Your CoastFIRE Number Is Probably Smaller Than You Think. Here's How to Calculate It.

What CoastFIRE Actually Means

CoastFIRE is a specific threshold, not a philosophy. It is the portfolio value at which compound growth alone, with zero additional contributions, will produce your full retirement number by a target date.

Once you hit that threshold, you have stopped needing to save. You can redirect every dollar you were forcing into index funds toward current spending, a career change, a business, or simply working less. The market does the remaining heavy lifting.

This is meaningfully different from traditional FIRE, which requires you to accumulate the entire retirement portfolio before you can stop working full-time. CoastFIRE only requires you to accumulate enough that time and compounding finish the job.

The distinction matters financially. Traditional FIRE targets for someone expecting $80,000 per year in retirement typically land around $2,000,000, using the 4% withdrawal rule. A CoastFIRE target for the same person, calculated at age 35 with a 30-year runway, might be $330,000 to $380,000 depending on your assumed real return.

Those two numbers represent a very different savings timeline.

The Formula

The CoastFIRE formula has two steps.

Step one: establish your retirement number.

Multiply your expected annual retirement spending by 25. This applies the 4% rule, the withdrawal rate supported by the Trinity Study and its subsequent revisions. A person expecting to spend $90,000 per year in retirement needs $2,250,000.

Step two: discount that number back to today.

Divide your retirement number by (1 + real annual return) raised to the power of years until retirement.

Written formally:

CoastFIRE Number = Retirement Target / (1 + r)^n

Where r is your assumed real annual return, net of inflation, and n is years until your target retirement age.

Use a real return of 5% to 7% for a broadly diversified stock-heavy portfolio. The S&P 500 has delivered approximately 7% real returns over rolling 30-year periods historically. Use 5% to 6% if you want a conservative buffer.

Worked Example 1: The 35-Year-Old with a 30-Year Runway

Scenario: Age 35. Target retirement age 65. Expected annual retirement spending: $80,000. Assumed real return: 6.5%.

Retirement target: $80,000 x 25 = $2,000,000.

CoastFIRE calculation: $2,000,000 / (1.065)^30

(1.065)^30 = 6.6144

$2,000,000 / 6.6144 = $302,361

This person needs $302,361 invested today, in a growth portfolio, to reach $2,000,000 by age 65 with zero additional contributions. If their current balance is $302,000, they have hit CoastFIRE. They can stop the aggressive saving that got them there.

What If Their Current Balance Is Lower?

Say the same person has $190,000 today. Their CoastFIRE gap is $302,361 minus $190,000, which equals $112,361. At a 6.5% real return, $190,000 grows to $190,000 x 6.6144, which equals approximately $1,256,736 by age 65. They are short by about $743,000 in today's money. They still need to save.

But the calculation tells them exactly how much time they are buying with each dollar contributed. That precision is actionable in a way that vague savings targets are not.

Worked Example 2: The 42-Year-Old Closer to the Finish Line

Scenario: Age 42. Target retirement age 60. Expected annual retirement spending: $110,000. Assumed real return: 6%.

Retirement target: $110,000 x 25 = $2,750,000.

CoastFIRE calculation: $2,750,000 / (1.06)^18

(1.06)^18 = 2.8543

$2,750,000 / 2.8543 = $963,466

This person needs $963,466 invested today to coast to $2,750,000 over 18 years without saving another dollar.

That is a meaningful number. It is not $2,750,000. If they are currently at $900,000 and contributing $30,000 per year to their portfolio, they are approximately two years away from CoastFIRE, not eight.

Many people in this position assume they must keep grinding at the same savings rate until they hit the full retirement number. The compound math says otherwise.

The Variables That Move Your Number the Most

Real Return Rate

The difference between a 5% and 7% real return assumption on a 30-year CoastFIRE target is substantial. Using a $2,000,000 retirement target:

• At 5%: CoastFIRE number = $2,000,000 / (1.05)^30 = $2,000,000 / 4.3219 = $462,762
• At 6.5%: CoastFIRE number = $2,000,000 / 6.6144 = $302,361
• At 7%: CoastFIRE number = $2,000,000 / (1.07)^30 = $2,000,000 / 7.6123 = $262,722

A one-point difference in assumed return moves your CoastFIRE number by $80,000 to $100,000. Be honest about your asset allocation. A 60/40 portfolio should not use a 7% real return assumption.

Years to Retirement

More time means a lower CoastFIRE number. A person with 35 years until retirement needs far less invested today than a person with 20 years. Every additional year of runway cuts your required balance by roughly 6% to 7% at typical return assumptions.

A 30-year-old planning to retire at 65 has a meaningfully easier CoastFIRE target than a 45-year-old with the same retirement goal, even if they earn the same income.

Retirement Spending Estimate

This drives the retirement target, which drives everything else. The 4% rule produces a $2,000,000 target at $80,000 per year and a $2,750,000 target at $110,000 per year. A $30,000 difference in annual spending produces a $687,500 difference in the retirement target, which flows directly into the CoastFIRE number.

Be specific about what you plan to spend. Include healthcare, which adds $5,000 to $12,000 per year per person in most retirement budget models.

What to Do With the Extra Cash Flow After CoastFIRE

Hitting your CoastFIRE number does not mean spending recklessly. It means redirecting forced retirement savings to uses with better immediate utility.

Common redirections:

• Funding a career transition to lower-paying, more satisfying work
• Paying down a mortgage early if you carry a rate above 6%
• Building a taxable brokerage account for flexibility before age 59.5
• Funding a business with excess cash rather than retirement accounts

The point is that you regain discretion. You are no longer obligated to maximize contributions to preserve a retirement you have already mathematically secured.

The Risk to Account For

CoastFIRE depends on a long-term real return assumption you cannot guarantee. A decade of poor equity performance, a significant lifestyle inflation event, or earlier-than-expected health costs can erode the math.

Three adjustments reduce this risk.

First, use a conservative return assumption. Model at 5.5% to 6% rather than 7%. Your CoastFIRE number will be higher and your buffer will be larger.

Second, revisit the calculation every two years. If your portfolio has underperformed, you may need to resume contributions for a period.

Third, maintain flexibility in your retirement spending estimate. A person who can reduce retirement spending from $90,000 to $75,000 without major lifestyle disruption has a much smaller retirement target and a much more achievable CoastFIRE number.

Run Your Actual Number

The worked examples above use clean figures for clarity. Your real CoastFIRE number depends on your current balance, your specific return assumptions, your target retirement age, and your actual projected spending.

The CalcMoney Retirement Calculator runs all four variables simultaneously and shows you the exact gap between where your portfolio sits today and where it needs to be for CoastFIRE. It also projects how your current contributions close that gap year by year, so you can see precisely when you can stop.

If you are within five years of your CoastFIRE number and still maxing every account on autopilot, the calculator will show you what you are giving up. That is the number worth knowing.

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