without any additional contributions, you've officially reached Coast FIRE.
Coast FIRE Examples at Different Ages and Spending Levels
Scenario A
Age 35, Retire at 65 (30 yrs)
Spending Target: $50,000
FI Number: $1,250,000
$164,183
Coast FIRE Number
Scenario B
Age 28, Retire at 60 (32 yrs)
Spending Target: $45,000
FI Number: $1,125,000
$136,900
Coast FIRE Number
Scenario C
Age 40, Retire at 65 (25 yrs)
Spending Target: $60,000
FI Number: $1,500,000
$274,880
Coast FIRE Number
Coast FIRE Reference Table
Find your FI Number in the left column, then look across to your years until retirement to see your Coast FIRE target today.
| FI Number | 20 years | 25 years | 30 years |
|---|---|---|---|
| $500,000 | $129,000 | $92,000 | $66,000 |
| $750,000 | $194,000 | $138,000 | $99,000 |
| $1,000,000 | $259,000 | $184,000 | $131,000 |
| $1,250,000 | $323,000 | $230,000 | $164,000 |
| $1,500,000 | $388,000 | $276,000 | $197,000 |
| $2,000,000 | $518,000 | $368,000 | $263,000 |
Coast FIRE vs Barista FIRE vs Regular FIRE
There are several ways to define "retirement" in the FIRE community. Each has a slightly different objective based on your ideal lifestyle:
| Type | What It Means | Still Need to Work? |
|---|---|---|
| Regular FIRE | Fully funded retirement | No |
| Coast FIRE | Investments coast to FI on their own | Yes — cover living costs only |
| Barista FIRE | Coast FIRE + part-time job with benefits | Yes — part-time |
| Lean FIRE | Full retirement on minimal spending ($25K–$40K) | No |
| Fat FIRE | Full retirement on high spending ($80K+) | No |
Coast FIRE and Barista FIRE suit people who genuinely like working but hate financial pressure. Regular, Lean, and Fat FIRE suit people whose primary goal is to stop working entirely — the difference is just spending level. None is more valid than another; it depends entirely on what you want your life to look like.
Barista FIRE is a specific version of Coast FIRE where you work part-time specifically to access employer benefits — most commonly health insurance in the US. The name comes from the idea of working part-time at a place like Starbucks — not because it's glamorous, but because certain employers offer health insurance to part-time workers, solving the healthcare problem that stops many from retiring early.
What Happens to Coast FIRE if Markets Drop?
Market volatility is the biggest threat to any Coast FIRE plan. The 7% growth assumption used in most calculations is a long-run average — it smooths over years where markets fell 30% and years where they rose 25%. If you calculate your Coast Number using 7% and then retire into a decade of flat returns, you may find you need to contribute more than you originally planned to stay on track.
Show the concrete difference: using 5% instead of 7% for Scenario A:
Coast Number = $1,250,000 ÷ (1.05)^30 = $289,271
That's $125,000 more than the 7% estimate. The conservative approach — using 5% or 6% as your planning rate — builds a meaningful margin of safety. While it might take you an extra 2-3 years to hit your "safe" coast number, it protects your future retirement from the sequence of return risk that most FIRE articles skip over.
Frequently Asked Questions
What is a Coast FIRE number?
Your Coast FIRE number is the amount you need invested today so that — without making any further contributions — compound growth will grow it to your full FI Number by your target retirement age. At 7% annual growth, $164,000 invested at age 35 becomes $1.25 million by age 65. Once you hit your Coast FIRE number, you only need to earn enough to cover current living expenses.
How do I calculate my Coast FIRE number?
First calculate your FI Number: multiply your expected annual retirement spending by 25. Then divide that figure by (1.07) raised to the power of your years to retirement. For example: $50,000 annual spending gives an FI Number of $1,250,000. With 30 years to retirement, divide by (1.07)^30 = 7.612, giving a Coast Number of $164,183. You can also use our FIRE calculator to run this interactively.
What is the difference between Coast FIRE and regular FIRE?
Regular FIRE means your portfolio is already large enough to fund your retirement now — you can stop working entirely. Coast FIRE means your portfolio isn't there yet, but it will get there on its own through compound growth if you stop contributing and wait. You still need to work during the "coasting" period to cover living expenses, but you no longer need to aggressively save for retirement.
What investment return should I use in my Coast FIRE calculation?
The standard assumption is 7% annual growth — the approximate inflation-adjusted long-term average of US stock market index funds. For a conservative calculation with a margin of safety, use 5% or 6%. The difference matters: at 5%, your Coast Number for a $1.25M FI target over 30 years is $289,000 rather than $164,000. Building in that buffer protects you if the next 30 years are less generous than the historical average.
Can I reach Coast FIRE in my 30s?
Yes — for many people with high savings rates in their 20s, Coast FIRE in the early 30s is achievable. At age 30 with 35 years to retirement, the Coast Number for a $1.25M FI target is around $130,000 at 7% growth. Someone who invested aggressively in their 20s and has $130,000 by 30 has technically hit Coast FIRE — they just need to cover living costs for the next 35 years without adding to their retirement portfolio.
What is Barista FIRE and how is it different from Coast FIRE?
Barista FIRE is a specific version of Coast FIRE where you work part-time specifically to access employer benefits — most commonly health insurance in the US. The name comes from the idea of a low-stress part-time job that happens to include insurance coverage. The investment math is identical to Coast FIRE; the difference is the deliberate choice of a benefits-providing employer to solve the healthcare cost problem that many early retirees face.
What happens if I keep contributing after hitting my Coast FIRE number?
You accelerate toward full FIRE. Every additional contribution shrinks the time until your portfolio reaches your full FI Number. If you hit your Coast Number at 35 but keep saving aggressively, you might reach full FIRE at 50 instead of 65. Coast FIRE is a floor — the minimum you need today to stop worrying about retirement saving — not a ceiling. Continuing contributions is always fine; it just means you have options sooner.
Coast FIRE isn't a perfect system — it relies on return assumptions that may not exactly match your future. A decade of poor market returns, a shift in spending needs, unexpected costs: any of these can mean you need to revisit the numbers. But as a framework for understanding when compound interest can do the heavy lifting for you, it's one of the most useful concepts in personal finance. To run your own numbers, try our compound interest calculator — enter your current savings, your FI Number target, and your years to retirement, and it'll show you how close you already are. For more on how compound interest works at a mechanical level, or to calculate your true hourly rate and understand how much your time is actually worth, see the rest of the Cashluom blog.
Sources & Citations: Content verified against official guidelines from the IRS (US), HMRC (UK), and ATO (AU). Information is reviewed for accuracy prior to publication.
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