Coast FIRE Calculator — Methodology

How This Calculator Works

"Coast FIRE" is the point at which the money you have already invested, with no further contributions, would grow to your full FIRE number by your chosen retirement age. After that point you only need to earn enough to cover current spending.

The calculator first works out your FIRE number from your spending and a safe withdrawal rate, then discounts it back to today at your expected return. It compares that figure with what you have invested now and shows the Coast number at every age between now and retirement.

Inputs and Assumptions

• Annual expenses in retirement: In today's dollars.
• Current invested assets: Everything already invested for retirement.
• Expected annual return: Default 7%. Because expenses are in today's dollars, this should be a real (after-inflation) return. Using a nominal return here would overstate how far your money coasts.
• Safe withdrawal rate (SWR): Default 4%. Sets the FIRE number.
• Current age and retirement age: Set the years of compounding.
• Part-time income (optional): Annual income in retirement for the Barista variant; it reduces the spending the portfolio must cover.

Formula

fireNumber       = annualExpenses / SWR
years            = retirementAge − currentAge
coastFireNumber  = fireNumber / (1 + realReturn)^years

isCoastFire      = currentAssets ≥ coastFireNumber
progress         = min(currentAssets / coastFireNumber, 100%)

// Barista variant
baristaFireNumber  = max(annualExpenses − partTimeIncome, 0) / SWR
baristaCoastNumber = baristaFireNumber / (1 + realReturn)^years

// Table: Coast number at each age a from now to retirement
coastNumber(a)   = fireNumber / (1 + realReturn)^(retirementAge − a)

Data Sources

• Bengen, W. P. (1994), "Determining Withdrawal Rates Using Historical Data," Journal of Financial Planning — origin of the 4% rule of thumb used as the default SWR.
• Cooley, Hubbard & Walz (1998), "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable" (the Trinity Study) — sustainability of fixed withdrawal rates over 30-year retirements.
• Return default — 7% real is a conventional round figure for long-run US equity returns after inflation. It is an assumption you should change, not a measured value.

Limitations

• Uses one constant return for every year. Real returns vary, and a poor decade just before retirement can leave a "coasting" portfolio short. Re-check periodically rather than treating Coast FIRE as permanent.
• The 4% SWR comes from studies of roughly 30-year retirements. Early retirees with longer horizons often use 3–3.5%.
• Does not model taxes on withdrawals, investment fees, Social Security, pensions or one-off future expenses.
• Assumes no withdrawals between now and retirement; the calculator does not check whether your income actually covers your spending in the meantime.
• The Barista variant assumes part-time income continues for the whole retirement.

Last Updated

October 2026 — Methodology page published. Previously this calculator linked to the FIRE Calculator's methodology.