Coast FIRE lets you front-load your retirement savings, then stop contributing while compounding finishes the job. Here’s how to know if your nest egg can already carry you to retirement.
What Coast FIRE Actually Means
Coast FIRE is a strategy within the broader FIRE (Financial Independence, Retire Early) movement, but it flips the usual playbook. Instead of grinding to save 25 times your annual expenses as fast as possible, you front-load a smaller sum early in your career and let compound growth carry it the rest of the way. Once your invested balance can reach your retirement target on its own, you’ve hit your coast number and can stop making new retirement contributions.
The distinction matters. A full-FIRE saver aims to leave work entirely, often decades early. A Coast FIRE saver keeps working but only needs to earn enough to cover current living expenses — rent, groceries, insurance — because the retirement piece is already handled. You’re no longer racing to build the nest egg; you’re protecting the runway you already built.
That shift changes what your job has to do for you. With the long-term math on autopilot, you can take a lower-paying role you enjoy, cut back to part-time, start a business, or take extended parental leave without panic. You’re not retired, and you may not touch the money for decades, but the heaviest lifting on your savings is behind you.
The Math Behind Your Coast Number
The engine of Coast FIRE is compound growth, and its most valuable ingredient is time. A dollar invested at 30 has roughly 35 years to grow before a traditional retirement at 65; a dollar invested at 50 has only 15. Because returns compound on prior returns, those early dollars do a disproportionate share of the work — which is exactly why stopping contributions early can still work.
A quick way to feel this is the Rule of 72: divide 72 by your assumed annual return to estimate how many years money takes to double. At a 7% return, money doubles roughly every ten years, so $200,000 invested at 35 could pass $400,000 by 45 and keep doubling from there — without a single new deposit.
To find your coast number, start with a retirement target. Many planners use the 25x rule, drawn from research on sustainable withdrawal rates, which suggests you might withdraw around 4% of a portfolio annually. Multiply your expected annual retirement spending by 25, then discount it back to today: coast number = target ÷ (1 + r)^n, where r is your assumed real (inflation-adjusted) return and n is years until retirement. Because those assumptions do the heavy lifting, stay conservative — historical averages are educational context, not a promise about your portfolio.
Running Your Own Coast Number
Consider a simplified example. Suppose you want to spend about $60,000 a year in retirement. At the 4% guideline, that implies a target near $1.5 million. If you’re 35 and plan to retire at 65, you have 30 years of compounding ahead of you — the raw material Coast FIRE runs on.
Using a cautious 5% real return assumption, you’d discount that target back 30 years: $1,500,000 ÷ (1.05)^30, or about $1,500,000 ÷ 4.32, which lands near $347,000. In other words, if you already have roughly $350,000 invested for retirement, the math suggests you could stop contributing and still reach your goal — provided your assumptions hold.
Age changes everything. Run the same calculation for a 25-year-old and the coast number drops sharply, thanks to ten more years of compounding; run it for a 50-year-old and it climbs, because there’s far less time. This is also why these figures are illustrative, not personalized — your account types, from traditional 401(k) and IRA to Roth or taxable brokerage, carry different tax treatment on withdrawal, so it’s worth running your own numbers and consulting a qualified professional for anything consequential.
Where Coast FIRE Can Break Down
The biggest vulnerability is sequence-of-returns risk. Because you’ve stopped contributing, a steep market decline in the years right after you coast hits harder — you’re no longer buying at lower prices with fresh money, so the portfolio has to recover on its own. A projection built on smooth average returns can look very different against a rocky first decade.
Optimistic assumptions are the second trap. Plugging in a 10% nominal return instead of a sober inflation-adjusted figure can make you feel finished years before you actually are. Inflation compounds against you too: the $60,000 lifestyle you price today will cost meaningfully more in three decades, so your target should be expressed in real terms.
Then there’s life. Lifestyle creep can quietly raise your future spending, a job loss can force you to tap savings early, and healthcare is a common blind spot — Medicare doesn’t begin until 65, so coasting before then means covering premiums yourself, often through the ACA marketplace. None of this makes Coast FIRE unworkable; it makes it something to revisit, with a buffer above the bare minimum and a willingness to resume contributions if markets disappoint.
Bridging the Coast Years
Reaching your coast number doesn’t end your financial planning — it narrows it to the present. During the coast years you still need income for current expenses, so the practical goal becomes earning enough to live on without dipping into the retirement money you’re trying to leave alone.
Two habits protect that runway. First, keep an emergency fund fully separate from your invested assets, so a surprise expense doesn’t force you to sell during a downturn. Second, think hard before walking away from an employer 401(k) match — even while coasting, a match is an immediate return on your contribution that’s tough to replicate elsewhere.
Access rules matter more once these accounts become central. Most retirement accounts impose a 10% penalty on withdrawals before age 59½, with exceptions such as the Rule of 55 for certain workplace plans, and some early retirees plan ahead with Roth conversion ladders — strategies with tax consequences worth understanding fully first. Treat coasting as a living plan rather than a finish line: an annual check-in to confirm you’re on track, and adjust contributions if not, is what turns a tidy spreadsheet into a retirement you can count on.
