What Is Coast FIRE? Definition, Math, and Examples

Coast FIRE is the point where what you have already invested will grow into a full retirement pot on its own. You keep working to pay today's bills, but you can stop saving for retirement, because compound growth does the rest. The number is your FIRE number discounted back by the years you have left. This article gives you the formula, your number by age, three worked examples with the arithmetic shown, and what the bad decades do to the plan.

Updated October 3, 2026: the by-age table now has two return assumptions and the real crossover ages, the examples are recomputed, the US account limits are the 2026 figures, and the return claims cite their sources.

Coast FIRE meaning, in one sentence

Invest enough, early enough, and compound growth carries that money to your retirement target without another dollar from you. From that moment on, your paycheck only has to cover your life, not your future.

The word "coast" means to glide on momentum. You push hard at the start, then you coast. It sits between two other shapes of financial independence: regular FIRE, where the portfolio replaces your whole income right now, and Barista FIRE, where part-time income covers the gap. Coast FIRE needs the smallest pile of the three, which is why it is the most reachable.

	Traditional FIRE	Coast FIRE
Goal	Replace all income from investments	Secure retirement, keep working for living expenses
How much you need today	Your full FIRE number (25 × annual spending)	$50K–$534K depending on age (table below)
What you do after	Work only if you want to	Work to cover current expenses, on your terms
Who it's for	High earners with extreme discipline	Practically anyone who starts early enough

In the FIRE guide I wrote about the full path. Mr. Money Mustache's shockingly simple math puts it at 22 working years on a 40 % savings rate and 12½ on 60 %, at 5 % real returns and a 4 % withdrawal. Not everyone wants, or can, save like that for a decade. Coast FIRE asks for a few years of it.

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The Coast FIRE formula

Coast FIRE number = (annual spending × 25) ÷ (1 + real return)^(years until retirement)

Three inputs.

Annual spending × 25 is the 4 % rule turned around. William Bengen's 1994 paper "Determining Withdrawal Rates Using Historical Data" found that a first-year withdrawal of 4 %, raised with inflation each year, had never exhausted a portfolio of half stocks and half intermediate-term Treasuries in under 33 years, in any US retirement starting from 1926 on. The Trinity Study (Cooley, Hubbard and Walz, 1998) tested the same idea across every historical window from 1926, and the authors' 2011 update, "Portfolio Success Rates: Where to Draw the Line", extended the data through December 2009. Spend $3,500 a month, that is $42,000 a year, times 25 is $1,050,000. That is the FIRE number every example below uses.

The real return is what the market gives you after inflation. Over 1900 to 2025, US equities returned 6.6 % a year after inflation (9.8 % before it, against 2.9 % inflation), per the UBS Global Investment Returns Yearbook 2026 summary. The tables below use the round 7 % that most Coast FIRE pages use, and show 5 % beside it, which is the rate I actually plan with; the section on risk explains why.

Years until retirement is the exponent, and it is doing most of the work. Every year you add to it shrinks the number you need today by about 6.5 %.

Because the return is already a real (after-inflation) figure, every number in this article is in today's dollars. When I write "$1 million", I mean $1 million of today's purchasing power. You do not need to adjust anything.

Where compound growth comes from

You invest $10,000, it earns 7 % a year after inflation, and you never add another dollar:

Year	What happens	Balance
0	You invest $10,000	$10,000
1	7 % of $10,000 = +$700	$10,700
2	7 % of $10,700 = +$749	$11,449
10	Interest on interest adds up…	$19,672
20	…and accelerates…	$38,697
30	…and accelerates…	$76,123
40	…and keeps accelerating	$149,745

In the first ten years you gained about $10K. In the last ten, from year 30 to 40, you gained $74K, with the same original money. The longer it works, the harder it works.

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Coast FIRE number by age

Finish line $1,050,000 ($3,500 a month × 12 × 25), retirement at 65, two return assumptions. One thing about the last column: your number rises every year you are still saving, because each year leaves fewer years to compound. A 25-year-old needs $70K today, but if it takes her nine years to get there she is 34, and the number at 34 is $129K. The crossover below is where a steady monthly contribution finally meets that moving target, not the frozen number from the first row.

Age today	Years to 65	Number at 7 % real	Number at 5 % real	Saving this much a month…	…you cross at (7 %)
20	45	$50,000	$117,000	$600	age 29
25	40	$70,000	$149,000	$850	age 34
30	35	$98,000	$190,000	$1,500	age 37
35	30	$138,000	$243,000	$1,500	age 46
40	25	$193,000	$310,000	$2,000	age 52
45	20	$271,000	$396,000	$2,500	age 59
50	15	$381,000	$505,000	$3,000	after 65
55	10	$534,000	$645,000	$4,000	after 65

All figures in today's dollars. At 5 % real the crossovers come much later (a 30-year-old on $1,500 a month crosses at 45, not 37), which is the plain reason the return assumption matters more than anything else on this page.

What the table says:

The earlier, the better. A 20-year-old needs $50K. A 45-year-old needs $271K, 5.4× more for the same result.

From 50 on, Coast FIRE stops being a strategy and becomes ordinary retirement saving. The target moves up faster than a normal contribution can chase it; the lever at that age is the contribution rate, not the compounding.

The numbers under 45 are achievable. Tens to low hundreds of thousands, not millions. The first two rows, $50K and $70K, are about a year of the median US worker's pay ($1,251 a week in the second quarter of 2026, roughly $65,000 a year); the rows to 40 are two to three years of it.

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Coast FIRE examples, with the arithmetic

Jake, 25

Jake has 40 years ahead of him. His number today is $1,050,000 ÷ 1.07^40 = $70,000. If he had it in hand this year and never added a dollar, it would grow to $1,050,000 by 65.

He does not have it in hand. He invests $850 a month. After six years he has about $75K, but by then he is 31 and his target has moved to $105K. He crosses the moving line at 34, with about $132K invested. From that day on, the pot reaches $1,050,000 at 65 on its own, and his $850 a month is his to redirect: a down payment, a sabbatical, or, as I would, more investing.

If you test his savings against the frozen $70K instead, he "arrives" at 31, but $70K at 31 grows to only about $698K by 65 at 7 %. The nine-year version is the one that reaches $1,050,000.

Michelle, 35

Michelle has 30 years. Her number today is $1,050,000 ÷ 1.07^30 = $138,000, almost double Jake's, because her money has ten fewer years to grow.

Investing $1,500 a month, she crosses the moving target at about 46, with roughly $289K. That is eleven years, not the six a frozen target suggests, and eleven years of a four-figure monthly contribution.

Sarah, 27, with a smaller target

Sarah spends $3,000 a month, so her FIRE number is $900,000 and her Coast FIRE number at 27 is $900,000 ÷ 1.07^38 = $69,000. On $1,200 a month she crosses the moving target a little before her 33rd birthday, with about $102K, and from there the pot reaches $900,000 at 65 with nothing more from her.

From then on the $1,200 a month is hers to redirect, and her retirement is funded whether or not she ever invests again.

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Calculate your own number

The arithmetic is three steps, and the Coast FIRE calculator does them for you, with your own age, spending, current portfolio and return assumption, and needs no sign-up. It also shows the moving target, so you see the real crossover rather than the frozen one.

By hand:

Step 1: Estimate your monthly expenses in retirement. Not sure? Take your current expenses minus your mortgage.

Step 2: Annual expenses × 25 = your FIRE number.

Step 3: Divide your FIRE number by the divisor for your age (7 % real, retirement at 65):

Your age	Divisor	Your age	Divisor
20	21.00	40	5.43
25	14.97	45	3.87
30	10.68	50	2.76
35	7.61	55	1.97

Example: you are 30, retirement expenses $3,500 a month. FIRE number = $3,500 × 12 × 25 = $1,050,000. Coast FIRE number = $1,050,000 ÷ 10.68 = $98,300.

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The cost of waiting

If you start at…	Instead of…	You need extra	How much more
25	20	+$20,000	+40 %
30	25	+$28,000	+40 %
35	30	+$40,000	+40 %
40	35	+$56,000	+40 %
45	20	+$221,000	+443 %

Every five years of waiting costs about 40 % more (1.07^5 = 1.40). Wait 25 years and you pay 5.4× for the same result.

Adam, Beth and Carlos make the same point from the other side. All three invest $500 a month at 7 % real. Adam does it from 20 to 30 and stops; Beth from 30 to 40 and stops; Carlos from 30 to 65 and never stops.

Person	Total invested	Value at 65
Adam (20–30, then stops)	$60,000	$910,000
Beth (30–40, then stops)	$60,000	$460,000
Carlos (30–65, never stops)	$210,000	$860,000

Carlos invested 3.5× what Adam did, for 25 more years, and still ended with less. Adam's ten early years did more work than Carlos's extra $150,000.

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Why I plan with 5 %, not 7 %

Here is the part the Coast FIRE pages leave out, and the reason I have not treated our own coast number as a finish line.

The 7 % is an average. Coast FIRE removes your last lever, the contribution, and then hands the result to whichever twenty or thirty years you happen to get. I pulled Robert Shiller's US stock market data (the open dataset is the same series) and worked out rolling 20-year real total returns, dividends reinvested. The median is about 6.8 %, a shade under the 7 % in the tables above. But:

• 22 % of all 20-year windows since 1871 came in under 4 % a year. The worst, mid-1901 to mid-1921, was slightly negative over the whole twenty years; January 1962 to January 1982 managed 0.7 %.
• Since 1926, 19 of the 78 twenty-year windows starting in January were under 4 %. None was negative, and that is the sentence people lean on. It does not help much: a 1 % decade-pair still leaves a coast plan at a third of its target.
• The "lost decade" was worse than flat. From January 2000 to January 2010 the S&P 500 lost about 3 % a year in real terms, roughly a quarter of its purchasing power.

Data: Robert J. Shiller, US equity real total returns 1871–2024. The same chart, and what it did to our plan, is in Coast FIRE isn't a permission slip.

So I plan with 5 %. It sits below the 5.2 % that world equities returned after inflation over 1900 to 2024 (the UBS yearbook figure I used in Coast FIRE isn't a permission slip), well below the US 6.6 %, and if it is wrong it is wrong in the direction I can live through. My wife and I have not crossed our coast number yet; it lands around 40, six to eight years out, and when we do we intend to keep investing and buy a four-day week rather than stop. Coast FIRE is the floor of a plan, the month it stops being fragile. It is not the month you are allowed to walk out.

The practical rule that follows: if the market hands you a bad decade, restart contributions for a few years. You do not have to coast no matter what.

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What Coast FIRE changes about work

Before: "I have to stay at this job because I need to save for retirement."

After: "My retirement is handled. I just need to cover my monthly expenses, and I can do that with work I actually enjoy, even if it pays less."

Once the floor is in, a pay cut for better hours, freelancing, a business, a four-day week, or a calm job hunt all stop being existential. You still need to earn a living, an emergency fund of three to six months, and a yearly recalculation, because lifestyle creep moves the FIRE number and therefore the coast number with it.

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US tax accounts: the 2026 limits

The US tax code is generous to long-term investors, and for Coast FIRE a tax-free wrapper means the compounding is never taxed on the way up.

Account	2026 limit	Why it matters for Coast FIRE
401(k)	$24,500 (+$8,000 catch-up at 50)	Take the employer match first; it is the best return you will get
IRA (Roth or traditional)	$7,500 (+$1,100 catch-up at 50)	A Roth compounds tax-free and comes out tax-free; the best Coast FIRE wrapper there is
HSA (with a high-deductible plan)	$4,400 self-only, $8,750 family	Deductible in, tax-free growth, tax-free out for medical costs; after 65, taxable withdrawals for anything
Taxable brokerage	no limit	Long-term gains are taxed at 0 % below $49,450 of taxable income for a single filer in 2026 ($98,900 married)

Order of operations: 401(k) up to the match, then the Roth IRA, then the HSA if you have one, then the brokerage account. What to invest in: total stock market index funds (VTI, VTSAX) or total world funds (VT, VTWAX). Broad and cheap.

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Common objections

"7 % a year? That's optimistic."

For the US it is slightly above the record since 1900 (6.6 % real), and the risk section above shows what the bad decades look like. Use 5 % if you want a plan that survives them; the calculator lets you set either.

"What if I need that money earlier?"

Your Coast FIRE portfolio is untouchable until retirement; that is what makes the exponent work. Emergencies come out of the emergency fund and your expenses out of income. If you have to raid the pot, recompute the number from what is left.

"Why would I stop investing?"

You do not have to. Coast FIRE is the floor, not the ceiling. Everything above it retires you earlier or richer, and it is the only defence you have against a 1962 or a 2000 start.

"$3,500 a month in retirement won't be enough."

Then use your own figure. In retirement most people have no mortgage, commute or childcare, so the number is usually lower than today's spending, but the formula works with any target.

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Conclusion

Coast FIRE is not about becoming a millionaire. It is about one thing: time is your greatest ally, and the sooner you put it to work, the less you have to do yourself. A few years of real commitment at the start, a return assumption you can defend, a yearly check that the target has not moved, and the math carries you.

Run your own numbers in the Coast FIRE calculator: age, spending, what you already have, and the return you believe in.

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Where to go next

1. Coast FIRE isn't a permission slip — what the worst two decades on record do to a coast plan, and what we intend to do on the day we cross ours
2. My FIRE journey — how my wife and I are doing it
3. The best index funds and ETFs — where to put the money
4. Understanding inflation — why every figure here is in today's dollars

New to all of this? Start with How to start a budget and Investing made simple.

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Questions? Email me at dennis.vymer@myfinancialfreedomtracker.com.