Want to retire before 65? You pay for it with a higher coast number — retiring 10 years earlier roughly doubles the amount you need invested today.
📈 Saving Up to Your Coast Number
Age: 35 | Current balance: $50,000 | Coast number (target $1.2M at 65): $157,641 | Coast by age: 45
Monthly savings needed:~$358/month
Your existing $50K grows to ~$98K on its own in 10 years. Adding just ~$358/month closes the gap to $157,641 — then you can stop saving forever.
Understanding the Coast FIRE Formula
Coast FIRE rests on one idea: compounding does the heavy lifting. Once your investments are large enough, their growth outpaces anything you could add by saving.
The Coast FIRE Number Formula
Coast FIRE Number = Retirement Target ÷ (1 + r)ⁿ
Where n = retirement age − current age, and r = expected annual real return
Example: $1,200,000 ÷ 1.07³⁰ = $157,641
The verified classic: age 35, retire at 65, 7% real return
Projecting Your Balance With Contributions
FV = P(1+r)ⁿ + C × [((1+r)ⁿ − 1) / r]
Future value of current balance (P) plus annual contributions (C) at return rate r over n years
Solving for Monthly Savings to Reach Coast
C = (Coast Number − P(1+r)ⁿ) × r ÷ ((1+r)ⁿ − 1)
The annual contribution needed so your balance reaches the coast number by your target coast age — divide by 12 for monthly
Key Coast FIRE Concepts
Retirement Target = Annual Spending × 25
The 4% rule from the Trinity Study — withdraw 4% of your nest egg in year one, adjust for inflation, and it historically lasts 30+ years
Real Return = Nominal Return − Inflation
Use real (inflation-adjusted) returns so your target is in today's dollars — 7% is the historical real return of US stocks
Step-by-Step Coast FIRE Planning
1
Set your retirement target: Estimate annual retirement spending and multiply by 25 (4% rule). For $50,000 of spending, your target is $1,250,000.
2
Choose your retirement age: Traditional retirement is 65, but any age works. Earlier retirement means fewer compounding years — and a much larger coast number.
3
Pick a real return assumption: 7% matches the long-run US stock average; conservative planners use 5-6%. Lower returns mean a higher coast number.
4
Calculate your coast number: Divide your retirement target by (1 + r)ⁿ. That's the amount needed invested today — once you have it, compounding finishes the job.
5
Compare your current balance: Already past the coast number? You can stop contributing now. Otherwise, use "Check Your Progress" to find the monthly savings needed to reach it.
6
Review annually: Recalculate each year as your balance, expenses, and return assumptions change. A market downturn can push your coast age further out — stay flexible.
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Coast Number Calculation
Find the exact amount you need invested today to stop saving and still reach your retirement target — using the verified Coast FIRE formula.
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Retirement Projection
Project your balance at retirement with current savings and monthly contributions, and see your surplus or shortfall versus your target.
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Dual Calculation Modes
Find your coast number from your target and time horizon, or check progress with contributions and a year-by-year growth table.
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4% Rule Helper
Turn a desired monthly retirement income into a target nest egg instantly (income × 12 × 25), and see the monthly savings needed to reach coast.
Coast FIRE is the point in your financial independence journey where the compounding growth of your existing investments alone is enough to reach your retirement number by your target age. You can stop saving — or downshift to a job you love — and still retire comfortably. The calculator finds that number: the balance that lets compounding finish the job.
The math is refreshingly simple. Your coast number is your retirement target discounted back to today at your expected real return:
Coast Number = Retirement Target ÷ (1 + r)ⁿ
Example: $1,200,000 ÷ 1.07³⁰ = $157,641 at age 35
Notice what this means: a 35-year-old needs only ~$158,000 invested to reach $1.2M by 65 at a 7% return. That is the power of 30 years of compounding — and why starting early beats saving more later.
The 4% Rule and Your Coast Number
The 4% rule, from the 1998 Trinity Study, found that a diversified portfolio can usually sustain ~4% annual withdrawals for 30 years. So your retirement target ≈ annual spending × 25.
Retirement Target = Annual Spending × 25
The inverse of 4% — if you spend $50,000/year, you need $1,250,000
Why 7%? The S&P 500 has returned roughly 10% nominal over the long run; subtracting ~3% inflation gives a ~7% real return. Conservative planners use 5-6%; aggressive ones 8%+. The rate is adjustable.
Caveat: coasting assumes consistent returns, but real markets are lumpy. A severe downturn near retirement can derail a coast plan — that is sequence-of-returns risk.
Use a conservative return (5-6%) so your coast number has built-in margin.
Recheck your number annually and be ready to save again after bad market years.
Keep an emergency fund separate so a job loss or big expense never forces you to sell investments early.
Coast FIRE vs Lean FIRE vs Barista FIRE
FIRE is a spectrum trading off how much you save, when you stop, and how much you spend. Coast FIRE sits in the middle: keep working, stop saving.
🌊 Coast FIRE
You've saved enough that compounding alone will hit your retirement number by your target age. You keep working to cover living expenses, but you can stop saving for retirement.
🌱 Lean FIRE
Full retirement on a minimal budget, typically under $40,000/year in spending. Requires a smaller nest egg ($750K-$1M) but demands strict frugality.
The key distinction: full FIRE (Lean, Fat, and traditional FIRE) means your portfolio covers 100% of your expenses today — you can retire immediately. Coast FIRE only requires enough to cover expenses at a future retirement age, using compounding to bridge the gap. Coasting trades years of saving for years of flexibility.
Frequently Asked Questions
What is Coast FIRE?
Coast FIRE is the point where your existing investments are large enough to grow into your full retirement number by your target age — with no additional contributions. You can then "coast": keep working to cover living expenses, but stop saving for retirement entirely. Compounding carries your portfolio the rest of the way..
How do I calculate my coast number?
Divide your retirement target by (1 + r)ⁿ, where n is the years until retirement age and r is your expected real return. Example: $1,200,000 target at 65, age 35, 7% → $1,200,000 ÷ 1.07³⁰ = $157,641. That's what you need invested today to stop saving.
Is 7% a realistic return?
The 7% figure is the long-run real (inflation-adjusted) return of the US stock market — roughly 10% nominal minus ~3% inflation. It's a reasonable central estimate over 30+ year horizons, but an average, not a guarantee. Conservative planners use 5-6%; aggressive ones use 8%. Your coast number is very sensitive to this assumption, so calculate with a conservative rate and treat the result as a floor, not a target..
What if I want to retire before 65?
Retiring early means fewer compounding years, which makes your coast number significantly larger. Retiring at 55 instead of 65 with a $1M target at 7% roughly doubles the coast number — from ~$184K to over $380K. Early retirees also plan for longer retirements, so many use a more conservative 3-3.5% withdrawal rate, which raises the target itself..
How is Coast FIRE different from regular FIRE?
Regular (full) FIRE means your portfolio covers 100% of your expenses forever, so you can stop working entirely. Coast FIRE only requires that your portfolio will reach that size by retirement age on its own — you still work, but only to pay today's bills, not to save. In short: full FIRE means you've quit your job; Coast FIRE means you've quit saving..
Do taxes affect my coast number?
Yes. Your retirement target should be based on after-tax spending — withdrawals from tax-deferred accounts (401(k), Traditional IRA) are taxed as ordinary income. Where your money lives matters too: Roth accounts grow and withdraw tax-free (great for coasting), while taxable accounts incur dividend and capital gains taxes that slightly reduce your effective return..
⚠️ Important Note: This Coast FIRE Calculator is for educational purposes only. It assumes consistent real returns; actual markets are volatile and taxes and fees reduce growth. Consult a financial advisor before making decisions.