Imagine hitting your early 40s with enough invested that you can stop saving for retirement entirely, yet still retire comfortably at a normal age. That is the core promise of Coast FIRE. Unlike traditional FIRE, which demands aggressive saving until you can quit work altogether, Coast FIRE focuses on a specific inflection point—your "Coast Number." Once you reach it, your existing investments, left untouched, will grow to support a full retirement by age 60 or 65. This article explains exactly how to calculate your Coast Number, which accounts really work for early compounding, and the real trade-offs involved (like needing to keep working to cover current expenses). We’ll use specific return assumptions, real fund examples such as Vanguard’s VTSAX, and walk through scenarios for different income levels. You’ll also learn common mistakes people make when estimating Coast FIRE—such as ignoring inflation or assuming constant market returns—so you can avoid them and build a realistic plan.
Coast FIRE is a subset of the Financial Independence, Retire Early (FIRE) philosophy. The term "Coast" means you've saved enough that you can stop adding new money to retirement accounts and just let compounding do the rest. You still need to work to pay living expenses, but you no longer need to save a penny for retirement. The typical target age is 40–45, with a traditional retirement age of 60–65. This approach appeals to people who want flexibility in their 40s—maybe to switch to a lower-stress job, start a business, or spend more time with family—without the extreme saving required for Lean FIRE or Fat FIRE.
Your Coast Number is the amount you need today so that, growing at a chosen rate, it reaches your Full FIRE Number by your target retirement age. Use this three-step process: First, estimate your annual retirement expenses (e.g., $40,000 in today’s dollars). Second, multiply by 25 to get your Full FIRE Number using the 4% rule ($40,000 x 25 = $1,000,000). Third, discount that future value back to the present using an assumed real return (after inflation). If you want to retire by 65 and you’re 40 now (25 years of growth), and you assume a 7% real return from a broad market index like VTSAX, the present value factor is 1 / (1.07^25) ≈ 0.184. Multiply $1,000,000 by 0.184 to get a Coast Number of about $184,000. That’s how much you need invested by age 40 to coast to a $1M portfolio in 25 years.
The 7% real return is optimistic for many; using 5% real (common for conservative balanced funds) gives a present factor of 1 / (1.05^25) ≈ 0.295, so you’d need $295,000 by age 40. To be safe, run projections at 4%, 5%, and 6% real returns. The Vanguard Total World Stock Index (VTWAX) has historically returned around 7.5% nominal, but after inflation (say 3%), that’s 4.5% real. No single number is precise—choose a reasonable range.
The best accounts for Coast FIRE are tax-advantaged ones with long growth horizons and low fees. Traditional IRAs and 401(k)s let contributions grow tax-deferred, while Roth accounts grow tax-free. For coasting, you want assets you won’t touch until age 59.5 to avoid early withdrawal penalties. A Roth IRA is ideal because contributions can be withdrawn tax-free at any time (you can access that part), but the growth stays locked until 59.5 unless you pay penalties—perfect for a coast strategy because you don’t plan to touch growth early. A taxable brokerage account offers flexibility if you think you might need the money earlier, but you’ll pay capital gains taxes. Specific low-cost options: use VTSAX (total US stock market, expense ratio 0.04%) or target-date index funds like Vanguard’s 2045 fund (expense 0.08%) to automate asset allocation. Avoid high-fee active funds or individual stocks that increase risk of underperforming your assumed returns.
Even with a solid Coast Number, mistakes can break the strategy. The first is ignoring sequence-of-returns risk—if the market crashes hard in your 40s and you can’t add new money, you may never recover enough to hit your target. Solution: use a slightly lower assumed real return (like 5%) as a buffer, and keep a small emergency fund in cash (6 months of expenses) to avoid selling stocks in a downturn. Another pitfall is assuming today’s expenses won’t change. If you plan to have kids in your 40s, or you want to travel more, your Coast Number will need to be higher. Also, inflation erodes purchasing power faster than you think: a 3% inflation rate over 20 years means $40,000 in expenses becomes ~$72,000. Recalculate your Coast Number every 3–5 years with updated expense estimates. Finally, many people forget that Federal and state income taxes will apply to Traditional 401(k) withdrawals. If you have $1M in a Traditional account, you’ll owe taxes on every dollar you withdraw—so your effective spending power is lower. Factor in a 15–20% effective tax rate for a typical retiree.
Take a married couple, both 30, with a household income of $100,000. Their annual spending is $55,000, leaving $45,000 for taxes and savings. They aim for a Coast Number by age 40. Step 1: Estimate retirement spending. They expect $45,000/year in today’s dollars (they plan to spend less and have a paid-off house). Step 2: Full FIRE Number at age 65: $45,000 x 25 = $1,125,000. Step 3: With 10 years to grow (from 30 to 40) and then 25 more years from 40 to 65, the total growth period is 35 years. Assuming 6% real return (using a balanced portfolio of 80% VTSAX and 20% BND), the present value factor for 35 years is 1 / (1.06^35) ≈ 0.130. Coast Number = $1,125,000 x 0.130 = $146,250. They need to save $146,250 by age 40. To get there in 10 years with $0 starting point, they must save ~$1,219 per month (assuming same 6% return during accumulation). At $100,000 income, that’s 14.6% savings rate—very doable. If they start at 25 with a $20,000 head start, the monthly amount drops to about $800. This underscores how starting early lowers the required and risk.
Coast FIRE assumes you stop adding to retirement at your Coast age, but life happens. Suppose at 40 you have $180,000 invested but then experience a market downturn (20% drop) and can’t add more. Your portfolio drops to $144,000, and with 25 years left, the future value at 6% real is $144,000 x (1.06^25) ≈ $618,000—far short of $1.125M. To recover, you have two options: work longer (to 70 instead of 65) or resume saving at a lower rate (say 5% of income) for a few years. A better approach is to build a margin of safety: aim for a Coast Number that’s 10–15% higher than calculated, and plan to keep saving a small amount (like 2% of income) even after hitting the Coast Number. This extra buffer protects against below-average returns. Some people call this “Coast Plus” FIRE: you coast but still add a trickle every month until age 50 or 55. It reduces risk without requiring a full savings commitment.
Using a good calculator keeps your plan honest. My personal favorite is the free online tool at WalletBurst (walletburst.com/tools/coast-fire-calc) – it lets you input current age, retirement age, expected returns, and current savings. Another solid option is the spreadsheet template from The Fioneers, which includes inflation adjustments and tax considerations. For tracking investments, link your accounts to Personal Capital or Empower—they provide a dashboard with asset allocation and performance vs. benchmarks. Through these tools, compare your portfolio’s growth to your projected Coast Number path. If you’re consistently below projections after 2–3 years, consider increasing saving rate or adjusting your retirement age upward by a few years. Also, use Vanguard’s annual Market Perspectives report to gauge realistic long-term return expectations (4–6% real for a balanced portfolio). Remember, calculators are only as good as your assumptions—update them every two years with actual spending and investment returns.
First, calculate your target retirement expenses. Track monthly spending for three months and multiply by 12. Add a 20% buffer for healthcare and lifestyle changes. Second, use a Coast FIRE calculator with 5% real return and your target retirement age (65 is standard). Write down your Coast Number and your current savings. Third, choose an allocation with at least 80% stocks for the growth phase – VTSAX or a total world index works. Set up automatic contributions to a Roth IRA or 401(k) to reach your Coast Number by your target age. Fourth, set a calendar reminder every year to recalculate: update expenses, actual returns, and adjust if needed. Fifth, once you hit the Coast Number, shift your work focus to something you enjoy, but keep earning enough to pay bills. And don’t forget to enjoy the process—Coast FIRE isn’t about watching a number; it’s about gaining freedom to choose how you spend your time in your 40s. Start today with an honest look at your numbers, and you’ll be on your way before you know it.
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