How Withdrawal-Rate Assumptions Change Your Coast FIRE Number

Four percent is one possible calculator assumption.

The Coast FIRE formula itself does not require 4%.

For the same $50,000 of modeled annual retirement spending:

3%

produces a retirement target of approximately:

$1,666,667

while:

4%

produces:

$1,250,000

and:

4.5%

produces approximately:

$1,111,111

The rate changes the target because it changes the denominator in the retirement-target calculation.

Because the model requires a larger future portfolio to generate the same modeled annual spending amount at a lower percentage.

Using $50,000 of annual retirement spending:

At 4%:

$50,000 ÷ 0.04 = $1,250,000

At 3%:

$50,000 ÷ 0.03 ≈ $1,666,667

The annual spending assumption did not change.

The larger retirement target then flows backward through the Coast FIRE calculation, producing a larger Coast FIRE number today.

Related: Coast FIRE Formula Explained

Different withdrawal models can use percentages in different ways.

The Coast FIRE calculator uses the selected withdrawal-rate assumption primarily to calculate a modeled retirement portfolio target.

For example:

$50,000 ÷ 4% = $1.25 million

That calculation does not by itself define the exact withdrawal schedule that would occur during retirement.

A retirement withdrawal strategy could involve different spending patterns, inflation adjustments, account types, taxes, market returns, and distribution rules.

Those are separate from the Coast FIRE target calculation.

A withdrawal-rate assumption does not guarantee that the original portfolio balance will remain unchanged.

Ending portfolio value depends on factors such as:

• investment returns;
• the sequence of those returns;
• withdrawals;
• inflation adjustments;
• fees;
• taxes;
• and the length of the withdrawal period.

The U.S. Government Accountability Office has reviewed historical retirement-income research involving systematic withdrawals, including examples beginning at 4% of the initial portfolio and increasing withdrawals with inflation.

The same GAO analysis explicitly notes that historical investment returns do not assure future investment returns.

So a 4% input should not be interpreted as a principal-preservation guarantee.

A historical research result is not a guarantee about a future portfolio.

A U.S. Government Accountability Office report summarized Congressional Research Service modeling of retirement drawdowns over different periods.

The reported outcomes changed when:

• the withdrawal percentage changed;
• the time horizon changed;
• and the model assumptions changed.

GAO also emphasized that the estimates relied on historical investment returns and that there was no assurance future investment returns would match historical returns.

That distinction matters for Coast FIRE.

The calculator can use 4% as an editable mathematical assumption without asserting that 4% will produce a particular future outcome.

Retirement duration and withdrawal-rate assumptions are separate model concepts.

A user can select:

• a retirement age;
• a spending level;
• a withdrawal-rate assumption;
• and other model inputs.

Changing the retirement age changes the Coast FIRE growth horizon.

Changing the withdrawal rate changes the modeled retirement target.

The calculator should not silently change one assumption because another input changed unless that behavior is explicitly part of its documented methodology.

They perform different jobs in the model.

For example:

Withdrawal-rate assumption: 4%

and:

Nominal investment-return assumption: 7%

do not describe the same thing.

The calculator should not subtract one directly from the other.

Related: How Return Assumptions Change Coast FIRE

Inflation measures changes in purchasing power and is used in the calculator's real-growth framework.

The withdrawal-rate assumption converts modeled retirement spending into a portfolio target.

For example, this site's current editable defaults include:

Inflation: 3%

and:

Withdrawal-rate assumption: 4%

Those values perform separate mathematical functions.

Changing inflation affects modeled real growth.

Changing the withdrawal-rate assumption affects the modeled retirement target.

Related: How Inflation Changes Your Coast FIRE Number

They are different concepts.

A Coast FIRE withdrawal-rate assumption is an input used by a mathematical retirement model.

A Required Minimum Distribution, or RMD, is a tax-law distribution requirement for certain retirement accounts.

The Internal Revenue Service defines RMDs as minimum amounts that certain retirement-plan and IRA owners generally must withdraw annually once the applicable distribution rules begin.

The IRS calculates RMDs using account balances and life-expectancy factors.

That is different from using a fixed percentage such as 4% to calculate a Coast FIRE retirement target.

The Coast FIRE calculation uses the percentage to construct a modeled retirement target.

Actual account distributions can involve additional questions such as:

• which account is used;
• when distributions occur;
• taxes;
• plan rules;
• required minimum distributions;
• and whether withdrawals are periodic or irregular.

The U.S. Department of Labor's Employee Benefits Security Administration distinguishes between retirement account balances and ways those balances can be converted into retirement income, including lifetime-income illustrations for defined contribution plans.

A Coast FIRE withdrawal-rate assumption is therefore not the same thing as an account-distribution instruction.

Because the withdrawal rate is an assumption rather than a universal constant in the Coast FIRE formula.

Different calculators may:

• use different defaults;
• allow the rate to be edited;
• use different retirement-spending definitions;
• model pensions or other income differently;
• use different inflation frameworks;
• or apply different rounding.

Two calculators using identical investment returns can therefore still produce different Coast FIRE numbers if their withdrawal-rate assumptions differ.

The relevant comparison is not just:

What percentage does the calculator show?

It is:

How does that percentage affect the retirement target in that calculator's methodology?