Coast FIRE: When You Can Stop Saving and Still Retire on Time

Most FIRE math answers one question: how much do you need, total, to retire. Coast FIRE answers a different one: how much do you need right now, at your current age, such that you never have to add another dollar to retirement accounts and still hit your number by the time you want to retire.

The distinction matters because it changes what you optimize for. Once you've coasted, the goal shifts from saving aggressively to covering your current cost of living, which for a lot of people means work becomes more flexible, not because they're financially independent yet, but because the retirement math is already solved in the background.

The mechanism: compound growth doing the remaining work

The core idea is that money invested early has more years to compound than money invested late, and at some point, the balance you already have, growing at a reasonable average rate, closes the remaining gap to your number on its own, without new contributions. The math is just the future value of a lump sum: current balance × (1 + growth rate)^years remaining. The SEC's compound interest calculator is a good place to sanity-check this kind of projection with your own numbers rather than trusting a single worked example.

A worked example

Say you're 30 years old, have $150,000 invested, and want to retire at 60 with a $1,500,000 portfolio (using a 4% withdrawal rate framework; see the 4% rule for the reasoning and its limits). Assume a 7% average annual real return, a common long-run planning assumption for a diversified stock portfolio, though not a guarantee.

$150,000 × (1.07)^30 ≈ $150,000 × 7.61 ≈ $1,141,500.

That's short of $1,500,000, so at 30 you haven't hit Coast FIRE yet. You'd still need either more savings, more time, or a lower spending target. Now suppose you keep contributing for five more years and reach $225,000 by age 35, with 25 years left until 60:

$225,000 × (1.07)^25 ≈ $225,000 × 5.43 ≈ $1,221,750.

Still short. Push to $280,000 by age 38, 22 years to go:

$280,000 × (1.07)^22 ≈ $280,000 × 4.43 ≈ $1,240,000.

Getting closer, but this illustrates the sensitivity: at 7%, reaching $1.5 million by 60 from a starting point in your late 30s requires a balance in the $300,000-$320,000 range at that age, with nothing added after. The exact crossover point depends entirely on your real return assumption, which is the biggest variable in this whole exercise, not a fixed input.

Solving for your own coast number

The formula rearranges easily to answer "what balance do I need today" instead of "what will my balance grow to": coast number = target number ÷ (1 + growth rate)^years remaining. Plug in your own target, your own age, and your own retirement age, and you get a single dollar figure: the amount that, sitting untouched and continuing to grow, gets you there without another contribution.

The table below uses the same $1,500,000 target and 7% assumption as the worked example, just solved in reverse, to show how much the "coast number" moves for each five years of head start:

| Current age | Years to compound (retiring at 60) | Coast number needed today | |---|---|---| | 25 | 35 | ≈ $140,000 | | 30 | 30 | ≈ $197,000 | | 35 | 25 | ≈ $276,000 | | 40 | 20 | ≈ $387,000 | | 45 | 15 | ≈ $543,000 |

Notice the pattern: each five-year delay raises the required balance by roughly 40%, because you're removing five years of compounding from the same target. That's the practical takeaway more than any single number in the table: the earlier you can front-load savings, the smaller the balance you need to hit before you're allowed to ease off.

Coast FIRE, Barista FIRE, and full FIRE aren't the same finish line

It's worth being precise about what "coast" means relative to nearby FIRE variants, since the terms get used loosely. Coast FIRE means your retirement accounts are on track without further contributions, but you're still covering today's living costs from today's income, full-time, part-time, or otherwise. Barista FIRE usually refers to stepping down to lower-stress or part-time work specifically because a partial nest egg plus reduced income needs makes that sustainable, often paired with employer health coverage. Full FIRE means the portfolio covers all spending and no work income is needed at all. Coast FIRE is compatible with either continuing full-time work or downshifting. It's a statement about the retirement accounts specifically, not a statement about your current job.

Checking whether you've already coasted

Some people run this calculation and discover they passed their coast number years ago without noticing, because they saved aggressively in their 20s and then let contributions taper as other priorities (a house, kids, a career change) took over. If that's you, the practical use of the exercise isn't to start coasting retroactively. It's to recognize that additional retirement contributions are optional rather than obligatory, which can free up cash flow for other goals: paying down a mortgage faster, funding kids' education, or simply spending more on the present without guilt, as long as the retirement number is genuinely already on track.

Why the growth-rate assumption is the whole ballgame

A 7% real return is a reasonable long-run planning assumption for a stock-heavy portfolio, based on historical U.S. market averages, but "average" hides a lot of variation, and some 20-year stretches have returned far less, some far more. Coast FIRE math is unusually sensitive to this input because it's compounding over decades with zero new contributions to smooth out a bad stretch. A more conservative assumption (5-6%) produces a higher, safer coast number; a more aggressive one (8-9%) produces a lower one that's riskier to actually rely on.

This is also why a single coast number calculated once, years before retirement, shouldn't be treated as permanent. A reasonable practice is to recheck the calculation every year or two using your actual current balance and actual years remaining, rather than trusting a projection made a decade earlier. If markets have run well ahead of the assumed rate, you may already be past your coast number sooner than planned; if returns have lagged, the number you need may have moved further out. Either way, the check is cheap to run and meaningfully better than assuming the original projection still holds.

Decision framework

• Pick your target number and target age first, using something like a FI number calculator, before working backward to a coast number. Coast FIRE only makes sense relative to a real target.
• Choose a growth-rate assumption you'd defend to a skeptical friend. Err conservative if you're going to stop contributing based on this number. You can't undo lost compounding years if the return comes in lower than planned and you'd already stopped saving.
• Decide what "coast" means for your actual asset allocation. Coasting doesn't mean ignoring the portfolio; it still needs to stay invested and appropriately allocated for the time horizon, and rebalanced periodically.
• Separate "stop saving for retirement" from "stop earning." Most people who hit Coast FIRE still work and still cover current living costs from current income; the accounts just don't need new retirement contributions. Confirm your current income comfortably covers current spending before treating this as license to downshift.

What to do with the freed-up cash flow

Once the retirement math is on track, the money that used to go toward aggressive retirement contributions doesn't have to sit idle. Common destinations: building the bridge account that covers spending before your retirement accounts are accessible, paying down a mortgage or other debt faster, funding a shorter-term goal like a home renovation or a child's education, or simply increasing current lifestyle spending deliberately, rather than by drift. The mistake to avoid is treating "I hit my coast number" as license to stop thinking about money entirely. It answers one question (is the long-term number on track) and leaves several others (near-term cash flow, health insurance, what work looks like) still open.

Limits and exceptions

This framework assumes a fairly steady, uninterrupted growth path and no withdrawals from the coasting balance before the target date. A market downturn plus an emergency withdrawal in year one can meaningfully change the trajectory, especially early, when there's less balance to absorb a shock (see sequence of returns risk for why the order of returns, not just the average, matters). It also doesn't account for inflation eroding your target number's purchasing power if you set the target in today's dollars but retire decades later. Run the calculation in either all-real or all-nominal terms consistently, not a mix. And it says nothing about health insurance, taxes, or other early-retirement logistics; hitting a coast number is a savings milestone, not a full retirement plan.

Coast FIRE is ultimately a reframe more than a formula: it tells you when the hardest saving years are behind you, so you can redirect effort toward the parts of the plan that still need it.