The Complete Coast FIRE Guide: the math, the mistakes, and whether you're already there

Coast FIRE is the least dramatic and most useful idea in the FIRE movement. It does not ask you to quit your job, move somewhere cheap, or live on rice and beans. It asks one question: do you already have enough invested that compound growth can carry you to a full retirement number by a traditional retirement age — even if you never save another dollar?

If the answer is yes, you have permission to stop sprinting. You still work. You still cover your bills. But the aggressive accumulation phase — the 40-to-60% savings-rate grind — can end. That is what “coasting” means: your portfolio does the heavy lifting from here, and your paycheck only has to cover your life.

This guide is the full treatment: the formula, worked numbers at every age, why the return assumption matters more than everything else combined, how Coast differs from Barista and full FIRE, the five mistakes that wreck Coast plans, and a checklist for deciding whether you are already there. Every example is illustrative — run your own numbers in the Coast FIRE calculator before changing your savings behavior.

The one formula that matters

If your full FIRE target is T, your assumed real (after-inflation) return is r, and the years until a traditional retirement age is n, then your Coast number ≈ T ÷ (1+r)^n. The formula discounts your future target back to today, the same way a bond is priced: money you will have later is worth less than money you have now, and compounding bridges the gap.

Each input deserves a sentence. T is annual spending divided by a withdrawal rate: $60,000 of yearly spending at a 4% withdrawal rate means T = $1,500,000. The r is the real return — after inflation, not the number on your brokerage statement. A portfolio earning 8% nominal in a 3% inflation world is earning roughly 5% real, and it is the real number that belongs in the formula. The n is your runway: 65 minus your current age, if 65 is your target retirement age.

The shape of the formula is the whole insight. A longer runway or a higher assumed return shrinks the number you need today — exponentially, not linearly. That is why Coast FIRE is a young person's superpower and a 55-year-old's modest discount.

Worked examples at every age

Take $60,000 of annual spending and a 4% withdrawal rate, so the full FIRE target is $1,500,000. Assume 5% real returns and a traditional retirement age of 65:

• Age 25 (40-year runway): $1,500,000 ÷ 1.05^40 ≈ $213,000. A 25-year-old with $213,000 invested may never need to save for retirement again — if the assumptions hold.
• Age 35 (30-year runway): $1,500,000 ÷ 1.05^30 ≈ $347,000.
• Age 45 (20-year runway): $1,500,000 ÷ 1.05^20 ≈ $565,000.
• Age 55 (10-year runway): $1,500,000 ÷ 1.05^10 ≈ $921,000.

Read the table in reverse, too. A 35-year-old with $500,000 already invested is past the Coast number: $500,000 compounding for 30 years at 5% real grows to about $2,160,000, comfortably above the $1.5M target. A 35-year-old with $200,000 is not coasting yet — but that $200,000 still grows to roughly $864,000 over 30 years at 5% real, which means more than half the target is already handled with zero future contributions.

Notice what the table does not say. It does not say you can retire. Every one of these examples assumes you keep working and covering all of your spending — housing, food, healthcare, everything — until 65. The portfolio is growing untouched in the background. Coast FIRE funds your 65-year-old self, not your 40-year-old self.

The return assumption is the whole game

The formula is simple. The input is not. Because the return is raised to the power of your runway, small changes in r move the Coast number enormously. Same $1.5M target, same 30-year runway, different real returns:

• 3% real → Coast number ≈ $618,000
• 4% real → Coast number ≈ $462,000
• 5% real → Coast number ≈ $347,000
• 6% real → Coast number ≈ $261,000
• 7% real → Coast number ≈ $197,000

The cautious 3% case demands more than three times the portfolio of the optimistic 7% case. Anyone who shows you a Coast number without showing you this sensitivity is selling certainty they do not have. The honest rule: run your number at 3% and 4% real before you change your savings behavior. If you are only “coasting” at 7% real, you are not coasting — you are hoping.

There is a second, subtler trap inside r: sequence risk applies to the growth phase, not just the withdrawal phase. The formula assumes a smooth 5% every year. Real markets deliver 5% average as a lumpy mix of great decades and lost decades. A bad decade early in your coast hurts far more than a bad decade late, because early losses compound against you for longer. That is why the Coast FIRE calculator's sequence stress test exists — use it to see ranges, not just a single number.

What a lost decade does to a Coast plan

Here is the lost decade made concrete. You are 35 with $347,000 — exactly your Coast number for a $1.5M target at 5% real over 30 years. The math checks: $347,000 compounding at 5% real for 30 years becomes $1,500,000 on the nose. Now suppose the first ten years deliver 0% real — a genuine lost decade, the kind markets have delivered before. Your $347,000 is still $347,000, but only 20 years remain. To still reach $1.5M, those 20 years must now deliver about 7.6% real every year. That is a heroic assumption stacked on top of a bad decade.

Two lessons. First, the Coast number is most fragile exactly when you stop paying attention to it — early, when compounding has the most time left to work for or against you. Second, margin is not optional: this is the mathematical reason for the “coast at 4%, confirm at 3%” rule. A plan that only works if the first decade cooperates is not a plan.

Coast vs Barista vs full FIRE

Three different questions, three different numbers. Mixing them up produces the wrong savings target and the wrong lifestyle story:

• Full FIRE asks: “Can I stop working forever, today?” Number = annual spending ÷ withdrawal rate. $60,000 at 4% → $1,500,000.
• Coast FIRE asks: “Can compounding finish the job by 65 with zero future contributions?” Number = full FIRE ÷ (1+r)^n. At 35 with 5% real → ≈ $347,000. You keep working and covering your bills.
• Barista FIRE asks: “Can I semi-retire now if part-time work covers part of my spending?” Number = (spending − work income) ÷ withdrawal rate. $60,000 spending with $20,000 of barista income → $40,000 gap ÷ 4% → $1,000,000.

Here is the intuition trap: Coast often produces a much smaller number than Barista, which makes it look like the easier path. It is not easier — it answers a different question. Coast keeps you in full earning years for decades while the portfolio compounds silently. Barista buys lifestyle freedom now, at the price of a bigger portfolio plus the ongoing need for work income. Choose by which question you are actually asking, not by which number is smaller.

Where the money lives matters

The formula treats all dollars as equal. The tax code does not. A $347,000 Coast number split across a 401(k), a Roth IRA, and a taxable brokerage behaves differently than $347,000 all in one pre-tax account — especially since you cannot touch retirement accounts before age 59 1/2 without penalties and exceptions, while the whole point of coasting is leaving the money alone until 65 anyway.

For most Coast plans this is a second-order concern, because you are not withdrawing during the coast — you are letting everything compound. But account placement still shapes the landing: pre-tax dollars will be taxed as ordinary income when withdrawn, Roth dollars will not, and the mix determines your tax bill in the retirement the portfolio is funding. When you run your number, run it against invested assets net of the taxes you will eventually owe, not the gross balance that makes you feel rich.

Mistake 1: treating “coast” as “retired”

The most common and most dangerous error. Hitting your Coast number means your 65-year-old self is funded. It does not fund a single year of spending between now and then. If you quit your job at 40 with a Coast number in hand and no income, you will spend the portfolio that was supposed to compound — and the math collapses. Coast FIRE requires covering 100% of your living costs from earnings until traditional retirement age. The day you stop covering your spending is the day you stop coasting.

Mistake 2: using nominal returns instead of real ones

“The market returns 10% historically” is the fastest way to a fantasy Coast number. Ten percent nominal minus three percent inflation is roughly seven percent real — and even that is generous for planning. Plug 7% real into a 30-year runway and your Coast number drops to $197,000 on a $1.5M target, which feels great and means nothing. Use 3–5% real for planning, and make your go/no-go decision at the low end of that range. Optimism is not a contribution.

Mistake 3: forgetting the paycheck still has three jobs

Once you stop saving aggressively, your paycheck must still do three things without fail: cover all spending, carry health insurance, and maintain an emergency fund. Coast plans die most often on the second one. An uninsured health event in your 50s does not just cost money — it raids the compounding portfolio, which is the entire engine of the plan. Before you downshift, price your healthcare explicitly and keep six months of expenses liquid. The Coast number assumes the portfolio is never touched early; your emergency fund is what makes that true.

Mistake 4: computing the number once and never revisiting it

A Coast number is a snapshot, not a tattoo. Markets fall, spending creeps, inflation runs hot, and assumptions age. Recheck annually: update your invested balance, update your spending (use 12 months of actuals, not a budget you wish were true), and rerun the formula at conservative returns. A plan you never revisit is a plan you will discover is broken at the worst possible time. The five-minute annual recheck is the cheapest insurance in personal finance.

Mistake 5: ignoring what happens if returns disappoint early

The formula's smooth compounding hides the real risk: a poor decade in your 30s damages a Coast plan far more than a poor decade in your 50s. Build margin deliberately. One practical rule: only downshift your savings when you coast at 4% real and are still on track at 3% real. That one-percentage-point buffer is the difference between a plan and a prayer. And keep the ability to restart contributions — the best Coast plans are reversible.

Checklist: are you actually coasting?

• Price your spending from 12 months of actuals, including healthcare, taxes, and the boring stuff. Add 10% if you are guessing.
• Set your full FIRE target: annual spending ÷ withdrawal rate. Use 4% as a starting point, 3–3.5% if your horizon is 40-plus years.
• Compute your Coast number at 4% real — then again at 3% real. Downshift only if both say yes.
• Count invested assets only. Home equity does not compound into retirement spending; neither does your emergency fund.
• Confirm your paycheck covers all spending plus healthcare plus an emergency buffer, with no portfolio withdrawals.
• Stress-test the growth phase for bad early decades, not just the average.
• Recheck every year. Spending, balances, and assumptions all move.

FAQ

• Does Coast FIRE mean I can quit my job? No. It means you can stop saving aggressively. Your earnings must still cover 100% of your spending until traditional retirement age.
• What real return should I assume? Plan at 3–4% real and treat 5% as the optimistic case. Equities have delivered more historically, but planning is about the bad decades, not the average one.
• What counts toward my Coast number? Invested assets you will not touch: 401(k)s, IRAs, taxable brokerage. Exclude home equity, emergency funds, and anything you plan to spend.
• What if I have a pension? A pension that covers part of your retirement spending lowers your full FIRE target T, which lowers your Coast number through the same formula. Price the pension conservatively.
• What if the market crashes right after I stop saving? That is the central risk — early bad returns compound against you longest. This is why you keep margin (the 3% test), keep an emergency fund, and keep the ability to restart contributions.
• Can I coast, then un-coast? Yes, and you should expect to. Career breaks, windfalls, and market crashes all change the math. Recompute yearly and adjust without drama.
• Does Coast FIRE work if I'm starting from zero at 40? The formula does not care about your past — it only prices the runway ahead. At 40 with 25 years to 65 at 5% real, a $1.5M target needs about $443,000 today ($1,500,000 ÷ 1.05^25). Starting from zero, the question becomes how fast you can save that first chunk; the Years to FIRE calculator models exactly that sprint.
• Should I count my house? No. Home equity does not compound into retirement spending unless you plan to sell or downsize, and most people do not. Count invested assets you can actually leave alone for decades.
• What about kids and college costs? They belong in your spending number, not as a footnote. If college will cost you $100,000 over four years in your 50s, that money is not compounding toward 65 — model it as spending, or fund it from a separate bucket.
• How is Coast FIRE different from just saving normally? It isn't, mechanically — that is the point. Coast FIRE is what normal retirement saving looks like once the balance is large enough that compounding outruns contributions. The label matters because it gives you permission to downshift spending on savings deliberately instead of guiltily, and a formula for knowing when that permission is earned.
• Can couples coast on one portfolio? Yes, with one adjustment: use household spending for T and the later of the two retirement ages for n. The partner with the longer runway sets the timeline, because the portfolio must last until both of you are done working. Price healthcare for two — it is the budget line that doubles fastest.
• When should I not coast? When the paycheck cannot reliably cover spending, when healthcare is unpriced, when the number only works at 6% real or better, or when you would have to raid the portfolio for anything in the next decade. Coast FIRE rewards the already-stable. If life is volatile, keep saving — flexibility beats a label.

The bottom line

Coast FIRE is permission to stop sprinting, not permission to stop. The math is one division problem; the discipline is everything around it — honest spending, conservative returns, healthcare covered, margin for bad decades, and a yearly recheck. Get those right and compounding does the rest. Run your number in the Coast FIRE calculator, stress-test it, and walk the checklist before you change anything about how you save. Educational only — not financial advice.