Imagine you’ve been contributing to your 401(k) faithfully for a decade, and someone asks: “What if you just… stopped? Could the money you already have grow into a full retirement nest egg on its own?” That’s the core question behind Coast FIRE — a milestone in the FIRE (Financial Independence, Retire Early) movement where your current retirement savings, invested today and left completely untouched, will compound to fund a traditional retirement by age 65. You’re not retiring now. You’re not even necessarily stopping work. You’re reaching the point where the math says your future self is covered, and any additional retirement contributions are optional. This article walks through the formula, embeds a live calculator, and gives you a clear decision framework for what to do once you know your number.
What Coast FIRE Actually Is (and Isn’t)
Full FIRE — the version that gets the headlines — means your portfolio is large enough right now to sustain withdrawals indefinitely, so you can quit your job today. Coast FIRE is something quieter and, honestly, more achievable for most people in their 30s and 40s.
The logic works like this: every dollar invested at age 35 has 30 years to compound before a traditional age-65 retirement. At a 5% real annual return (that’s growth after inflation is stripped out — the number that actually matters for future purchasing power), $1 invested today becomes roughly $4.32 by age 65. That multiplier means you need far less today than you’ll need at retirement — and once your current balance hits that threshold, time does the rest.
Coast FIRE is not:
- Permission to raid your retirement accounts
- A guarantee you’ll retire comfortably (it depends heavily on your assumptions)
- The same as being financially independent today
It is a meaningful checkpoint that tells you: “Your retirement savings are already working for you. You have flexibility to redirect current cash flow toward other goals — a house, a child’s education, paying off debt, or simply earning less.”
The Coast FIRE Formula (Show Your Work)
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The formula has two parts:
Step 1: Your Retirement Target
This is the lump sum you’ll need at retirement. The most common shorthand is the 25x rule — multiply your expected annual spending in retirement by 25. This derives from the widely cited 4% safe withdrawal rate, a research-backed guideline suggesting a diversified portfolio can sustain 4% annual withdrawals over a long retirement horizon. If you expect to spend $60,000/year in today’s dollars, your retirement target is $1,500,000.
Understanding how compounding powers this growth over time is foundational. The SEC’s Investor.gov explains how compound interest works and why time in the market is the most powerful lever available to retirement savers — you can explore those concepts at Investor.gov’s introduction to compound interest.
Step 2: Discount Back to Today
Once you have your retirement target, you solve for what you’d need invested right now to hit it with zero additional contributions:
Coast Number = Retirement Target ÷ (1 + r)^n
Where:
r= your assumed real annual return (e.g., 0.05 for 5%)n= years until retirement (e.g., 30 if you’re 35 and targeting age 65)
Example: Retirement target of $1,500,000, 30 years out, 5% real return:
Coast Number = $1,500,000 ÷ (1.05)^30 = $1,500,000 ÷ 4.3219 ≈ $347,000
If your current retirement accounts total $347,000 or more at age 35, you’ve hit your Coast FIRE number. Every additional dollar you contribute to retirement is building a buffer — not a necessity.
By the Numbers
| Age Today | Years to 65 | 5% Real Return Multiplier | Coast Number (for $1.5M target) |
|---|---|---|---|
| 30 | 35 | 5.52× | ~$272,000 |
| 35 | 30 | 4.32× | ~$347,000 |
| 40 | 25 | 3.39× | ~$443,000 |
| 45 | 20 | 2.65× | ~$566,000 |
The earlier you hit the threshold, the more flexibility you gain. At 30, you need roughly $75,000 less than at 35 to achieve the same outcome — that’s the compounding runway at work.
The Assumptions Baked Into Your Coast Number
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No calculator is neutral. Every Coast FIRE estimate is only as good as three inputs, and all three deserve scrutiny before you make any real decision based on the output.
1. Real Return Rate
The U.S. stock market has delivered roughly 10% nominal annual returns over long periods, but inflation (running near 2.5–3% as of mid-2026) erodes that. A 5–6% real return is a reasonable central estimate for a diversified equity-heavy portfolio, but 4% is a conservative backstop and 7% is an optimistic ceiling. We strongly recommend running your Coast number across that full range — at 4%, 5%, and 6% — before drawing any conclusions. A band of estimates is more honest than a single figure.
2. Retirement Spending
Most people underestimate healthcare costs in retirement. The Employee Benefit Research Institute (EBRI), in its annual Retirement Confidence Survey, consistently identifies healthcare as the largest wildcard in retirement budgeting. Build in a 10–15% buffer above your current spending estimate if you’ll be retiring before Medicare eligibility at age 65.
3. Retirement Age and Social Security
Your Coast number assumes you’re drawing down savings starting at 65. But Social Security benefits can meaningfully reduce how much your portfolio needs to cover. The Social Security Administration publishes detailed guidance on how your benefit amount is determined by your earnings history and claiming age in its document Retirement Benefits (SSA Publication No. 05-10035), available through SSA.gov. If your projected Social Security benefit is $24,000/year and your total spending need is $60,000, your portfolio only needs to fund the $36,000 gap — which drops your retirement target from $1,500,000 to $900,000, and your Coast number shrinks proportionally. Always run both versions.
You can review your personal earnings record and get a benefit estimate through your my Social Security account at SSA.gov, which is the most reliable way to plug an accurate figure into your Coast calculation.
Tradeoffs You Need to Name Explicitly
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Coast FIRE is a permission structure, not a mandate. Here’s what the decision actually looks like in practice:
If you’ve hit your Coast number and carry high-interest debt, redirecting contributions toward payoff is almost certainly the right call. A 7–9% guaranteed return from eliminating debt beats a projected 5–6% real market return every time — and the market projection has variance that debt repayment doesn’t.
If you’ve hit your Coast number and want to pay down a mortgage, the math is murkier. With 30-year fixed rates in the mid-6% range as of May 2026, prepaying principal returns roughly 6% risk-free — slightly better than the conservative end of real equity returns, but you lose liquidity and tax-advantaged compounding. This is a values decision as much as a math decision.
If you’ve hit your Coast number and want to fund a 529, know that 529 contributions don’t reduce your federal taxable income (unless your state offers a deduction), but the tax-free growth on withdrawals for qualified education expenses is substantial. IRS Publication 970 details the full rules for tax benefits on education savings, including qualified expense definitions and contribution treatment. Redirecting retirement savings to a 529 makes sense only if you’re genuinely past your Coast threshold and confident in your return assumptions.
If you haven’t hit your Coast number, the 2026 contribution limits are meaningful levers. Per the IRS retirement plan contribution limits page, the 2026 401(k) employee contribution limit is $24,500 ($31,000 if you’re 50 or older under catch-up provisions), and the IRA limit is $7,000 ($8,000 if you’re 50+). Maxing both accelerates your Coast timeline faster than almost any other move for W-2 earners.
According to Vanguard’s How America Saves 2025 report, the median 401(k) balance for participants aged 35–44 is approximately $76,000 — well below most Coast FIRE thresholds at that age range. That means most people in their late 30s are still building toward their Coast number, not past it. Understanding where you stand relative to the threshold is itself useful information.
How to Use the Coast FIRE Calculator Below
The embedded calculator on this page asks for five inputs:
- Current retirement savings — sum of all tax-advantaged accounts (401(k), IRA, Roth IRA, 403(b)) and any taxable investment accounts earmarked for retirement
- Current age
- Target retirement age (default: 65)
- Expected annual spending in retirement (in today’s dollars)
- Real annual return assumption (default: 5%; we recommend also running at 4%)
The calculator outputs your Coast FIRE number and tells you whether your current savings exceed, meet, or fall short of it. If you fall short, it shows the monthly contribution required to hit the threshold by age 40, 45, or 50 — giving you a concrete savings target rather than just a gap.
[Coast FIRE Calculator — interactive embed renders here]
The Decision Rule
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Here’s the clean if/then framework:
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If current savings ≥ Coast number: You have earned the right to redirect contributions. Don’t stop investing entirely — but you can reduce to employer match only, fund other goals, or take a lower-paying job you actually like without derailing retirement.
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If current savings are within 15% of your Coast number: Don’t redirect yet. You’re close enough that 12–24 more months of focused contributions will close the gap. The math of compounding means closing this gap now is worth far more than closing it at 42.
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If current savings are more than 15% below your Coast number: Coast FIRE is a motivating target, not an imminent option. Prioritize maxing your 401(k) to the IRS limit, capturing any employer match (that’s an immediate 50–100% return), and opening a Roth IRA if you’re income-eligible — the IRS IRA deduction limits page has current phase-out thresholds by filing status and income level. Come back to the calculator every 12 months.
The most common mistake practitioners make with Coast FIRE is treating the number as a binary: either you’ve “made it” or you haven’t. In reality, every dollar past the threshold is margin of safety — and given the uncertainty in 30-year return assumptions, building a meaningful buffer above your technical Coast number is just sound engineering.
Download the Coast FIRE planning worksheet below to stress-test your number at 4%, 5%, and 6% real returns simultaneously, and to build the Social Security offset into your calculation — that’s where most people find their actual Coast number is closer than they thought.