What Is Coast FIRE? Definition, Formula, and How to Calculate Your Number

Table of Contents

• Quick answer
• Introduction
• What "Coast FIRE" Actually Means
• The Coast FIRE Formula
• Worked Example
• Coast FIRE vs Lean FIRE vs Fat FIRE vs Barista FIRE
• Why Age Changes Your Coast FIRE Number So Much
• What Coast FIRE Doesn't Account For
• How to Calculate Your Own Coast FIRE Number
• Frequently Asked Questions

Quick answer

Coast FIRE is a version of financial independence where you've saved and invested enough at a young enough age that compound growth alone, with no further contributions, will grow your portfolio to a full retirement number by a traditional retirement age. Once you hit your Coast FIRE number, you no longer need to save for retirement. You still need to work to cover today's living expenses, but the pressure to keep contributing to retirement accounts is gone, which is where the name comes from: your investments can "coast" the rest of the way on growth alone.

Introduction

Most financial independence, retire early (FIRE) strategies focus on a single number: how much you need saved to stop working entirely. Coast FIRE asks a different question: how much do you need saved right now, at your current age, so that compound growth alone gets you to that number by a traditional retirement age like 60 or 65, even if you never contribute another dollar?

That reframing matters because it changes what "progress" looks like. Instead of measuring progress purely by your current net worth against a distant target, Coast FIRE measures whether your current savings, left alone to grow, are already on track to reach that target on their own. Watching both over time is easier with a FIRE tracking app that projects your portfolio forward.

This guide explains what Coast FIRE means, the formula behind it, a full worked example, how it compares to Lean, Fat, and Barista FIRE, and what the strategy does and doesn't account for.

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What "Coast FIRE" Actually Means

Reaching your Coast FIRE number doesn't mean you stop working. It means you stop needing to save for retirement, because the money already invested, given enough time and an assumed rate of return, will compound into a full retirement number on its own by your target age.

You still need income to cover current living expenses, rent or mortgage, food, insurance, and everything else, so most people who hit Coast FIRE keep working. What changes is the relationship to that work: retirement contributions become optional rather than necessary, which opens the door to a lower-paying job, fewer hours, a career change, or simply less anxiety about whether you're saving "enough" every month.

The mechanism behind Coast FIRE is the same one behind all compound growth: money invested earlier has more time to grow, so it takes a smaller amount invested early to reach the same target than it would investing the same amount later. Coast FIRE is essentially the point where you've front-loaded enough of that early compounding that time can finish the job without further contributions.

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The Coast FIRE Formula

The calculation works backward from your full FIRE number using the standard compound growth formula, solved for the present value needed today.

Coast FIRE Number = Full FIRE Number / (1 + Rate of Return)^Years Until Retirement

Where:

Full FIRE Number is your target retirement portfolio, typically calculated as annual expenses × 25, based on a 4% safe withdrawal rate.

Rate of Return is the assumed average annual investment growth rate, commonly modeled at 7% for a diversified stock portfolio.

Years Until Retirement is the number of years between your current age and your target traditional retirement age.

The table illustrates the core insight behind Coast FIRE: the earlier you calculate it, the smaller the number required, because there's simply more time for growth to do the work.

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Worked Example

A 28-year-old wants to retire at age 65, a 37-year time horizon. Their target annual spending in retirement is $50,000/yr.

Full FIRE number at a 4% withdrawal rate:

$50,000 / 0.04 = $1,250,000

Assuming a 7% average annual return, their Coast FIRE number is:

$1,250,000 / (1.07)^37 ≈ $102,261

Input	Value
Target annual spending	$50,000
Full FIRE number	$1,250,000
Years until retirement (65)	37
Assumed annual return	7%
Coast FIRE number needed today	≈ $102,261

If this person already has roughly $102,261 invested at age 28, they don't need to contribute another dollar to retirement accounts for that money to grow into $1,250,000 by age 65, assuming a steady 7% average annual return. Anything they save beyond that point is either accelerating their timeline, increasing their eventual spending target, or building a cushion against a lower actual return.

Use the Compound Interest Calculator to try different ages, return rates, and target numbers.

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Coast FIRE vs Lean FIRE vs Fat FIRE vs Barista FIRE

Coast FIRE and Barista FIRE both describe a middle stage rather than a final destination, but they answer different questions. Coast FIRE is about your investment number: have you saved enough that you never need to invest another dollar? You keep working, often full-time, to cover today's expenses, and your portfolio stays untouched and compounding. Barista FIRE is about your income number today: have you reduced your required income enough that part-time work covers it, while you may be drawing on some of the portfolio's growth rather than leaving it fully untouched?

Lean FIRE and Fat FIRE describe the size of the eventual number relative to lifestyle. Coast FIRE describes a relationship between savings, time, and growth on the way to any of those numbers, whether the eventual target is a lean number or a fat one.

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Why Age Changes Your Coast FIRE Number So Much

The single biggest input in the Coast FIRE formula isn't your savings rate or even your target spending, it's how many years of growth you have left before your target retirement age. Because growth compounds, small differences in age translate into large differences in the Coast FIRE number:

• A 25-year-old targeting a 65-year age needs a smaller Coast FIRE number than a 35-year-old targeting the same retirement age and spending level, purely because of the extra decade of compounding.
• Someone who wants to retire earlier, say at 55 instead of 65, needs a larger Coast FIRE number today, since there's less time left for growth to close the gap.
• Reaching Coast FIRE later in life, in your 40s or 50s, is still mathematically possible, but requires either a larger portfolio already in place, a higher assumed return, or a later target retirement age.

This is also why Coast FIRE calculations are highly sensitive to the assumed rate of return. A 7% assumption versus a 5% assumption produces meaningfully different numbers over a multi-decade horizon, which is worth stress-testing rather than anchoring to a single optimistic figure.

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What Coast FIRE Doesn't Account For

It assumes a consistent average return over decades. Real markets don't grow in a straight line. A Coast FIRE calculation done at a single point in time doesn't capture sequence-of-returns risk, the possibility of a prolonged downturn early in the growth period, or years of below-average returns that push the timeline out.

It doesn't factor in inflation unless you build it in. The formula above uses today's dollars. A target spending level and full FIRE number decades in the future should typically be adjusted for expected inflation, or modeled using a real (inflation-adjusted) rate of return instead of a nominal one.

It says nothing about current living expenses. Coast FIRE tells you when you can stop contributing to retirement accounts, not whether your current income covers your current lifestyle. Someone can be "Coast FIRE" on paper and still be living paycheck to paycheck if their day-to-day expenses aren't under control.

It doesn't model taxes or account type. Growth inside a tax-advantaged account like a 401(k) or IRA behaves differently than growth in a taxable brokerage account, and required minimum distributions or early withdrawal penalties can affect when the money is actually accessible.

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How to Calculate Your Own Coast FIRE Number

Start with your target retirement age and target annual spending in retirement, then calculate your full FIRE number using the 25x rule (or your target spending divided by your chosen safe withdrawal rate). From there, work backward using an assumed rate of return to see how much needs to be invested today, then compare that to your actual current savings and investments.

A single static calculation is a reasonable starting point, but it doesn't capture how sensitive the result is to changes in your assumed return, your target retirement age, or a future decision to retire earlier or later. Calm Sea's projection tools let you enter your current savings, contribution rate, and target retirement age, then see year by year whether your portfolio is on a Coast FIRE trajectory, and how that trajectory shifts if your return assumptions, timeline, or spending target change.

No account linking required. Free tier available.

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Frequently Asked Questions

Calm Sea is a personal finance planning tool. Nothing in this article constitutes financial advice. All projections and calculations are illustrative estimates based on publicly available market data. Always conduct your own due diligence and consult a qualified financial adviser before making retirement decisions.