What is Coast FIRE?
means you have already invested enough that your portfolio, left alone, will grow into a full retirement fund by the age you plan to stop working. You still earn a living, but you no longer need to save. That frees you to downshift to a lower-stress job, take a sabbatical, or start a business. Planning as a couple? The couples calculator shows which of you can cut back first.
The coast number is the portfolio size today that makes this true. The calculator finds yours and returns a verdict: coasting, on track, or behind with a concrete fix.
What is my coast fire number?
Your coast fire number is the amount that, invested now and never added to, compounds into your full retirement fund by your retirement age. It is far smaller than the retirement number itself, because time does most of the work: in the example below, a $900,000 FIRE number at 65 needs only about $256,000 invested at 32. For the number at every age from 25 to 55, see the Coast FIRE number by age table.
Age and retirement age set the exponent: each extra year before retirement divides by another (1 + r), so coast numbers fall steeply through your thirties and forties. Spending sets the FIRE number underneath, and a pension splits the target into two phases.
How to calculate your coast fire number
Divide your FIRE number by one year of real growth for each year until retirement:
coast number = FIRE number ÷ (1 + real return)(years to retirement)
- FIRE number = gross annual spending ÷ safe withdrawal rate. Spend $36,000 a year at 4% and you need $900,000. A retirement tax rate grosses spending up, and a pension or Social Security splits the target into a bridge phase and a post-pension phase.
- Years to retirement is the exponent, not years of retirement.
- Real return is converted from nominal by division, never by subtraction. All figures are in today's dollars.
- Progress = current investments ÷ coast number. At 100% you are coasting. Below it, the solver finds the age your contributions catch up: your coast age.
The calculator flags impossible inputs, such as a negative real return, instead of printing a nonsense verdict. Full derivations are on the methodology page.
Why real return is calculated by division
The correct conversion is r = (1 + nominal) ÷ (1 + inflation) − 1, not nominal minus inflation. With the classic 7% return and 3% inflation, the difference looks tiny (3.88% vs 4%), but it compounds to roughly a 3–4% overstatement of your portfolio over 30 years, enough to make you think you have hit your coast number years early.
A worked example
Maya is 32 with $150,000 invested. She spends $3,000 a month and plans to retire at 65. She assumes a 7% nominal return and 3% inflation, so her real return is about 3.88%.
- FIRE number: $36,000 ÷ 4% = $900,000 at age 65.
- Coast number at 32: $900,000 ÷ 1.038833 ≈ $256,000. Maya is about 59% of the way there.
- Contributing $700 a month, her portfolio crosses the moving coast number around age 50. After that, every dollar she saves is optional.
The verdict does not say Maya can retire at 50. It locks in her retirement fund, not her monthly bills.
Reading your verdict
- ✓ Coasting (coral): your investments already exceed your coast number. Keep saving only if you want a buffer.
- On track (emerald): not coasting yet, but at your current contribution rate you will be. The subline names the exact age.
- Behind (amber, never red): at this pace you would fall short. The panel shows the extra monthly contribution that lands you on time, or the later retirement age your pace supports.
The chart plots both futures: solid if you keep contributing, dashed if you stop today. Flip the crash test to replay your verdict with your portfolio cut 30%.
When Coast FIRE doesn't work
Coast math assumes steady average returns over decades, so it is weakest in three cases:
- Short horizons, under about 10 years, where one bad decade can dominate.
- Spending that is high relative to what you can save.
- Volatile career income, where you may not be able to stop saving. Treat your coast number as a floor, not a finish line.
This page is education, not personalized investment advice.
Coast FIRE vs the other FIRE flavors
Coast FIRE is one of five common flavors. They differ on whether you keep working and whether you keep saving.
| Flavor | The idea | Still working? | Still saving? |
|---|---|---|---|
| Coast FIRE | Enough invested that compounding alone reaches your retirement number. | Yes — to cover today's bills | No |
| Barista FIRE | Part-time income plus withdrawals bridge the gap until full retirement. | Yes — part-time | Usually not |
| Lean FIRE | Fully retired on a deliberately minimal budget. | No | No |
| Chubby FIRE | Fully retired with real comforts priced in. | No | No |
| Fat FIRE | Fully retired on a luxury budget. | No | No |
Frequently asked questions
What is the difference between Coast FIRE and regular FIRE?
Regular FIRE means your portfolio already covers your full spending today — you can stop working for good. Coast FIRE is the earlier checkpoint where you have just enough invested that compounding alone will grow it into your full FIRE number by your planned retirement age. You still work to pay your bills, but you no longer need to save.
How is the coast FIRE number calculated?
Your FIRE number is your gross annual spending divided by your safe withdrawal rate (spending $36,000 a year at a 4% SWR gives $900,000). Your coast number today is that FIRE number discounted back to the present: FIRE number ÷ (1 + real return)^years until retirement. With roughly 39 years of compounding ahead, $900,000 becomes about $190,000–$260,000 depending on the return you assume.
What return rate should I assume?
The calculator defaults to a 7% nominal return with 3% inflation — close to long-run global equities — but everything is adjustable in Advanced settings. The key detail: we convert nominal to real using (1 + nominal) ÷ (1 + inflation) − 1, not simple subtraction, so every number you see is in today's dollars.
Does the coast FIRE number include taxes and Social Security?
It can. Set a retirement tax rate and we gross up your spending so your portfolio targets the pre-tax income you actually need. Add an expected pension or Social Security amount and its start age, and the engine computes a two-phase FIRE number: a bridge fund that covers you until the pension starts, plus a smaller nest egg for the years after.
What if the market crashes right after I hit my coast number?
Flip the crash-test toggle to replay your verdict with your investments cut 30%. A crash delays compounding but contributions and time usually recover it — the toggle shows honestly whether your plan survives. If the amber verdict worries you, a lower assumed return or a later retirement age is the conservative fix.
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Asking a different question entirely? Turn your savings rate into years to FI, stress-test the 4% rule, count down your remaining workdays, clear debt first, price a rental property, or settle lump sum vs DCA — or head back to all FireVerdict calculators.
この計算機の日本語版もあります： コーストFIRE 計算機
Also available with local tax wrappers and pensions: Canada, UK and Australia editions.