Common Coast FIRE Mistakes

A calculator default is an input used to produce a mathematical scenario.

It is not automatically a prediction.

For example, this site's demonstration framework uses:

• nominal return assumption: 7%
• inflation assumption: 3%
• investment fees: 0.10%
• withdrawal-rate assumption: 4%

These values produce a particular Coast FIRE result.

Changing them produces another result.

The U.S. Securities and Exchange Commission states that past performance does not necessarily predict future results.

That distinction matters when a historical return, a calculator default, and a future expectation are discussed as if they are the same number.

They are not.

Related: How Return Assumptions Change Coast FIRE

A nominal return and a real return do not represent the same thing.

A nominal return is measured before inflation.

A real return reflects an inflation adjustment.

For example:

7% nominal return

with:

3% inflation

does not mean:

7% real return

If a retirement target is expressed in today's purchasing power while growth is modeled using an unadjusted nominal return, the calculation can mix two different dollar frameworks.

The reverse can also happen: a future-dollar retirement target can be combined with a real-return assumption without the corresponding conversion.

A consistent model can use:

today's-dollar target + real growth

or:

future-dollar target + nominal growth

The problem is mixing the two frameworks without accounting for the conversion.

Related: Real Return vs Nominal Return in Coast FIRE

Inflation can be represented more than once if a calculator's framework is not understood.

Suppose a calculator:

• asks for nominal return;
• asks separately for inflation;
• converts growth into real terms;
• and displays results in today's dollars.

Now suppose the retirement-spending input is also manually increased for the same inflation assumption before being entered.

The same inflation concept may then be represented in more than one part of the calculation.

The opposite problem is possible too: nominal growth can be used with today's-dollar spending without another inflation adjustment.

The question to check is:

Where is inflation already being handled in the model?

The U.S. Bureau of Labor Statistics publishes the Consumer Price Index as a measure of average price change over time. A current inflation observation is different from a long-term calculator assumption.

Related: How Inflation Changes Your Coast FIRE Number

Total net worth and current invested assets are not automatically the same input.

Net worth can contain:

• retirement accounts;
• brokerage investments;
• cash;
• primary-home equity;
• vehicles;
• business value;
• rental property equity;
• money assigned to college costs;
• other assets.

A Coast FIRE calculation compares its modeled retirement target with the assets included in its retirement-investment model.

On this site's main calculator, that input is:

Current Investments

not:

Total Net Worth

This distinction matters because adding an asset to the investment balance changes what the calculation represents.

A primary residence and an invested retirement portfolio are different asset types.

A home may have substantial value.

That does not automatically mean its full market equity is part of the calculator's current investment balance.

A separate scenario could explicitly model:

• selling the property;
• downsizing;
• converting part of the value into invested assets;
• or changing future housing spending.

That would be a different model.

Simply adding home equity to current retirement investments without another assumption changes the meaning of the input.

The Coast FIRE target and the projected portfolio path answer different questions.

The basic Coast FIRE target asks:

How much would need to be invested now for modeled growth alone to reach the selected future retirement target?

Future contributions are not required to define that target.

They can be added separately to project another portfolio path.

For example:

Current investments: $300,000

Future contributions: $0

and:

Current investments: $300,000

Future contributions: $500/month

produce different projected portfolio paths.

But if retirement spending, retirement age, return, inflation, fees, and withdrawal-rate assumptions stay unchanged, the underlying Coast FIRE target does not change merely because the contribution scenario changed.

Investor.gov's Compound Interest Calculator similarly treats the initial investment, monthly contribution, length of time, and estimated interest rate as separate inputs.

These are different values.

The Coast FIRE target is the amount the model calculates as needed at the current age under the selected assumptions.

The projected portfolio estimates how an entered portfolio could grow under a contribution and growth scenario.

For example:

Modeled Coast FIRE target: $340,403

Current investments: $300,000

means the current mathematical comparison is:

$40,403 gap

A projected portfolio chart showing a larger future balance does not change what the current Coast FIRE target means.

It is a separate projection.

A Coast FIRE target calculated several years ago may not represent the current calculation.

The target depends on:

• current age;
• selected retirement age;
• retirement spending;
• return assumptions;
• inflation;
• fees;
• withdrawal-rate assumption.

Even if every selected assumption remains unchanged, the number of years until retirement declines as time passes.

For example:

At age 30, retirement at 65 provides:

35 years

of modeled growth.

At age 35, the same retirement age provides:

30 years.

An old target and a current portfolio may therefore be based on different time horizons.

Related: When Have You Reached Coast FIRE?

Two calculators can receive similar-looking inputs and still produce different results.

Possible differences include:

• nominal versus real return;
• exact versus approximate inflation adjustment;
• investment fees;
• withdrawal-rate assumptions;
• today's dollars versus future dollars;
• contribution timing;
• pension treatment;
• Social Security treatment;
• rounding;
• whether other retirement income is included.

A difference between two Coast FIRE numbers does not by itself identify which calculation step caused the difference.

The useful comparison is:

Which assumptions and formulas produced each result?

That is why methodology should be visible alongside the output.

Fees and investment returns are separate variables.

If one calculation uses:

7% nominal growth before fees

and another effectively uses:

7% after fees

the percentages do not describe the same model.

Investor.gov explains that fees and expenses reduce the amount of money remaining in an investment portfolio to earn returns and that even relatively small ongoing fees can have a meaningful effect over long periods.

This does not establish one future fee level.

It explains why two return assumptions should not be treated as equivalent if one model includes fees and the other does not.

A future income source should represent one component of the model, not two copies of the same resource.

For example, suppose a calculator reduces modeled portfolio-funded retirement spending because it separately includes a pension.

If the same pension is then also converted into an additional investment balance and added to current investments, the model may represent the same future income twice.

The same issue can arise with Social Security.

The Social Security Administration provides retirement-benefit estimates based on an individual's earnings record, future earnings assumptions, and claiming age. SSA also notes that future estimates can differ from estimates produced today.

A Coast FIRE calculator should follow its documented method for integrating an estimated future benefit.

A recurring income stream and a current invested portfolio balance are not automatically the same model input.

A 4% withdrawal-rate input performs a mathematical job in the Coast FIRE model.

For example:

Annual retirement spending: $50,000

Withdrawal-rate assumption: 4%

produces:

$1,250,000 modeled retirement target

because:

That calculation does not guarantee how a future retirement portfolio will perform.

It does not guarantee:

• a particular future return;
• a specific portfolio ending value;
• principal preservation;
• or a fixed retirement duration.

The U.S. Department of Labor uses assumptions to produce retirement-income illustrations for defined-contribution accounts and explicitly describes those illustrations as estimates rather than guarantees.

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

A portfolio can be above a modeled target at one point and below it later.

For example:

Modeled Coast target: $340,403

Current investments: $360,000

Result:

$19,597 surplus

If the current investment value later becomes:

$300,000

while the modeled target remains $340,403, the new comparison becomes:

$40,403 gap

The earlier calculation was not erased.

The current portfolio value changed.

The target itself can also change if the inputs change.

So:

Above Modeled Coast Target

describes a current mathematical comparison.

It does not mean the portfolio is guaranteed to remain above that target.

Coast FIRE and immediate financial independence answer different questions.

The Coast FIRE calculation generally asks whether current retirement investments could mathematically grow toward a future retirement target under selected assumptions.

It does not automatically establish whether current expenses can be funded without employment income today.

That distinction also means reaching the modeled Coast target does not require:

• leaving work;
• changing careers;
• reducing hours;
• or stopping contributions.

Those are separate scenarios.

A consistent demonstration model might use:

The modeled retirement target is:

$1,250,000

The modeled fee- and inflation-adjusted real growth rate is approximately:

3.79%

The modeled Coast FIRE target is approximately:

$340,403

A useful calculation check is then: