What Is a Coast FIRE Number and How Do You Calculate Yours?

What Is a Coast FIRE Number?

Your Coast FIRE number is the amount of money you need invested right now — at your current age — such that, even if you never contribute another dollar, your portfolio will grow to your full retirement target by the time you reach traditional retirement age.

Once you hit it, you've removed the retirement savings pressure from your financial life. You still need to earn enough to cover day-to-day expenses. But you no longer need to save for the future — the future is already funded. Your investments are "coasting" to the finish line.

It's a different kind of milestone than full FIRE. It doesn't mean you can stop working entirely. But it means the retirement question is essentially answered, which changes your relationship to your career and financial decisions in meaningful ways.

For a deeper understanding of how Coast FIRE fits into the broader landscape of FIRE approaches, see our Coast FIRE explainer.

The Math Behind It

Coast FIRE uses the future value of money formula, rearranged to solve for the present value you need today.

The future value formula:

FV = PV × (1 + r)^n

Where:

• FV = Future value (your full FIRE number at retirement)
• PV = Present value (what you need invested today — your Coast FIRE number)
• r = Annual return rate (as a decimal)
• n = Years until retirement

Rearranging to solve for the present value (your Coast FIRE number):

Coast FIRE Number = FIRE Number ÷ (1 + r)^n

This formula answers: how much money, invested today at a given return rate, will compound to your target FIRE number in n years?

A Worked Example

Setup:

• Current age: 32
• Target retirement age: 65
• Planned annual retirement expenses: $55,000
• Full FIRE number: $55,000 × 25 = $1,375,000
• Expected annual return: 7% (a reasonable inflation-adjusted long-term estimate for a diversified stock portfolio)
• Years to retirement: 33

Coast FIRE Number = $1,375,000 ÷ (1.07)^33

First, calculate (1.07)^33 = approximately 9.32

Coast FIRE Number = $1,375,000 ÷ 9.32 = $147,600

At 32, if you have $147,600 invested in index funds and never contribute another dollar, you will have approximately $1,375,000 at age 65 at a 7% average return.

That's the power of time and compounding. $147,600 today does the work of nearly a million dollars over three decades.

Use our Coast FIRE Calculator to calculate your exact number — no manual math required.

Coast FIRE Numbers by Age and Target

The table below shows Coast FIRE numbers at different ages, assuming a 7% annual return and a target retirement age of 65.

A few things stand out from this table:

Starting early has an outsized impact. A 25-year-old with a $1.5M target needs only $100,900 in Coast FIRE savings. A 40-year-old with the same target needs nearly $276,000 — almost three times as much. Every year you wait costs you in compound growth.

The jump from 25 to 35 is steep. Between age 25 and 35, the required Coast FIRE number roughly doubles. The decade from 25 to 35 is the highest-leverage decade for reaching Coast FIRE.

A lower target retirement age raises your Coast number. The table uses 65 as the target. If you want to retire at 55, your n drops from, say, 30 years (at age 35) to 20 years — which means the denominator in the formula is smaller, and your Coast FIRE number is higher.

What to Do After Reaching Your Coast FIRE Number

Reaching your Coast FIRE number doesn't require any single dramatic action. But it does create options:

Option 1: Stay the course

Keep saving and investing as before. You'll reach full FIRE faster and with a larger portfolio. Coast FIRE is a floor, not a ceiling.

Option 2: Reduce work hours

If you only need to cover current living expenses (not save for the future), a lower-income job might suffice. Many people shift to part-time, freelance, or lower-stress work once they hit Coast FIRE. Read about Barista FIRE for a similar approach.

Option 3: Change careers

High-stress careers often demand high salaries. Once you've hit Coast FIRE, you can afford to pursue lower-paying work that's more meaningful, less demanding, or better aligned with your values — because you've already funded retirement.

Option 4: Take extended time off

A sabbatical, extended travel, or time raising young children becomes more viable when you're not worried about setting back your retirement savings. Your existing portfolio keeps compounding while you're away from the workforce.

Important Assumptions to Revisit

The Coast FIRE formula depends heavily on three inputs:

The return rate assumption: 7% is a commonly used real (inflation-adjusted) long-term return for a diversified stock portfolio, based on historical U.S. market performance. The actual number could be meaningfully higher or lower. Some planners use 6% for more conservative projections. If you use 5% instead of 7% in the example above, your Coast FIRE number increases substantially.

The retirement age assumption: Coast FIRE only works as described if you actually let the money grow undisturbed until your target retirement date. Withdrawing early, even for emergencies, disrupts the calculation.

The FIRE number assumption: Your Coast FIRE number is derived from your full FIRE number, which is based on your projected retirement expenses. If your lifestyle changes significantly, your Coast number needs to be recalculated.

This is why the calculator matters: small changes in assumptions produce large changes in the output. Run multiple scenarios with different return rates and target retirement ages to understand the range you're working within, not just a single number.

This article is for educational purposes only and does not constitute financial or investment advice. Investment returns are not guaranteed and will vary. Consult a qualified financial professional for guidance specific to your situation.