Regular FIRE asks “how fast can I get to a number?” Coast FIRE asks a calmer question: is the number already taken care of? If your savings, left completely alone, will compound to your FIRE number before you need them, you never have to contribute again. You can keep working — but now it’s for rent and groceries and fun, not for retirement.
The engine is two lines: your FIRE number (spending ÷ withdrawal rate), and that same number discounted back to today at your assumed return. The discounted figure is the coast number — what your current savings must already equal. Above it: coasting, and the result card shows your surplus. Below it: the gap, stated in dollars and as a percentage of the coast number you’ve reached.
How the math works
Step 1 — the FIRE number. Annual spending divided by withdrawal rate. $60,000 at 4% means a $1,500,000 target — same as the FIRE calculator.
Step 2 — grow what you have. Your current savings compound for the years until retirement: balance × (1 + return)^years. No contributions are added — zero, by definition. The growth multiple is what makes coasting possible.
Step 3 — compare. If the grown balance meets or beats the FIRE number, the surplus row appears — you are coasting. If it falls short, the shortfall row and the exact coast number tell you where the line is: the lump sum you’d need today to coast. If you’re below it, contributions still rule — the FIRE calculator will schedule them.
FIRE number = annual spending ÷ withdrawal rate
grown = current savings × (1 + return)^years
coast number = FIRE number ÷ (1 + return)^years
coasted = current savings ≥ coast numberThe growth multiple does all the work here. At a 7% return over 25 years, every dollar today becomes about $5.43 — which is why the sample below, with $350,000 saved, grows past the $1.5M target with room to spare. Extend the horizon or raise the return and the coast number collapses; shrink either and it climbs. Time is the entire trick of Coast FIRE.
The sample, end to end. You have $350,000 saved, plan to spend $60,000 a year, and use a 4% withdrawal rate — so your FIRE number is $1,500,000. Left untouched at a 7% return for 25 years, your savings grow to about $1,899,601 — a 5.43× multiple on what you started with. That clears the target by $399,601, and your coast number — the minimum needed today — was only $276,374. You passed it 21% ago. From here, every paycheck is a choice, not a requirement.
Terms & definitions
Coast FIRE
The point where current savings alone, compounded to retirement, reach your FIRE number — no further contributions required.
Coast number
The FIRE number discounted back to today: FIRE number ÷ (1 + return)^years. If your savings equal it, you can coast.
FIRE number
The portfolio size that funds retirement: annual spending divided by your withdrawal rate. At 4%, it is 25× spending.
Barista FIRE
A stage before Coast FIRE where investments cover most expenses and part-time work covers the remainder.
Frequently asked questions
What if I’m already past my coast number?
Then the calculator shows a surplus and your progress bar exceeds 100% — retirement is funded by growth alone, permanently. You can keep contributing for a bigger margin or an earlier retirement, but the required part is done. That is the entire appeal of Coast FIRE: the savings obligation switches off while the compounding keeps running.
What if the calculator says I can never coast?
There is no literal “never” while returns are positive and time is available — but if the coast number is out of reach today, the honest answers are: extend the horizon (more years = smaller coast number), raise the gap between income and spending, or accept that contributions remain part of the plan and use the FIRE calculator to schedule them. Coasting is a milestone you reach, not a starting point.
How is Coast FIRE different from Barista FIRE?
Coast FIRE means investments alone will fund retirement — you may still work, but only to cover current expenses, with zero going to savings. Barista FIRE is one step earlier: investments cover most expenses, and part-time work (the “barista”) covers the rest while the portfolio keeps compounding. Coast is about stopping contributions; barista is about supplementing spending.
How sensitive is the coast number to the return assumption?
Extremely — because it is raised to the power of years. At 7% for 25 years, $1 today becomes about $5.43; at 5% only about $3.39; at 9% about $8.62. That is why the default is labeled an assumption, not a prediction: rerun the calculator at 5% and 9% before treating “coasted” as final. A margin of safety above the coast number is not optional.
References
- IRS — Retirement plans and FAQs for individuals: irs.gov/retirement-plans/retirement-plan-faqs
- SSA — Period life table (retirement horizon framing): ssa.gov/oact/STATS/table4c6.html
- RetirePlot — How much do I need to retire, with the math: /guides/how-much-do-i-need-to-retire/