Coast FIRE Calculator

Last reviewed: June 16, 2026

What is Coast FIRE?

“Coast FIRE” is a term from the FIRE (Financial Independence / Retire Early) community for a specific milestone: the point at which your invested portfolio is large enough to grow on its own — with no further contributions — to your full retirement number by the time you plan to retire. Once you've hit that number, you can “coast”: stop adding to investments, cover living expenses from earned income, and let compound growth do the rest.

The term is widely used in personal-finance communities but has no single academic definition. What it describes, however, is entirely standard mathematics: present value. The coast number is your retirement target expressed as a lump sum in today's dollars — the amount that, compounded at your expected real return over the years remaining, exactly reaches your FIRE number at retirement. No black box. Just discounting.

The coasting phase is not retirement. You still need income to cover daily expenses until you stop working. The coast milestone simply ends the race to grow your portfolio — your investments are now large enough to cross the finish line on their own.

Why it's just present value

Your FIRE number is a future value — a target balance at retirement. Present value is the standard financial tool for asking: “what is that future amount worth in today's dollars?” The formula is:

• PV — present value (your coast number)
• FV — future value (your FIRE number in today's dollars)
• r — real annual return (nominal return minus inflation, roughly)
• t — years until retirement

Using the calculator's defaults — $40,000 annual spending, 4% withdrawal rate, age 30 retiring at 60, 7% nominal return, 3% inflation:

• FIRE number = $40,000 / 0.04 = $1,000,000
• Real return ≈ 3.88% (see next section)
• Years to retire = 30
• Coast number = $1,000,000 / (1.0388)^30 ≈ $318,862

That $318,862 invested today at a 3.88% real return for 30 years grows to exactly $1,000,000 in today's purchasing power — your FIRE target. Every number is in today's dollars, so no inflation adjustment is needed when you compare to your current portfolio.

The real-return derivation — explained plainly

This calculator takes two separate inputs — nominal return and inflation — rather than asking you to derive the real return yourself. There's a good reason: a portfolio earning 7% nominal while inflation runs at 3% grows your purchasing power at roughly 3.88%, not 7%. The 7% gain buys you 3% less each year in real goods, so only the excess is genuine real growth.

The standard formula for converting is the Fisher equation:

For 7% nominal and 3% inflation: (1.07 / 1.03) − 1 = 0.04 / 1.03 ≈ 3.8835%. The calculator derives this for you and shows it explicitly in the results panel, so you can always see which rate is actually driving the math. All outputs — FIRE number, coast number, projected portfolio — are in today's purchasing power, making them directly comparable to your current account balance.

The FIRE number and the withdrawal-rate caveat

The FIRE number is derived from your annual spending divided by a withdrawal rate: how large a portfolio do you need so that withdrawing a fixed percentage each year covers your expenses without depleting the principal? At 4%, you need 25× your annual spending.

The 4% figure originates from William Bengen's 1994 paper in the Journal of Financial Planning, which analyzed historical U.S. stock and bond returns over 30-year periods and found that a 4% annual withdrawal rate had never exhausted a diversified portfolio. The Trinity Study (Cooley, Hubbard & Walz, 1998) reinforced this finding.

The rate is contested. International evidence is mixed, longer horizons reduce the safe rate, and sequence-of-returns risk — getting bad returns early in retirement — can sink a plan that looks fine on paper. Some researchers argue 3–3.5% is more conservative; others suggest 4–5% is fine with flexible spending. This calculator exposes the withdrawal rate as an editable input so you can model your own assumption. The Safe Withdrawal Rate Calculator (coming soon) unpacks the evidence in full.

Worked example

Using the calculator's defaults — age 30 targeting retirement at 60, $40,000 annual spending, 4% withdrawal rate, 7% nominal return, 3% inflation, $50,000 already invested, $1,000/month contribution:

• FIRE number: $40,000 / 0.04 = $1,000,000 in today's dollars
• Real return: (1.07 / 1.03) − 1 ≈ 3.88% per year
• Coast number: $1,000,000 / (1.0388)^30 ≈ $318,862 — the amount needed today to reach the FIRE number with zero further contributions
• Current position: $50,000 invested, gap of about $268,862 to coast
• With contributions: adding $1,000/month in real terms (growing with inflation in nominal terms), the projected portfolio crosses the coast curve at the age shown in the results panel and chart — check the highlighted row in the by-age table

The by-age chart plots two lines: your projected portfolio (solid teal, rising with contributions and growth) against the coast curve (dashed, rising from the coast number today to the full FIRE number at retirement). Where they intersect is the coast age. After that point, contributions are optional — the portfolio does the work.

What happens after you coast?

Once you've hit the coast number, your investment portfolio is on autopilot — it compounds at the real return without additional contributions and reaches your FIRE number by retirement. You still need income to cover living expenses until then. Many people in the coasting phase shift to lower-stress or part-time work (sometimes called Barista FIRE), taking advantage of the freedom that comes from knowing retirement is already funded. The coast milestone doesn't mean you stop working; it means you stop racing.

What this calculator assumes

Every projection requires assumptions. These are labeled rather than hidden:

• Today's-dollars framing throughout. Every monetary figure — FIRE number, coast number, projected portfolio — is in real (inflation-adjusted) terms. The nominal future FIRE number appears as a secondary, clearly labeled figure. Comparing your current account balance directly to the coast number is valid because both are in the same purchasing-power terms.
• Constant real return. The real return is held fixed over the entire projection. Actual returns vary year to year; this model does not capture sequence-of-returns risk. A coasting portfolio that earns the average real return on paper can still run short if bad returns cluster early in retirement.
• Constant real contributions. Monthly contributions are constant in today's purchasing power, which implies nominal contributions grow with inflation. This is a planning approximation, not a guarantee.
• Constant inflation. The inflation rate is fixed for the entire projection period. Actual inflation is variable.
• The withdrawal rate is an assumption, not a guarantee. The 4% default is a starting point from historical U.S. data over 30-year horizons — not a promise. Edit it to stress-test your plan.
• No taxes. Returns are pre-tax. Tax treatment depends on account type (401k, Roth IRA, taxable) and jurisdiction.
• Sequence-of-returns risk is not modeled. This is a deterministic projection. The Safe Withdrawal Rate Calculator (coming soon) addresses this directly.

Frequently asked questions

Is my coast number the same as my FIRE number?

No. Your FIRE number is what you need at retirement — the full balance that funds your withdrawals. Your coast number is a smaller amount you need today; it grows into your FIRE number over time. At retirement, the two converge: the coast number at retirementAge equals the FIRE number, because there are zero years of growth remaining.

Why does the coast number change with my withdrawal rate?

The withdrawal rate sets your FIRE number (spending ÷ rate), which is the future value you're discounting. A lower withdrawal rate raises the FIRE number and therefore raises the coast number. A 3% withdrawal rate requires a larger nest egg (33× spending instead of 25×) and a larger coast number. This is why the withdrawal rate is the most consequential assumption in a FIRE plan.

What if my real return is zero or negative?

If your expected nominal return is at or below inflation, the real return is non-positive. The calculator warns you and sets the coast number equal to the full FIRE number — there's no discounting benefit when money doesn't grow in real terms. For diversified equity portfolios over multi-decade horizons, a positive real return is the historical norm, but it's not guaranteed.

Does this account for taxes?

No — results are pre-tax. Your effective tax rate in retirement depends on account types (traditional vs. Roth), income sources, and jurisdiction. A common approach is to use after-tax spending and after-tax expected returns as inputs, or to add a tax buffer to your FIRE number.

My contributions aren't constant — what should I enter?

Use your average monthly contribution in today's dollars. The model treats contributions as constant in real terms (equivalent to nominal contributions that grow with inflation). If your contributions are irregular or expected to change, the projection is approximate — a useful planning benchmark, not a forecast.