Coast FIRE vs FIRE: What's the Difference?

The difference is when the portfolio is expected to reach the full modeled retirement target.

In a basic Coast FIRE calculation:

In a basic FIRE calculation:

Coast FIRE therefore relies on a remaining growth period.

FIRE evaluates the full modeled target without requiring that future Coast growth period first.

Use one consistent scenario:

The modeled retirement target is:

Under the same assumptions, the modeled Coast FIRE target at age 30 is approximately:

So the two model values are:

The difference between those modeled targets is:

That difference exists because the Coast FIRE calculation gives the $340,403 portfolio another 35 years of modeled growth before the selected retirement age of 65.

The FIRE target does not first discount the $1.25 million target across that future Coast period.

See the exact assumptions: Coast FIRE Calculator Assumptions & Methodology

In the example above, both calculations relate to the same future portfolio target:

That target comes from:

divided by:

The difference is what happens next.

For the FIRE comparison, $1.25 million is the full modeled portfolio target.

For Coast FIRE, that future target is discounted across the remaining 35-year growth period to calculate how much would be required at age 30 under the selected growth assumption.

That produces approximately:

The Coast target is therefore not a different retirement-spending goal in this example.

It is a different point in time in the same mathematical scenario.

Because the Coast FIRE calculation includes time for modeled compound growth.

The basic relationship is:

In the example:

produces a Coast FIRE target of approximately:

The Coast calculation therefore asks whether a smaller current amount could grow toward the larger future target.

Investor.gov defines compound interest as earning interest on principal and accumulated interest and provides a calculator that separately models an initial investment, contributions, time, and an estimated interest rate.

The result remains a projection based on assumptions.

The basic Coast FIRE calculation models the portfolio toward a future retirement date.

It does not establish that the existing portfolio can fund current living expenses immediately.

For example, someone can be:

The calculation gives the portfolio 35 modeled years to grow toward the selected future target.

That is different from having the full $1.25 million modeled portfolio target at age 30.

Related: When Have You Reached Coast FIRE?

Reaching a modeled FIRE target and leaving employment are separate concepts.

The calculation can compare current invested assets with a modeled portfolio target.

It does not require a person to:

• leave a job;
• reduce hours;
• stop earning income;
• stop contributing;
• or begin withdrawals immediately.

Traditional FIRE is often discussed in terms of employment income no longer being required to fund modeled spending.

But whether someone continues working is not determined by the portfolio-target formula.

What the basic model does show is that current retirement spending before the selected future retirement date is not being funded by withdrawals from the Coast portfolio in the basic calculation.

Those current expenses therefore have to be represented somewhere outside that retirement-growth test.

That could involve employment income or another source of current cash flow.

The Coast FIRE calculation itself does not choose the source.

The basic Coast test sets future retirement contributions to:

because that isolates the question:

Could the existing portfolio grow toward the future retirement target without additional retirement contributions?

A person can still model another scenario with:

Investor.gov's Compound Interest Calculator similarly treats:

• initial investment;
• monthly contribution;
• length of time;
• and estimated interest rate

as separate variables.

Changing contributions changes the projected portfolio path.

It does not automatically change the Coast FIRE target when the target inputs remain the same.

That can occur in different mathematical scenarios.

For example, the portfolio could:

• grow over time;
• receive additional contributions;
• experience different investment returns;
• or have its future target change because the model inputs change.

Suppose the model begins at age 30 with:

and the selected future target is:

The Coast FIRE calculation is constructed so that $340,403 grows toward that target under the selected growth assumptions and $0 future contributions.

If contributions continue, that is a different projected portfolio path.

The calculator does not guarantee when or whether an actual portfolio will reach the modeled FIRE target.

Someone can calculate:

and separately calculate:

The Coast target identifies the current amount associated with growth toward the future target under the selected assumptions.

The full target identifies the portfolio amount associated with the modeled annual spending and withdrawal-rate assumption.

What happens after the Coast target is reached is outside that comparison.

A person might:

• continue contributing;
• leave contributions unchanged;
• change retirement age;
• change spending assumptions;
• or simply keep using Coast FIRE as a reference point.

The formula does not choose among those possibilities.

Job type is not part of the Coast FIRE formula.

The calculation does not require:

• part-time work;
• a lower salary;
• freelancing;
• a lower-stress job;
• a second career;
• or a career change.

Those are employment scenarios sometimes discussed alongside Coast FIRE.

They are not mathematical requirements for reaching the Coast FIRE target.

This is one of the most important distinctions.

In the basic Coast FIRE model:

The model does not assume that the Coast portfolio is currently paying the annual retirement-spending amount.

In the full FIRE comparison:

Using the example:

The Coast target of $340,403 is not being treated as a $50,000-per-year current-spending portfolio.

It is being modeled as an amount that could grow toward the $1.25 million future target.

The multiple depends on the selected withdrawal-rate assumption.

With:

the reciprocal is:

so:

With a different withdrawal-rate assumption, the modeled portfolio target changes.

That is why the withdrawal rate should be treated as an explicit model input rather than a guaranteed property of FIRE.

Related: How Withdrawal-Rate Assumptions Change Your Coast FIRE Number

A consistent comparison can use the same withdrawal-rate assumption for both.

In this guide:

Both therefore use the same:

The difference comes from the Coast calculation discounting that future target across the remaining growth period.

If two calculators use different withdrawal-rate assumptions, they may produce different targets even when all other inputs appear similar.

The projected portfolio path changes.

Suppose current investments equal:

One scenario uses:

Another uses:

The second projection includes an additional source of portfolio growth.

That could cause the modeled portfolio to reach the full $1.25 million target earlier than the zero-contribution projection under the same growth assumptions.

It does not change what the original Coast FIRE test means.

The Coast test still answers the zero-additional-contribution question.

Under this guide's assumptions, the full modeled FIRE target is larger.

At age 30:

The Coast target is lower because it has:

of modeled compound growth before the selected retirement date.

If the remaining growth period becomes shorter, the Coast target moves closer to the full target.

At the selected retirement date, there is no remaining Coast growth period to discount.

Retirement age has a direct effect on the Coast FIRE target because it changes the remaining growth period.

For example:

Retirement at:

provides:

of modeled growth.

Retirement at:

provides:

With the same future retirement target and growth assumptions, the shorter horizon produces a larger current Coast FIRE target.

The full modeled retirement target can remain the same if annual spending and the withdrawal-rate assumption remain unchanged.

Related: Coast FIRE by Age

A current portfolio can move below either modeled target.

For example, a portfolio may be:

on one date and:

later if its market value changes.

A portfolio at the full modeled FIRE target can also later have a different market value.

The U.S. Securities and Exchange Commission states that investment targets and projections are hypothetical and that past performance cannot predict future performance.

Neither Coast FIRE status nor FIRE status makes future market values fixed.

Both are calculations based on selected assumptions.

Actual future outcomes can differ because of:

• investment returns;
• inflation;
• investment fees;
• spending;
• withdrawal patterns;
• retirement timing;
• contributions;
• and other changes to the model inputs.

The U.S. Department of Labor's Employee Benefits Security Administration similarly requires assumption-based lifetime-income illustrations to explain that estimates are illustrative and are not guarantees.

A Coast FIRE or FIRE projection should therefore be read as a model result rather than a promised future outcome.

Age 65 in the example is a Coast FIRE model input.

It is not being presented as a Social Security rule.

The Social Security Administration states that full retirement age is 67 for people born in 1960 or later.

SSA also explains that the age someone stops working and the age they begin receiving Social Security benefits are often different.

That distinction is useful for Coast FIRE vs FIRE because:

do not have to occur on the same date.

The distinction is mathematical.

It is not a ranking of which approach is better.