Short answer: you’ve hit Coast FIRE when the money you’ve already invested will compound — with zero further contributions — into your full retirement target by your target age. Example below: a 30-year-old who wants to spend $40,000 a year from 65 needs $263,555 invested today (assuming a 3.88% real return); with $275,000 invested, they’re already past the line. This calculator tells you your number, your gap, and the age your current savings would cross the full target on their own.
| Your full FIRE number at retirement (today’s dollars) | $0 |
| You have invested today | $0 |
| Gap (negative means you are past the line) | $0 |
| Today’s savings projected to retirement age, adding $0 more | $0 |
| Age your current savings alone would reach the full target | — |
Year-by-year: today’s savings left to compound alone (today’s dollars)
| Age | Projected balance | % of full target |
|---|
Assumptions, stated loudly: Coast FIRE means your existing investments, left untouched, compound to your full retirement target by your target age — so you only need to cover living costs from here, not save another dollar. The math discounts the target back at a real (inflation-adjusted) return and assumes that return is smooth, spending stays flat in today’s dollars, and taxes/Social Security are ignored. Falling short does not mean failing — it just means the “coast” line is further away; the full FIRE calculator shows the monthly saving that closes the gap. Educational estimate only, not financial advice. Your numbers never leave this page.
How this calculator works
Full target = annual retirement spending ÷ withdrawal rate (4% default → 25×). Coast number = that target discounted back to today at a real (inflation-adjusted) return: (1 + return) ÷ (1 + inflation) − 1. If your invested savings exceed the coast number, compounding alone finishes the job in this model. Everything is in today’s dollars. Assumptions, stated loudly: the real return is smooth and constant (real markets deliver their average in a lumpy order that matters); spending stays flat in today’s dollars; taxes, Social Security, and pensions are ignored; and “coasting” still means covering all your living costs from work until retirement — it is not retiring early, it’s retiring from saving.
Worked example
Age 30 · retire at 65 · $275,000 invested · $40,000/yr retirement spending · 7% return · 3% inflation · 4% withdrawal rate: – Full FIRE target: $1,000,000 – Coast number (needed today): $263,555 – Today’s $275,000 left alone grows to $1,043,427 by 65 – Verdict: past the line by $11,445 — current savings cross the full target around age 63.9
FAQs
If I’m past Coast FIRE, can I stop contributing? In this model, yes — that’s the definition. Most people don’t fully stop; coasting usually buys flexibility instead: a lower-paying job you like, part-time work, or redirecting savings to nearer goals. Your mileage varies with markets. What if I’m behind? The gap shown is the extra lump that would put you on the line today. More useful for most people: the full FIRE calculator shows the monthly contribution that reaches the target directly. Why does starting age matter so much? Because discounting is brutal in reverse: at a 3.88% real return, a 30-year-old needs about 26 cents invested per future dollar of target, while a 45-year-old needs about 47 cents. Time is the whole mechanism. Is Coast FIRE safer than regular FIRE? Differently risky: you keep working (so income risk remains covered), but the plan leans entirely on decades of uninterrupted compounding — a bad first decade hurts more than the smooth projection suggests.
Disclaimer
Educational content only — not financial advice. Projections use assumed smooth real returns and are not guarantees.
Sources: Investor.gov compound interest calculator (method), BLS Consumer Price Index (inflation reference) Figures last verified Report a wrong number