What Is Your Coast FIRE Number and How to Calculate It?

Coast FIRE is the point at which your investment portfolio has grown large enough that you can stop contributing and let compounding carry you to your full retirement goal. It represents a middle ground between full-time employment and complete financial independence—you "coast" to your target by halting contributions while your existing savings compound at market returns. This article explains how to calculate your Coast FIRE number and why it matters for retirement planning.

Quick definition: Your Coast FIRE number is the portfolio size that, when invested for a specified number of years at your expected return, will grow to your target retirement number without any additional contributions.

Key takeaways

• Coast FIRE lets you reduce work (part-time, freelance, less stressful) while maintaining your full retirement target.
• The Coast FIRE number depends on your target retirement number, years until full retirement, and expected investment returns.
• Coast FIRE works best with long time horizons (20+ years); shorter horizons require higher current balances.
• Reaching Coast FIRE is a psychological milestone: you've "won" financially, even if you continue working.
• Trade-offs exist between reaching Coast FIRE early and achieving other financial goals (home purchase, education, debt payoff).

The logic behind Coast FIRE

Traditional early retirement (FIRE) requires two conditions: your portfolio is large enough to generate your full annual expenses today, and you have complete financial independence. Coast FIRE relaxes the second condition. You don't need full independence yet; you need only enough to stop actively saving and let time do the work.

The math is straightforward: Future Value (FV) = Present Value (PV) × (1 + r)^n, where r is annual return and n is years. Rearranging: PV = FV / (1 + r)^n. Your Coast FIRE number is the present value that compounded grows to your retirement target.

Example: Your retirement target is $2 million. You're 35 and plan to retire at 65—a 30-year horizon. Assuming a 7% annual real return (after inflation), your Coast FIRE number is:

Coast FIRE = 2,000,000 / (1.07)^30
Coast FIRE = 2,000,000 / 7.612
Coast FIRE = $262,650

At age 35 with $262,650 invested, you can stop saving, work part-time, and reach $2 million by age 65 through compound growth alone. Every dollar above $262,650 is "extra"—money you can spend, gift, or invest in higher-risk vehicles.

Calculating your personal Coast FIRE number

Three inputs define your Coast FIRE number:

1. Target retirement number. This is your full retirement goal—typically derived from the 4% rule (annual expenses / 0.04) or a more conservative approach. If you need $100,000 annually and use the 4% rule, your target is $2.5 million. This is your FV in the formula.

2. Years to retirement. How many years until you want to stop working entirely? This is n in the formula. If you're 40 and target 70, use 30. If you're 35 and target 60, use 25. Shorter horizons require higher current balances; longer horizons allow smaller current balances.

3. Expected real return. This is your annual investment growth rate. Nominal returns (7–10% for stocks) minus inflation (2–3%) yields real returns of 4–7%. Many retirees use 5–6% for mixed portfolios (60/40 stocks/bonds). Use after-inflation returns to match your inflation-adjusted retirement target.

Putting it together:

Target	Years	Return	Coast FIRE Number
$1,000,000	30	5%	$231,377
$1,000,000	30	7%	$131,367
$2,000,000	30	5%	$462,755
$2,000,000	30	7%	$262,651
$2,000,000	20	5%	$751,314
$2,000,000	20	7%	$517,456

Notice: Longer time horizons and higher assumed returns both lower your Coast FIRE number. A 7% return over 30 years is much more powerful than 5%; you need a smaller starting balance. Conversely, a 20-year horizon demands 2–2.5× larger starting balance than 30 years, all else equal.

Coast FIRE versus full FIRE

Full FIRE (like the 4% rule) means your portfolio generates your full annual expenses today. You can retire immediately.

Coast FIRE means your portfolio will eventually generate your expenses, but not yet. You must continue earning income—though often at reduced intensity.

The financial difference: if your target is $2 million and you're currently $262,650 (at 30 years to retirement, 7% return), you're at Coast FIRE but need $1.737 million more to reach full FIRE. That gap shrinks dramatically with time. After 5 years of no contributions, $262,650 grows to $367,627; the gap to $2 million shrinks to $1.632 million. After 10 years, it shrinks to $1.365 million. Compound interest does the heavy lifting.

The lifestyle difference: at full FIRE, you stop working entirely (or work only for passion/purpose). At Coast FIRE, you might:

• Shift to part-time work (20–30 hours/week instead of 40+).
• Switch to freelance or consulting (more control over hours).
• Take a lower-stress job (reduced compensation, higher well-being).
• Focus on passion projects that don't pay well.

This is powerful: you've solved your retirement mathematically, but you retain the option to work in a way that aligns with your values.

Reaching Coast FIRE through savings

Rather than waiting passively, you can accelerate reaching Coast FIRE through higher savings rates. This is the classic trade-off: save aggressively for 5–10 years, hit Coast FIRE early, then work part-time for decades.

Example: You're 30, your target is $2 million at 65 (35 years), and you assume 6% real return. Your natural Coast FIRE number is:

Coast FIRE = 2,000,000 / (1.06)^35 = $179,058

But suppose you're currently at $50,000 and saving $20,000/year. At 6% return:

• After 5 years: $50,000 × (1.06)^5 + $20,000 × (((1.06)^5 - 1) / 0.06) = $267,000 + $115,000 = $382,000

At $382,000 in year 5, you exceed your Coast FIRE target of $179,058. From year 5 to 65 (30 more years), your $382,000 grows to:

$382,000 × (1.06)^30 = $2,449,000

You reach your $2 million goal and can work part-time starting age 35. The 5 years of aggressive saving (50%+ of income) collapsed a 30-year work requirement into a 5-year sprint plus a 30-year coast.

Visual decision tree: when to pursue Coast FIRE

Real-world examples

Case 1: Tech worker pursuing coast at 35

Alex is 35, earns $120,000, and has saved $400,000. His target retirement expense is $60,000/year, giving a target number of $1.5 million (4% rule). With 30 years to 65 and a 6% real return, his Coast FIRE target is:

Coast FIRE = 1,500,000 / (1.06)^30 = $264,034

Alex is already at Coast FIRE ($400,000 > $264,034). He can immediately switch to a $60,000/year consulting gig, dropping his income but gaining control of his schedule. His $400,000 portfolio will grow to nearly $2.1 million by age 65—well above his $1.5 million target.

Case 2: Couple working toward Coast FIRE together

Jamie and Morgan, both 32, have combined savings of $200,000 and household income of $150,000. Their target retirement spending is $80,000/year (target number: $2 million). With 33 years to 65 and 5.5% real return, Coast FIRE is:

Coast FIRE = 2,000,000 / (1.055)^33 = $298,000

They're $98,000 short. If they save $15,000/year for 7 years, they'll have $200,000 + $110,000 = $310,000, exceeding Coast FIRE. One partner can then reduce hours or switch to part-time remote work while the other maintains full employment. Their household stress and burnout decline while their financial timeline accelerates.

Case 3: Late start, realistic coast horizon

Keisha is 50 with $350,000 saved and wants to retire at 67 (17 years). Her target is $2 million. With 5% real return, Coast FIRE is:

Coast FIRE = 2,000,000 / (1.05)^17 = $877,193

She's well short ($350,000 vs. $877,193). Coast FIRE doesn't work in a 17-year window; she needs to save $44,000+ annually or extend her retirement to 72. Alternatively, she can accept a lower retirement target ($1.2 million for $48,000 annual spending) and reach Coast FIRE faster. Coast FIRE works best with long horizons; short horizons require high savings rates.

Common mistakes

Underestimating the required return. Many assume 8–10% real returns (after inflation), but this is optimistic for retirees who shift to lower-risk allocations. A 60/40 portfolio historically returns 5–6% real; a 40/60 portfolio returns 4–5%. Overstating returns shrinks your calculated Coast FIRE number, leading to a nasty surprise at retirement if actual returns lag.

Ignoring tax-deferred and taxable account differences. Coast FIRE is typically reached first in tax-deferred accounts (401k, IRA) that have high contribution limits. But to reach full FIRE later, you'll likely need both accounts. Don't assume your Coast FIRE number is the same across all account types—tax drag is higher in taxable accounts, so you may need slightly more there.

Failing to account for sequence of returns risk. Your Coast FIRE calculation assumes steady 6–7% returns. In reality, markets are volatile. If you coast into a bear market in year 28 (age 63, two years before retirement), you may fall short of your $2 million target. Consider a conservative buffer (assume 1–2% lower returns) or a later retirement age to insure against this risk.

Forgetting lifestyle creep. When you reach Coast FIRE and shift to part-time work, your income drops. But does your spending? Many people coast on $50,000/year while their spending remains $70,000, requiring continued portfolio withdrawals. Ensure your Coast FIRE transition includes a realistic spending plan that aligns with your reduced income.

Conflating Coast FIRE with retirement. Coast FIRE is not retirement; it's a way station. You still need income to cover expenses. Only when your portfolio reaches full FIRE (generates all expenses via the 4% rule) are you truly retired. Coast FIRE is the point where your future is secured, but your present still requires work.

FAQ

Is Coast FIRE the same as Financial Independence?

Not quite. Financial Independence (FI) means your portfolio generates all your expenses; you can retire. Coast FIRE means your portfolio will eventually generate all expenses, but not yet. FI is "retire now"; Coast FIRE is "can work part-time now, retire fully later."

What return rate should I use for Coast FIRE calculations?

Use real return (after inflation). For a diversified portfolio, 5–6% is realistic. If you're more conservative (40/60 stocks/bonds), use 4–5%. If you're more aggressive (80/20), use 6–7%. Check historical data for your intended allocation—Vanguard and Morningstar publish long-term return assumptions.

Can I Coast FIRE multiple times?

Yes. You might hit Coast FIRE at 35 with a $300,000 portfolio, work part-time 10 years, then hit "lean FIRE" (partial retirement) at 45 with a $600,000 portfolio. Multi-stage coast strategies work for those with extended time horizons and flexible careers.

What if my target retirement number changes?

Recalculate. If inflation or lifestyle changes push your target from $1.5 million to $2 million, your Coast FIRE number rises. If you can live on less, it falls. Every 5 years, revisit your target and your current Coast FIRE position to stay on track.

Should I Coast FIRE or save more aggressively?

This is a values question. Coast FIRE trades additional savings for reduced work intensity and more present-day well-being. If you're burning out at your current job, Coast FIRE is worth aggressive early saving. If you enjoy your work and have strong future earning power, standard retirement savings may feel less urgent.

What if I'm behind on my Coast FIRE number?

If you're 40 and your Coast FIRE number is $400,000 but you have only $150,000, you have options: (1) Save more aggressively; (2) Extend your retirement age; (3) Lower your retirement target; (4) Assume higher returns (though risky). Most commonly, a mix of these applies—a modest increase in savings, a one-year extension, and a slight lifestyle adjustment.

How does Coast FIRE interact with 401k and IRA limits?

Favorably. Your 401k (up to $24,000/year as of 2025) and backdoor Roth IRA ($7,000/year) fill quickly. Once you hit Coast FIRE, you can redirect the money that would have gone to these accounts into taxable index funds, Roth conversion ladders, or household debt payoff. This flexibility is a huge advantage of reaching Coast FIRE early.

Related concepts

• Understanding the 4% Rule
• How to Calculate Your Retirement Number
• Inflation and Your Retirement Number
• Chapter 14 Overview: Early Retirement and FIRE
• Glossary

Summary

Your Coast FIRE number is the portfolio size that, when invested for the years remaining until your target retirement age, will grow to your full retirement goal without further contributions. Calculating it requires three inputs: your target retirement number, years to retirement, and expected real return. For those with long time horizons (20+ years), Coast FIRE is often achievable through just 5–10 years of aggressive saving, enabling a 20–30 year coast on part-time or passion work. The key is to use realistic return assumptions, account for sequence-of-returns risk, and ensure your Coast FIRE transition includes a spending plan that aligns with reduced income. As with all retirement calculations, rules and economic assumptions change—revisit your Coast FIRE number every few years to confirm you're still on track.

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