FIRE vs Coast FIRE: What's the Difference?

Full FIRE means you have enough invested to live off portfolio withdrawals right now, so paid work becomes optional; Coast FIRE means you have enough invested that its growth alone should reach your FIRE number by your target retirement age, so you no longer need to add new money and your job only has to cover today's expenses.

FIRE (financial independence, retire early) and Coast FIRE describe two different milestones on the same road. One is the destination, having a portfolio large enough to fund your life. The other is an earlier checkpoint, having invested enough that time and compounding can carry you the rest of the way without further contributions. They are not competing strategies so much as two points on a single timeline.

This guide explains what each milestone means, walks through a worked example where the two numbers differ sharply, and shows what each is useful for, under a clear set of assumptions rather than as advice about which to aim for.

The short answer

Both milestones are defined against the same target: your FIRE number, the portfolio size that could fund your annual expenses through withdrawals. What differs is how much you need invested today to be "there."

• Full FIRE is reached when your portfolio already equals your FIRE number. At that point, under the withdrawal assumptions you have chosen, the portfolio itself can cover your expenses, and continuing to work is optional.
• Coast FIRE is reached earlier, when your current portfolio is large enough that its expected growth alone, with no further contributions, should compound up to your FIRE number by your chosen retirement age. Until then, you still work, but only to cover your current living costs, not to keep investing.

The practical difference is large because compounding does the heavy lifting between Coast FIRE and Full FIRE. The further you are from retirement age, the smaller the Coast FIRE number is relative to the full number, because growth has more years to work. This relies entirely on Compound Interest: reaching Coast FIRE is really the point at which existing investments, left alone, are projected to grow into the full target.

Side-by-side comparison

Each row is a difference in what the milestone represents, not a ranking. Which milestone is relevant depends on your age, target retirement age, and expenses, none of which the table decides for you.

Full FIRE, explained plainly

Full FIRE is the classic destination: your invested portfolio is large enough that, under your chosen withdrawal assumptions, it can pay your annual expenses indefinitely, or at least across your planned retirement horizon.

The usual shorthand starts from a withdrawal rate. If you assume you can withdraw 4% of the portfolio in the first year, your FIRE number is your annual expenses multiplied by 25 (because 1 divided by 0.04 is 25). A more conservative 3.5% assumption raises that multiple to about 28.6 times expenses. The multiple is just the inverse of the withdrawal rate you assume, which is exactly the trade-off explored in the 3.5% vs 4% Withdrawal Rate article.

Once your portfolio reaches that number, paid work is optional under those assumptions. You estimate a full FIRE target with the FIRE Calculator, and test how different withdrawal rates would draw it down with the Safe Withdrawal Rate Calculator.

Coast FIRE, explained plainly

Coast FIRE is the earlier checkpoint. You reach it when the money you already have invested is projected to grow, on its own and with no further contributions, into your full FIRE number by the age you plan to retire.

The plain-text relationship is:

Coast FIRE number = FIRE number divided by (1 + r) to the power of the years until retirement

where r is your assumed annual growth rate. In words, it is your full target discounted back to today by the growth you expect between now and retirement. The more years of compounding you have ahead, the smaller that present-day number is.

The name captures the idea: once you hit it, you can "coast." You still work, but only enough to pay your current bills, because you no longer need to add to retirement savings for the portfolio to reach the target on schedule. It shifts the job from wealth-building to expense-covering. You can find your own Coast FIRE number for a given age, target, and growth assumption with the Coast FIRE Calculator.

A worked example

Assume your annual expenses are $40,000, and you use a 4% withdrawal assumption, so your FIRE number is $1,000,000 (25 times expenses). Assume a 7% annual growth rate, that you are 40 years old, and that you plan to retire at 65, which leaves 25 years of growth.

Full FIRE requires the whole target today:

$1,000,000 invested now.

Coast FIRE requires only the amount that would grow to $1,000,000 in 25 years at 7%:

$1,000,000 divided by (1.07) to the power of 25 = about $184,000.

Under these assumptions, roughly $184,000 invested at age 40, left completely alone, is projected to compound to about $1,000,000 by age 65, without another dollar of contributions. Full FIRE at the same moment would require $1,000,000.

The gap between about $184,000 and $1,000,000 is the work compounding is expected to do over 25 years. It also shows why the Coast FIRE number is so sensitive to its assumptions: change the growth rate, the target, or the years remaining, and the number moves substantially. A lower assumed return or fewer years until retirement raises the Coast FIRE number toward the full one.

Which to use when

The two milestones answer different planning questions, so it is less "which to pick" than "which are you asking about," under your own assumptions:

• Full FIRE is the relevant frame when you want to know the portfolio that could actually fund your expenses through withdrawals, and when paid work could become fully optional. It depends heavily on the withdrawal rate you assume.
• Coast FIRE is the relevant frame when you want to know whether your existing investments, left to compound, are already on track to reach that full number by your target age, so you could stop adding to retirement savings and let your job cover only current costs.
• The numbers depend on your assumptions. The Coast FIRE figure in particular hinges on the assumed growth rate and the years until retirement, both of which are uncertain, so it is best read as a projection to test rather than a fixed threshold.

The reliable habit is to treat both numbers as scenarios driven by assumptions you can change, expenses, withdrawal rate, growth rate, and retirement age, and to re-run them as those inputs evolve. Projected compounding is not a guarantee of future returns.