How to calculate your Coast FIRE number — without fooling yourself

By GrowThenDraw Editorial Team · Updated July 28, 2026 · 13 min read · Editorial policy

A Coast FIRE number is the amount that would need to be invested now to grow into your retirement target without further contributions. It is a useful checkpoint, but only after every assumption is made visible.

The calculation itself is straightforward. The difficult part is choosing spending, retirement income, return, inflation, costs and a withdrawal rate without quietly making the answer easier than reality.

Start with the meaning: Coast FIRE is not retirement today

Reaching a modeled Coast FIRE number means existing retirement investments could grow to a future target if the entered assumptions occur. It does not mean current living costs are funded. Most people who coast still work, earn business income or otherwise cover every expense between today and retirement.

It is also not a permission slip to stop contributing. The result is a deterministic scenario while real investment returns arrive unevenly. Continuing to save creates resilience against lower returns, career changes, health costs and a retirement date that moves forward.

Step 1: calculate the spending your portfolio must fund

Estimate desired annual retirement spending in today's purchasing power. Then subtract reliable retirement income that is not drawn from the portfolio, also stated in today's money. A cautious pension or Social Security estimate can belong here; an uncertain inheritance should not.

For example, desired spending of $60,000 minus $15,000 of reliable annual income leaves $45,000 for the investment portfolio. Keeping both numbers in today's money prevents a future pension estimate from being mixed with today's spending.

Step 2: move today's spending to the retirement date

If retirement is 25 years away and inflation is assumed to be 2.5%, the portfolio-funded spending is multiplied by 1.025 to the power of 25. In the example, $45,000 becomes about $83,400 in first-year retirement dollars.

This inflation step can make the future target look enormous. That is expected: the target is expressed in future dollars, while the final Coast number will be discounted back to today's date.

Step 3: turn annual spending into a retirement target

Divide future portfolio-funded spending by the selected withdrawal rate. At 4%, $83,400 implies a retirement target near $2.09 million. At 3.5%, the same spending implies about $2.38 million. A lower selected rate requires more capital because each dollar of portfolio is assumed to provide less first-year income.

The 4% figure is associated with William Bengen's research using historical US returns and specified portfolio and retirement conditions. It is not a legal standard, a promise or a rate that fits every country and horizon. FINRA likewise emphasizes that withdrawal planning depends on longevity, market conditions, inflation and portfolio decisions.

Step 4: discount the target back to today

Subtract annual investment fees and any simplified tax-drag assumption from the nominal return, convert that net annual assumption to the calculator's monthly rate, and discount the future target over every month until retirement.

With a 7% nominal return, 0.5% annual fees and no tax drag, the example's $2.09 million target discounted over 25 years produces a Coast number around $414,000 under GrowThenDraw's monthly convention. This is not an expected market value; it is the balance mathematically consistent with those assumptions.

The compact Coast FIRE formula

A compact version is: Coast number today = retirement target ÷ (1 + monthly net return)^(months to retirement). The retirement target equals inflation-adjusted annual portfolio spending ÷ withdrawal rate.

GrowThenDraw models monthly contributions at the beginning of each month. If contributions increase annually, the higher amount starts after each completed 12-month period. Fees and optional tax drag reduce the entered nominal return before monthly compounding.

How the modeled Coast age works

Today's Coast threshold is not fixed. Each month closer to retirement leaves less time for compounding, so the required balance rises. The calculator projects planned contributions and compares the balance with that moving threshold.

The first month in which planned investments equal or exceed the threshold is the modeled Coast point. If that crossing never occurs before retirement, the page reports that the plan has not reached Coast rather than inventing a date.

Avoid the five most common double-counting errors

• Do not include the same pension as both an invested asset and reliable retirement income.
• Do not add home equity unless the retirement plan genuinely includes selling, borrowing against or renting the home.
• Do not enter future-dollar pension income beside spending stated in today's money.
• Do not subtract investment fees twice if the return assumption is already net of all costs.
• Do not count an employer contribution in the current balance before it has actually been invested.

Run three cases, not one

A single output encourages false precision. Run a cautious case with a lower return, lower withdrawal rate, higher fees and higher inflation; a middle case; and an optimistic case that you do not rely on for irreversible decisions.

Pay attention to how much today's Coast number and the modeled Coast age move. If a one-percentage-point return change adds many working years, the plan is sensitive and continued contributions have high insurance value.

Country and account rules still matter

The global formula does not know whether assets sit in a US 401(k), UK pension or ISA, Canadian RRSP or TFSA, Australian super fund, Singapore CPF-linked arrangement, Malaysian EPF, New Zealand KiwiSaver or a taxable account. Withdrawal access, contribution limits and tax treatment differ.

Use tax drag only as a transparent approximation for recurring leakage. It cannot reproduce brackets, allowances, employer matching, contribution relief, benefit taxation or changes in law. After testing the global plan, use a country-specific calculator where GrowThenDraw provides one.

A sensible way to use the result

Treat the Coast number as a progress marker: it can show whether flexibility is growing and which assumptions matter most. It can support conversations about reducing hours, changing careers or redirecting some future savings, but it does not settle those decisions.

Before stopping contributions or making a major employment decision, consider regulated advice in your jurisdiction and a plan that includes emergency savings, debt, insurance, taxes and multiple return paths. The calculator answers a narrow mathematical question; real financial resilience is wider.