Coast FIRE Explained: How Compounding Alone Can Get You to Retirement

If you've spent any time reading about FIRE and found the savings rates intimidating — 50%+ of after-tax income for 15-20 years just to retire at 45 — Coast FIRE is the variant that makes the math more accessible to people on normal incomes.

I started thinking about Coast FIRE specifically in 2018, when I realized that the savings I'd already accumulated would compound to a "real" retirement number by 60-65 even if I stopped contributing entirely. That realization changed how I thought about work and risk for the next decade. This article is what I wish I'd read earlier.

What Coast FIRE actually means

Coast FIRE is the state where you've saved and invested enough that compounding alone — without any further contributions — will grow your portfolio to your traditional retirement number by traditional retirement age. You're not financially independent today; you're on track to be by 60-65, without needing to save another euro.

The metaphor in the name is precise. You've reached the top of the hill (the savings phase), and now you can coast down (just covering current expenses with current income, letting the existing portfolio grow on its own). You're not retired and not financially independent — but you're freed from the obligation to keep saving aggressively.

The practical implication: once you've reached Coast FIRE, your career flexibility increases dramatically. You can take a lower-paying job you actually enjoy. You can start your own business knowing the runway pressure is reduced. You can take a sabbatical, work part-time, or change directions entirely. The portfolio is going to do its work in the background regardless.

Coast FIRE is structurally different from full FIRE because you're not aiming to live off investment income today — you're just letting investments compound to a future target. The tension between "current expenses" and "investment portfolio" is decoupled.

The math, in plain language

The math behind Coast FIRE is the compound-interest formula running in reverse from your target retirement date.

The forward question (traditional retirement planning): if I save €X per month at Y% return, how much will I have at age 65?

The Coast FIRE question (reverse): if I want €Z at age 65 and I'll get Y% return, how much do I need today to compound to €Z without any more contributions?

The reverse formula is: Today's amount = Future target / (1 + r)^n, where r is the annual return rate and n is the number of years until retirement.

Worked example for a European investor:

• Target retirement age: 65
• Current age: 35
• Years to retirement: 30
• Target retirement spending: €40,000/year (in today's purchasing power)
• Target portfolio at 65: €40,000 × 25 = €1,000,000
• Real return assumption: 5% (after inflation)
• Coast FIRE number today: €1,000,000 / (1.05)^30 = ~€231,000

So if you have €231,000 invested today at age 35, and you achieve 5% real returns, compounding alone gets you to €1,000,000 by age 65 — without saving another euro.

What changes the number:

• Younger you are, lower the Coast FIRE number (more years of compounding). At 25 with the same target, you need only ~€142,000.
• Older you are, higher the Coast FIRE number (less time for compounding). At 50 with the same target, you need ~€481,000.
• Higher target spending = higher Coast FIRE number. €60K spending instead of €40K means €1.5M target instead of €1M, so 1.5× the Coast FIRE number.
• Lower assumed return = higher Coast FIRE number. 4% real instead of 5% real raises your Coast FIRE today by ~30% for a 30-year horizon.

Why Coast FIRE is more achievable than full FIRE

Full FIRE requires you to save enough to support your entire retirement spending, today, indefinitely. That's typically 25-30× annual expenses — €1M+ for a €40K spending target.

Coast FIRE requires you to save enough to get to full FIRE through compounding alone. For a 30-year-old, that's typically 7-15× annual expenses — €280K-€600K for the same €40K spending target. Roughly half to a third of full FIRE.

The mathematical reason: a 30-year horizon at 5% real returns multiplies your money by 4.3×. So you need 4.3× less to start with than to be done.

The practical reason this matters: full FIRE typically requires sustained 50%+ savings rates for 15-20 years. Coast FIRE typically requires 30-40% savings rates for 8-12 years (during which you reach Coast FIRE), then the savings rate can drop dramatically. For most European single-income households, Coast FIRE is achievable; full FIRE often isn't.

Realistic Coast FIRE numbers for Europe

Let me run a few realistic European household scenarios.

Scenario 1: Single 30-year-old earning €45K net, spending €30K

• Target retirement spending: €30K (assuming retirement-spending = current-spending)
• Target retirement portfolio at 65: €30K × 25 = €750K
• 5% real return assumption, 35-year horizon
• Coast FIRE number today: €750K / (1.05)^35 = ~€136K
• Years to reach Coast FIRE saving 30% of net (€13.5K/year) at 5% real: ~9 years
• Coast FIRE achieved at age 39

Scenario 2: Two-income household, both 32, combined €100K net, spending €60K

• Target retirement spending: €60K
• Target retirement portfolio: €1.5M
• Coast FIRE number at 32: €1.5M / (1.05)^33 = ~€307K
• Years saving 30% (€30K/year) at 5% real: ~9 years
• Coast FIRE achieved at age 41

Scenario 3: Older starter, 45 years old, earning €55K net, spending €40K

• Target retirement portfolio: €1M
• 20-year horizon to age 65
• Coast FIRE number: €1M / (1.05)^20 = ~€377K
• If they're starting from €100K, saving 30% (€16.5K/year): they reach €377K at age 56
• Coast FIRE achieved at age 56

Pattern: Coast FIRE for European earners is typically reachable in 8-12 years of disciplined saving, ending in your late 30s or early 40s for most people who start in their 20s, or mid-50s for late starters. Full FIRE is often 5-10 more years on top.

The behavioral implication: hitting Coast FIRE in your 30s gives you 25+ years of "coasting" before traditional retirement, during which you have full career flexibility but don't need new contributions. That's an enormous lifestyle benefit relative to the additional savings discipline required to reach full FIRE.

How to know if you've reached Coast FIRE

The check is straightforward:

1. Pick your target retirement age and target retirement spending. Most readers should use 65 and a realistic spending estimate (start with current spending; adjust if you expect lifestyle changes).
2. Multiply target spending by 25 (for traditional retirement) or 30 (for early retirement). This is your future target portfolio.
3. Apply the reverse compound formula: Coast FIRE = Future target / (1 + real return)^years. Use 5% real return as a reasonable European assumption.
4. Compare your current invested portfolio to that number. If your invested portfolio ≥ Coast FIRE number, you're at Coast FIRE.

A quick reference table (assuming 5% real return, target portfolio = €1M):

| Current age | Years to 65 | Coast FIRE number | |---|---|---| | 25 | 40 | €142K | | 30 | 35 | €181K | | 35 | 30 | €231K | | 40 | 25 | €295K | | 45 | 20 | €377K | | 50 | 15 | €481K | | 55 | 10 | €614K | | 60 | 5 | €784K |

Scale linearly for different target portfolios. €750K target = multiply Coast FIRE numbers by 0.75. €1.5M target = multiply by 1.5.

The Coast FIRE risks worth understanding

Coast FIRE depends on the compounding actually happening. Three risks worth thinking through:

Sequence-of-returns risk during the coasting phase. If markets crash early in your coast and stay down, your portfolio can shrink instead of grow for several years, putting your "compounding alone gets you there" assumption at risk. The standard mitigation: you can resume saving if needed. Coast FIRE isn't a one-way commitment.

Real return below assumption. I used 5% real (above inflation). Historically European/global markets have produced roughly 5-7% real over long horizons, but past performance isn't future returns. If real returns are 3% instead of 5% over the next 30 years, the same Coast FIRE number actually arrives 5-7 years late.

Lifestyle inflation during the coasting phase. When you free up the income that used to go to savings, the temptation to spend it on lifestyle upgrades is real. If your spending grows faster than your salary during the coast phase, your retirement target grows alongside, and your Coast FIRE number — calibrated to a lower spending target — might no longer be enough.

The functional response: Coast FIRE is a meaningful milestone, not a final answer. Continue investing modestly during the coast phase even if not aggressively (€5-10K/year is much less than full-FIRE saving but provides margin). Re-evaluate every 2-3 years against the actual portfolio trajectory.

How Coast FIRE compares to other FIRE variants

The full FIRE landscape:

• Lean FIRE: live very modestly, retire on a small portfolio (€500-750K). Achievable on single income; requires sustained frugality.
• Coast FIRE: stop saving but keep working at lifestyle level. Portfolio compounds to traditional retirement size by 65. €150-500K depending on age and target.
• Barista FIRE: like Coast FIRE but with a part-time job covering most or all of current expenses. The portfolio coasts; the part-time work bridges to retirement age. Useful in countries without universal healthcare; less critical in Europe.
• Standard FIRE: 25-30× expenses, full retirement at 40-55. €1M+ for most people.
• Fat FIRE: high-spending retirement (€80K+/year), €2M+ target. Requires high income or business equity.

Read the FIRE Movement guide for the broader framework.

My personal take: Coast FIRE is the most underrated variant because it's the most accessible and provides the most career flexibility per euro saved. Hitting Coast FIRE in your late 30s means you have 25+ years of optionality before traditional retirement, without the extreme savings discipline that full FIRE requires. For most European readers on normal incomes, this is the variant most worth aiming for first.

When Coast FIRE makes sense (and when it doesn't)

Coast FIRE makes sense when:

• You enjoy your current career or are on a career path you want to continue, but want flexibility
• You have the discipline to save aggressively for 8-12 years up front
• Your spending target is realistic (matches current spending or modest growth)
• You're comfortable letting compounding do most of the work over 20+ years

Coast FIRE doesn't make sense when:

• You hate your current job and the goal of FIRE is escape from work entirely (full FIRE is the right target instead)
• You're already past 55 and don't have time for compounding to multiply your portfolio meaningfully
• You expect significant lifestyle inflation during the coast phase (target keeps moving)
• Your investment portfolio is too aggressive or too conservative to reasonably expect 5% real returns

FAQ

Verdict

Coast FIRE is the most accessible and most underrated variant of the FIRE movement for normal-income European earners. The math is straightforward, the savings discipline is achievable on single incomes, and the lifestyle benefit — 25+ years of career flexibility without needing aggressive ongoing contributions — is genuinely valuable in a way that the more extreme FIRE variants aren't.

For most European readers in their 20s and 30s reading this, Coast FIRE is the more sensible target than full FIRE. Reach Coast FIRE through 8-12 years of 30-40% savings rates, then enjoy career flexibility for the long coast phase. If your circumstances later push you toward full FIRE, you can resume aggressive savings; if they don't, you've still done the harder mathematical work and traditional retirement is on track.

The simplified European playbook: calculate your Coast FIRE number using the formula above (target portfolio at 65 / 1.05^years to 65), set up monthly automated investing into a broad-market UCITS ETF on Trade Republic or DEGIRO, save 30-40% of after-tax income consistently, and check your progress every 2-3 years. The compounding does most of the work; your job is to start the process and not interrupt it.

For the broader context, see the FIRE movement guide, and for the technical underpinnings see the 4 percent rule and sequence of returns risk.