Enter your current and target retirement ages, your monthly expenses by category, your current portfolio value, your expected after-inflation return, and a safe withdrawal rate (4% is the conventional starting point — see the safe withdrawal rate guide for the trade-offs). The calculator does two things: it totals your annual spending, and it works backward to the portfolio you need today so that compound growth alone gets you to the full FIRE number by your target age.
That second number — the "Coast FIRE Number" — is the one that matters. It's the point at which you can stop saving aggressively and let the market do the work. For most people it lands somewhere between 30% and 60% of the full FIRE number, which is the whole appeal of the strategy: a much smaller, much earlier target than the conventional one.
• Taxes on withdrawals. The 4% rule assumes you can withdraw without income tax in perpetuity, which is only true in a Roth account. A traditional 401(k) or IRA will owe income tax on every dollar you take out.
• Healthcare. If you're not on employer coverage at retirement, Medicare doesn't start until 65, and the gap between your retirement age and 65 is either a bridge job, a spouse's plan, or a subsidy. None of those are in the number above.
• Sequence-of-returns risk. A 4% withdrawal rate that works in a good market can deplete a portfolio that hit a 2008-style drawdown in its first five years. The calculator assumes a smooth average return; reality is lumpy.
• Inflation variability. The "after-inflation return" input is a single number. In practice, inflation is front-loaded in healthcare and back-loaded in general prices, and the two don't move together.
• Housing-specific costs. Property tax, HOA, maintenance — these scale with your asset base in a way that a flat "housing" line doesn't capture.
Median US household spending is roughly $60,000-$70,000/year. A typical full FIRE target for that spending level is $1.5M-$1.75M at a 4% safe withdrawal rate. A Coast FIRE target for the same spending, starting in your late 20s with a 25-year runway to retirement, is often $400,000-$700,000 — a number that is genuinely reachable on a moderate income if you start early and keep fees low. If your output is far outside those bands, double-check your inputs before you trust the date.
• Using gross instead of net income. The calculator wants take-home reality, not pre-tax figures. Enter what actually leaves your account.
• Forgetting irregular costs. Car registration, property tax (if not in your mortgage), annual insurance premiums, the big maintenance year. Annualize them into the monthly lines.
• Counting future salary as a negative expense. The Coast FIRE strategy assumes you stop saving, not that you stop earning. Don't subtract expected future income from your target — that's a different plan.
• Using a 7% "after-inflation" return. 7% is a gross, pre-inflation long-run estimate for equities. After inflation and fees, a realistic planning number is 4-5%.
• Entering your current 401(k) balance but not your expected match. The match is real money. Count it, or treat it as a future contribution — but pick one and be consistent.
CoastFIREWhen — Educational content. Not financial advice. See our editorial policy.