Free COAST FIRE calculator: find your FIRE number, the amount to invest today, and monthly savings needed. Check if you have reached Coast FIRE.
Disclaimer: Pure math illustration: uses the 25x rule and compound growth at the real return you enter. It ignores inflation beyond the real return, taxes, fees, Social Security, and market volatility. Not financial advice.
Coast FIRE means your retirement is already funded by the growth of money you have invested today — you only need to earn enough to cover your current bills. If your portfolio can compound into 25 times your annual spending by retirement age, you have "coasted." This free COAST FIRE calculator computes your FIRE number, your coast number, the gap to close, and the monthly savings needed to close it.
Coast FIRE is a milestone on the path to financial independence: the point where you never need to save another dollar for retirement because compound growth will do the rest. You keep working to pay today's expenses, but retirement saving becomes optional. It is the middle ground between starting from zero and full FIRE (Financial Independence, Retire Early).
The math rests on the 25x rule (the inverse of the 4% rule): a portfolio of 25 times your annual spending can historically sustain 4% yearly withdrawals indefinitely. Your FIRE number is therefore annual spending × 25. Your coast number is that FIRE number discounted back to today at your expected real return — the return after inflation — over the years until retirement.
Coast FIRE is powerful because compounding does the heavy lifting early. A dollar invested at age 30 at 7% real return becomes about $10.68 by age 65 with no further contributions. That is why the verdict can flip from "not yet" to "coasting" with just a few years of aggressive early saving.
FIRE number = annual spending × 25
Coast number = FIRE number ÷ (1 + r)years
Gap = coast number − amount invested now
If there is a gap, the monthly savings to close it is the annuity payment that grows to the gap over the remaining years:
Monthly savings = gap × (r/12) ÷ ((1 + r/12)12×years − 1)
Worked example: Age 30, retiring at 65 (35 years), spending $50,000/year, 7% real return, $40,000 invested. FIRE number = $50,000 × 25 = $1,250,000. Growth factor: 1.0735 ≈ 10.68. Coast number = $1,250,000 ÷ 10.68 = $117,077. Gap = $117,077 − $40,000 = $77,077. Monthly rate = 0.07 ÷ 12 = 0.005833; periods = 420; (1.005833)420 ≈ 11.50. Monthly savings = $77,077 × 0.005833 ÷ (11.50 − 1) = $449.63 ÷ 10.50 ≈ $42.82/month. Verdict: not coasting yet — but only $43 a month away from it.
Priya is 40, wants $60,000/year in retirement at 60, expects 6% real returns, and has $150,000 invested. FIRE number = $60,000 × 25 = $1,500,000. With 20 years left, 1.0620 ≈ 3.21, so her coast number = $1,500,000 ÷ 3.21 = $467,708. Her gap is $317,708, which means saving about $688/month for 20 years — very achievable on a professional salary.
Second scenario — already coasting: Daniel is 28 with $200,000 invested, plans to retire at 65 spending $45,000/year, and assumes 6% real returns. FIRE number = $45,000 × 25 = $1,125,000. With 37 years to grow, 1.0637 ≈ 8.61, so his coast number = $1,125,000 ÷ 8.61 = $130,635. He already has $69,365 more than that — he has reached Coast FIRE and never needs to save for retirement again.
Retirement Calculator — Project your full retirement savings timeline beyond the Coast FIRE milestone.
401(k) Calculator — See how employer-plan contributions accelerate your path to your coast number.
Social Security Calculator — Estimate the Social Security benefits that can lower the spending your portfolio must cover.
Salary Calculator — Work out how much of your pay you can redirect toward closing your coast gap.
Personal Loan Calculator — Weigh the cost of debt against the compounding you give up by not investing.
This page is a free COAST FIRE calculator. Key facts: Coast FIRE is the point where current investments, compounding at an assumed real return, will grow to 25× annual retirement spending by a target retirement age with no further contributions; FIRE number = annual spending × 25; coast number = FIRE number ÷ (1 + real return)^years; the verdict is "coasting" when invested amount ≥ coast number, otherwise a monthly savings figure closes the gap. Example: age 30, retire 65, $50k spending, 7% real return, $40k invested → coast number ≈ $117,077, gap ≈ $77,077, ≈ $42.82/month to close.
Coast FIRE means you have invested enough that compound growth alone will fund your retirement, so you no longer need to save for it. You keep working to cover current living expenses, but every future retirement dollar comes from growth, not new contributions.
Regular FIRE means you have the full 25x annual spending invested today and can retire now. Coast FIRE means you have a smaller amount today, the coast number, that will grow into the full 25x by your retirement age. You still work, but retirement saving becomes optional.
Use a return above inflation. For a stock-heavy portfolio, 5% to 7% real is a common planning range; the long-run US stock market has delivered roughly 7% after inflation. Use 4% to 5% if you want a conservative estimate, and never use a nominal return without subtracting inflation.
This calculator does not include Social Security. In practice, expected Social Security benefits reduce the spending your portfolio must cover, which lowers your FIRE number and your coast number. Treat the calculator result as the conservative, portfolio-only case.
Coast FIRE requires some invested money for compounding to work on, so from zero the answer is always not yet. But the monthly savings figure shows exactly how to get there: even small monthly contributions in your 20s and 30s compound into a coast number surprisingly fast.
Cite this page as: "COAST FIRE Calculator." Sigma Calculator. Accessed October 20, 2026. https://sigmacalculator.net/financial-calculators/coast-fire-calculator/
Last updated: October 20, 2026