Coast FIRE Calculator: How Much You Really Need (And When to Stop Saving)

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title: "Coast FIRE Calculator: How Much You Really Need (And When to Stop Saving)" description: "Complete guide to Coast FIRE: calculate your number, understand the math, know when to coast vs accelerate, and use this flexibility strategy for late-starters." date: "2025-12-09" author: "Jonathan" category: "Calculators" readingTime: "13 min"

Coast FIRE changed how I think about my FIRE timeline.

When I discovered it at 40, I realized I didn't have to choose between "grind until $1.5M" or "give up on FIRE entirely." There was a middle path: get to a specific number, then stop aggressive saving and let compound growth carry me to full FIRE.

It's the flexibility strategy that makes late-starting FIRE actually sustainable.

This post explains exactly what Coast FIRE is, how to calculate your Coast FIRE number, when it makes sense, and when you should keep accelerating instead.

What Is Coast FIRE?

Coast FIRE is the point where you have enough invested that — with no additional contributions — your portfolio will grow to your full FIRE number by traditional retirement age (typically 65-67).

Once you hit your Coast FIRE number, you can:

• Stop aggressive saving
• Take a lower-paying but more enjoyable job
• Work part-time to cover current expenses
• Reduce stress and burnout
• Let your investments "coast" to full FIRE while you live your life

The key difference:

• Traditional FIRE: Save aggressively until you have 25× annual expenses, then retire completely
• Coast FIRE: Save aggressively until you have enough for compound growth to do the rest, then ease off the gas
• Barista FIRE: Work part-time to cover expenses while your portfolio grows
• Lean FIRE: Retire early with a smaller portfolio and reduced lifestyle

Coast FIRE sits between "still grinding" and "fully retired." It's the off-ramp for people who want freedom now without waiting for the full FIRE number.

The Coast FIRE Formula

The formula is simple. You're solving for the Present Value needed today so that it grows to your target portfolio by retirement age.

Basic Formula:

Coast FIRE Number = Target Portfolio ÷ (1 + r)^n

Where:

• Target Portfolio = Your full FIRE number (annual expenses × 25, or whatever multiplier you use)
• r = Expected annual return (e.g., 0.06 for 6%)
• n = Number of years until traditional retirement age (e.g., 25 years if you're 40 and planning for 65)

Example:

Let's say:

• Your FIRE number is $1,500,000 (to support $60,000/year at 4% withdrawal rate)
• You're 40 years old, planning for retirement at 65 (n = 25 years)
• Expected return: 6% annually

Coast FIRE Number = $1,500,000 ÷ (1.06)^25

Breaking it down:

• (1.06)^25 = 4.2919
• $1,500,000 ÷ 4.2919 = $349,530

Interpretation: If you have $349,530 invested today and earn 6% annually with no additional contributions, it will grow to $1,500,000 in 25 years.

That's your Coast FIRE number.

Real-World Scenarios: What Your Coast Number Might Be

Let me show you scenarios for different ages, FIRE numbers, and assumptions.

Scenario 1: Age 35, Targeting $1.2M by 65

Assumptions:

• Current age: 35
• Retirement age: 65
• Years to grow: 30
• Target FIRE number: $1,200,000
• Expected return: 6%

Coast FIRE Number = $1,200,000 ÷ (1.06)^30

• (1.06)^30 = 5.7435
• Coast Number = $208,900

What this means: If you're 35 and have $209K invested, you can stop contributing and still hit $1.2M by 65 at 6% returns.

Scenario 2: Age 40, Targeting $1.5M by 65 (My Scenario)

Assumptions:

• Current age: 40
• Retirement age: 65
• Years to grow: 25
• Target FIRE number: $1,500,000
• Expected return: 6%

Coast FIRE Number = $1,500,000 ÷ (1.06)^25

• Coast Number = $349,530

Current status: I have $285K. I need another $65K to hit Coast FIRE.

At my current savings rate ($45K/year): I'll hit Coast FIRE in ~1.5 years (age 41-42).

Scenario 3: Age 45, Targeting $2M by 67

Assumptions:

• Current age: 45
• Retirement age: 67
• Years to grow: 22
• Target FIRE number: $2,000,000
• Expected return: 6%

Coast FIRE Number = $2,000,000 ÷ (1.06)^22

• (1.06)^22 = 3.6035
• Coast Number = $554,980

What this means: If you're 45 with $555K invested, you can coast to $2M by 67.

Scenario 4: Age 50, Targeting $1M by 65 (Conservative)

Assumptions:

• Current age: 50
• Retirement age: 65
• Years to grow: 15
• Target FIRE number: $1,000,000
• Expected return: 5% (more conservative)

Coast FIRE Number = $1,000,000 ÷ (1.05)^15

• (1.05)^15 = 2.0789
• Coast Number = $481,020

What this means: You need nearly half your FIRE number already invested because you have less time for compounding.

The Impact of Return Assumptions (This Is Critical)

The expected return rate massively impacts your Coast FIRE number. Let's see what happens if you use different return assumptions.

Coast Number Sensitivity Analysis (Age 40, $1.5M target, retire at 65)

| Expected Return | Coast FIRE Number | Difference from 6% | |-----------------|-------------------|---------------------| | 4% (conservative) | $562,500 | +$213K | | 5% | $443,580 | +$94K | | 6% (realistic) | $349,530 | Baseline | | 7% | $276,780 | -$73K | | 8% (optimistic) | $219,960 | -$130K |

Key insight: A 2% difference in expected return changes your Coast number by $100K+.

My approach: I calculate Coast FIRE using three scenarios:

1. Conservative (4-5% real return): This is my "worst case" number. If I hit this, I'm confident even in a bad market.
2. Realistic (6% real return): This is my planning number. Historically reasonable for 70/30 or 80/20 portfolio.
3. Optimistic (7-8% real return): Nice if it happens, but I don't count on it.

I aim for the conservative number and track the realistic number.

When Coast FIRE Makes Sense (And When It Doesn't)

Coast FIRE Is Great If:

✅ You're burned out and need a break from aggressive saving
If you're grinding 60-hour weeks to save $50K/year and it's destroying your mental health, hitting Coast FIRE gives you permission to downshift.

✅ You want to pursue a lower-paying passion
Maybe you want to teach, start a creative business, or work in a nonprofit. Coast FIRE lets you do that without sacrificing your retirement.

✅ You're starting late (35-45) and need milestones
The full FIRE number can feel impossibly far away. Coast FIRE is an achievable intermediate goal that proves you're on track.

✅ You have 15-25 years until traditional retirement age
This is the sweet spot for Coast FIRE. Enough time for compounding, but not so long that the number is trivially small.

✅ You value flexibility and optionality
Coast FIRE is a backstop. You can always keep contributing and accelerate to full FIRE. But knowing you could coast reduces stress.

Coast FIRE Might Not Be Right If:

❌ You want to retire in the next 5-10 years
If you're close to full FIRE, just push through. Coast FIRE is for people with longer timelines.

❌ You're under 35 with high income
If you're 28 making $150K, your Coast number is probably $80K. You'll hit that accidentally. Just keep maxing retirement accounts and aim for full FIRE.

❌ You don't trust your discipline to avoid lifestyle creep
Coast FIRE requires discipline not to increase spending once you stop aggressive saving. If you'll just inflate your lifestyle, it won't work.

❌ You have significant debt
Pay off high-interest debt first. Coast FIRE assumes you're investing, not paying 6-20% interest on credit cards or loans.

❌ You hate your job and need to escape NOW
Coast FIRE still requires working (just less stressfully). If you need to quit ASAP, aim for Barista FIRE or Lean FIRE instead.

My Coast FIRE Plan (What I'm Actually Doing)

Current numbers (age 40):

• Portfolio: $285,000
• Annual savings: $45,000
• FIRE number: $1,587,000 (to support $69K/year at 4.3% withdrawal)
• Coast FIRE number (6% return, retire at 65): $370,000

Timeline:

• 2026 (age 41): Hit Coast FIRE number ($370K)
• 2028 (age 43): Hit "Aggressive Coast" number ($500K for 5% return assumption)
• 2030 (age 45): Decide whether to downshift or push to full FIRE

Decision point at age 45:

By then, I'll have been grinding for 5 more years. I'll assess:

1. Am I burned out? If yes, Coast FIRE activated. Reduce to part-time roofing ($20K/year to cover expenses).
2. Am I still energized? If yes, keep saving aggressively and aim for full FIRE by 50-52.
3. Did something change? (Health, family, new opportunity) Adjust accordingly.

Why I like this plan:

It gives me optionality. I'm not locked into "must work until 53 or fail." I have off-ramps at 41 (Coast number), 45 (decision point), and 50-52 (full FIRE).

Psychologically, this is huge. Knowing I could coast at 41 reduces the pressure to optimize every dollar.

Advanced Concept: "Coast FIRE Plus" (The Hybrid Strategy)

Here's a strategy I'm planning that goes beyond basic Coast FIRE:

Coast FIRE Plus = Coast number + part-time income covering expenses

The idea:

1. Hit your Coast FIRE number
2. Reduce to part-time work that covers current expenses (~50% of current income)
3. Let your portfolio compound untouched
4. Enjoy 20-30 hours/week of work instead of 50-60

My version:

Once I hit $370K (Coast number), I'll:

• Cut roofing business to 15 hours/week (~$20K/year)
• Cover $69K annual expenses with part-time income ($20K) + investment income ($25K dividends/interest) + flexible gig work ($24K)
• Let the $370K grow untouched to $1.5M+ over 20-25 years
• Fully retire around 60-65 with way more than I need

Why this works:

I get freedom and flexibility now (age 41-45) without sacrificing full FIRE later. It's Barista FIRE + Coast FIRE combined.

Calculating Your "Accelerated Coast" Number

What if you don't want to wait until 65? You can calculate an "Accelerated Coast" number for early retirement.

Example: Coast to FIRE by 55 (instead of 65)

Assumptions:

• Current age: 40
• Target early retirement: 55 (15 years away)
• FIRE number: $1,500,000
• Expected return: 6%

Accelerated Coast Number = $1,500,000 ÷ (1.06)^15

• (1.06)^15 = 2.3966
• Accelerated Coast = $625,820

Comparison:

• Coast to 65: Need $349,530 today
• Coast to 55: Need $625,820 today

Difference: $276K more needed to retire 10 years earlier.

What this tells me: If I want to retire at 55 instead of 65, I need to save significantly more before I can coast. But it's still way less than the full $1.5M.

How to Use a Coast FIRE Calculator (Or Build Your Own)

DIY Spreadsheet Method:

1. Input your variables:

   • Current age
   • Target retirement age
   • FIRE number (annual expenses × 25)
   • Expected return (4-8%)

2. Calculate years to retirement:

   • n = retirement age - current age

3. Apply the formula:

   • Coast Number = FIRE Number ÷ (1 + return)^years

4. Run sensitivity analysis:

   • Test with 4%, 5%, 6%, 7%, 8% returns
   • See how your Coast number changes

Using Our Coast FIRE Calculator:

(I'm planning to build an interactive calculator for the site. It will let you input your numbers and see:

• Your Coast FIRE number
• How much more you need to save to reach it
• Timeline to hit Coast FIRE at your current savings rate
• Visual chart showing portfolio growth with vs. without additional contributions)

Common Mistakes People Make with Coast FIRE

1. Using Too Aggressive Return Assumptions

Mistake: "I'll use 10% returns because that's the S&P 500 historical average!"

Reality:

• 10% is nominal (includes inflation)
• Real returns are ~7% after inflation
• You need to use real returns for retirement planning
• Future returns may be lower than historical averages

Fix: Use 4-6% real returns for conservative planning.

2. Forgetting About Taxes

Mistake: Calculating Coast FIRE based on pre-tax accounts without considering future tax liability.

Reality: If your $350K Coast number is all in Traditional IRA, you'll owe taxes on withdrawals. Your after-tax Coast number is lower.

Fix:

• If using Traditional IRA/401k: Multiply Coast number by 1.15-1.3 to account for future taxes
• Or focus on Roth + taxable accounts where Coast number = actual take-home

3. Not Accounting for Lifestyle Inflation

Mistake: Hit Coast FIRE, stop aggressive saving, then inflate lifestyle and dip into the portfolio.

Reality: Coast FIRE only works if you don't touch the principal. If you inflate spending and start withdrawing, you're undoing the compounding.

Fix:

• Set a strict budget for living expenses
• Track expenses monthly
• Keep Coast portfolio in separate accounts you don't touch

4. Ignoring Healthcare Costs Pre-65

Mistake: Planning to Coast at 45 without budgeting for healthcare.

Reality: Healthcare costs $6K-$12K/year if you're not on employer insurance. That's a big chunk of part-time income.

Fix: Add healthcare costs to your "Coast FIRE expense budget" and ensure your part-time income covers it.

5. Overestimating Part-Time Income Sustainability

Mistake: "I'll just make $30K/year doing freelance work forever!"

Reality: Freelance/part-time income can dry up. Markets change. Your skills become outdated. Injury or illness happens.

Fix: Build a larger Coast number or a separate emergency fund. Don't assume part-time income is guaranteed.

Coast FIRE vs. Other FIRE Variants

Let me compare Coast FIRE to the other major FIRE strategies:

| Strategy | Portfolio Needed | Work Required | Risk Level | Best For | |----------|------------------|---------------|------------|----------| | Traditional FIRE | 25-30× expenses | None (fully retired) | Low | People who can save aggressively and want full retirement | | Coast FIRE | 10-15× expenses (depends on age) | Part-time to cover expenses | Medium | People with 15-25 years to retirement who want flexibility now | | Barista FIRE | 15-20× expenses | Part-time for expenses + benefits | Medium | People who want to work part-time but reduce stress | | Lean FIRE | 20-25× expenses (lower spending) | None | Medium-High | People willing to live frugally in early retirement | | Fat FIRE | 30-40× expenses | None | Low | High earners who want luxurious retirement |

Where Coast FIRE shines: It's the lowest barrier to entry for meaningful flexibility. You don't need $1M+. You need $200K-$500K (depending on age) and the discipline to let it grow.

When to Pivot from Coast FIRE to Full FIRE

Coast FIRE isn't necessarily the endgame. It's a milestone. Here's when you might decide to push from Coast to full FIRE:

Pivot to Full FIRE if:

✅ You get a windfall (inheritance, business sale, stock options vest)
Suddenly you're 70-80% to full FIRE. Push through the last bit and retire fully.

✅ Part-time work becomes unsustainable (health, burnout, market changes)
If you can't rely on part-time income, you need the full FIRE number for safety.

✅ Your expenses drop significantly (paid off mortgage, kids finish college)
Your FIRE number just decreased. You might already be at full FIRE without realizing it.

✅ You're still energized and earning well
If you're enjoying work and earning $80K+ while coasting, you might as well accelerate to full FIRE and retire earlier.

Stay at Coast FIRE if:

✅ Part-time work is fulfilling and sustainable
If you genuinely enjoy your 20-hour/week gig and it covers expenses, why rush to full FIRE?

✅ You value having structure and purpose
Some people need work for identity and routine. Coast FIRE gives you that without the grind.

✅ You want to keep building wealth beyond FIRE
Maybe your goal isn't early retirement—it's financial security and optionality. Coast FIRE provides that.

My Take: Coast FIRE Is the Late-Starter's Best Friend

Starting FIRE at 40, I don't have 20 years of compound growth ahead of me before I burn out.

Coast FIRE gives me a realistic milestone: $370K. That's achievable in 1-2 years at my current savings rate.

Once I hit it, I'll have options:

• Keep grinding if I'm energized
• Downshift to part-time if I'm burned out
• Pivot to a passion project that pays less
• Semi-retire and travel while doing remote contract work

The beauty of Coast FIRE is that it doesn't lock you into a single path. It's a safety net that says, "You've done enough. The rest is optional."

For late-starters, that psychological relief is priceless.

Action Steps

1. Calculate your full FIRE number (annual expenses × 25 or your chosen multiplier)
2. Determine your target retirement age (65, 60, 55, etc.)
3. Calculate years until retirement (retirement age - current age)
4. Run the Coast FIRE formula with 4%, 5%, and 6% return assumptions
5. Compare Coast number to current portfolio — how far away are you?
6. Calculate time to Coast FIRE at your current savings rate
7. Decide whether Coast FIRE is your goal or just a milestone on the way to full FIRE

Related Resources

• How to Calculate Your FIRE Number (Late Starter Edition) — Calculate your target portfolio
• Solo 401(k) Guide for Self-Employed FIRE Seekers — Accelerate your path to Coast FIRE with tax-advantaged contributions
• Roth vs Traditional IRA for FIRE at 40 — Optimize your Coast FIRE portfolio for taxes

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Disclaimer: I'm not a financial advisor or investment professional. I'm a 40-year-old self-employed business owner sharing my research and personal strategy. Coast FIRE involves assumptions about future returns, inflation, and personal circumstances—all of which are uncertain. This post is educational, not financial advice. Consult qualified professionals before making major financial decisions. For full legal disclaimers, see our disclaimer page.