Most retirement advice sounds like a life sentence.
Save 15% of every paycheck. For 40 years. Maximize your 401k. Don’t touch it. Work until 65.
Coast FIRE proposes something different — and the math behind it is genuinely surprising once you see it clearly.
The idea: if you invest enough money early enough, compound interest alone will grow it to your retirement number by the time you reach retirement age — without you contributing another dollar.
Once you hit that “Coast FIRE number,” you only need to earn enough to cover your current expenses. No more aggressive saving. No more retirement contributions. Just cover your life — and let what you’ve already invested compound to the finish line.
That’s Coast FIRE. Here’s exactly how it works.
What Coast FIRE Actually Means
FIRE stands for Financial Independence, Retire Early. Traditional FIRE requires accumulating 25x your annual expenses in invested assets — enough that a 4% annual withdrawal covers your lifestyle indefinitely.
Coast FIRE is a milestone on the path to full FIRE — or a destination in itself for people who want to reduce financial pressure without fully retiring early.
The core mechanism:
Money invested in a diversified stock market portfolio has historically averaged approximately 7% real returns annually over long periods (after inflation). At 7% growth, money doubles roughly every 10 years.
If you need $1,000,000 at age 65 to fund retirement, and you’re currently 35, you have 30 years of compounding ahead. At 7% real returns, money doubles approximately every 10 years — so it doubles three times between 35 and 65.
Working backwards: $1,000,000 / 2 / 2 / 2 = $125,000.
If you have $125,000 invested at age 35, you’ve theoretically hit your Coast FIRE number — the amount that, left untouched and growing at 7% real returns, reaches $1,000,000 by age 65 without another contribution.
How to Calculate Your Coast FIRE Number
Step 1: Determine your FIRE number
Annual expenses × 25 = FIRE number
If you spend $50,000/year: $50,000 × 25 = $1,250,000 FIRE number
Step 2: Determine your years until retirement
How old are you?? When do you want to retire?? The difference is your compounding runway.
Age 30, retire at 65: 35 years of compounding
Step 3: Calculate your Coast FIRE number
Coast FIRE Number = FIRE Number ÷ (1.07)^years
For the example above:
$1,250,000 ÷ (1.07)^35 = $1,250,000 ÷ 10.68 = $117,044
If you have $117,044 invested at age 30 in diversified index funds — and never contribute another dollar — it theoretically grows to $1,250,000 by age 65.
[INSERT Calculator: Coast FIRE number calculator — inputs: annual expenses, current age, retirement age, expected return rate → output: Coast FIRE number]
Coast FIRE Numbers by Age
Assuming $50,000 annual expenses and 65 retirement age:
| Current Age | Years to Retire | Coast FIRE Number |
|---|---|---|
| 25 | 40 | $83,200 |
| 30 | 35 | $117,000 |
| 35 | 30 | $164,000 |
| 40 | 25 | $231,000 |
| 45 | 20 | $325,000 |
| 50 | 15 | $457,000 |
The earlier you start, the lower your Coast FIRE number — because more years of compounding do more of the work.
What Happens After You Hit Coast FIRE
This is the part that makes Coast FIRE genuinely interesting as a life strategy.
After hitting your Coast FIRE number, you no longer need to save for retirement. Your existing investments handle that. You only need to cover your current living expenses.
The practical implications:
Lower income requirements. If you’re currently earning $80,000 and saving $15,000/year for retirement, hitting Coast FIRE means you only need to earn $65,000 — the amount that covers your actual expenses. You could take a lower-paying job you prefer, work part-time, freelance, or pursue work that’s meaningful rather than maximally compensated.
Reduced financial pressure. The anxiety of “am I saving enough??” — which 53% of Americans experience daily according to recent surveys — largely resolves once you know retirement is funded and you just need to cover today.
More flexibility. Geographic moves become more viable when income requirements drop. Career pivots become less financially risky. Sabbaticals become possible.
The Honest Limitations of Coast FIRE
The 7% return assumption isn’t guaranteed.
Historical averages are real. Future returns are not guaranteed to match historical averages. A significant market downturn early in your Coast FIRE period — before compounding has done substantial work — could require continued contributions to stay on track.
This is why Coast FIRE works best as a milestone to celebrate and use as permission to reduce financial pressure — not as a reason to completely stop monitoring your investment trajectory.
Inflation changes the math.
The $50,000 annual expenses you have today may not represent the same lifestyle in 35 years. Building inflation into your expense projections — using 2.5-3% annual expense growth — produces a more conservative and realistic Coast FIRE number.
Healthcare before 65 is expensive.
If Coast FIRE allows you to leave employer-sponsored health insurance before Medicare eligibility at 65, healthcare costs need to be factored into your annual expense calculation. This is a significant variable for anyone planning to Coast FIRE in their 40s or 50s.
Coast FIRE vs Other FIRE Variants
| Variant | What It Means | Best For |
|---|---|---|
| Traditional FIRE | Full financial independence — 25x expenses invested | People who want complete work optionality |
| Lean FIRE | FIRE on minimal expenses (under $40k/year) | Frugal minimalists |
| Fat FIRE | FIRE with high spending ($100k+/year) | High earners who don’t want to reduce lifestyle |
| Barista FIRE | Part-time work covers expenses, investments grow | People who want to semi-retire early |
| Coast FIRE | Hit investment milestone, work only covers current expenses | People who want reduced financial pressure without full retirement |
Coast FIRE is the most accessible variant for most people — because the target number is significantly lower than full FIRE and achievable earlier in a career.
FAQ
What is Coast FIRE??
Coast FIRE is a financial independence milestone where you’ve invested enough money that compound growth alone will fund your retirement — without additional contributions. After hitting your Coast FIRE number, you only need to earn enough to cover current expenses, not save for retirement.
How do I calculate my Coast FIRE number??
Multiply your annual expenses by 25 to get your full FIRE number. Then divide by (1.07) raised to the power of your years until retirement. The result is the amount you need invested today to reach your FIRE number through compound growth alone.
What do I do after hitting Coast FIRE??
You stop mandatory retirement saving and work only to cover current expenses. Many Coast FIRE achievers use this milestone to transition to lower-paying work they prefer, reduce hours, freelance, or take career risks they couldn’t afford previously.
Is Coast FIRE realistic??
For people who start investing early and consistently, yes. The math is straightforward — the challenge is accumulating the initial investment milestone. Building the financial foundation first, eliminating high-interest debt, and then aggressively investing creates the conditions for Coast FIRE.
What are the risks of Coast FIRE??
The primary risks are sequence-of-returns risk (market downturns early in the coasting period), inflation eroding purchasing power, and healthcare costs if retiring from employer coverage before Medicare eligibility at 65. These risks are manageable with conservative assumptions and periodic portfolio monitoring.