Your coast fire number is the specific dollar amount you need invested today — untouched — to coast to your retirement target. Plug in your numbers below to calculate it.
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Estimates only, based on a constant annual return and constant contributions — real markets don't move in a straight line. Not financial advice.
Your coast fire number is the exact dollar amount you need invested today — with no further contributions — for compound growth alone to carry it to your full retirement number by your target retirement age. It comes from two steps: first find your retirement number (desired annual spending divided by your safe withdrawal rate), then discount that number back to today using your expected annual return, compounded over the years remaining until retirement.
Say you want to spend $60,000/year in retirement, using a 4% safe withdrawal rate, 30 years from now, at a 7% expected annual return:
Retirement number: $60,000 ÷ 0.04 = $1,500,000
Coast fire number: $1,500,000 ÷ 1.07^30 = $197,051
That means roughly $197,051 invested today, left alone for 30 years at a 7% return, would grow into the full $1,500,000 needed to retire on $60,000/year. The calculator above does this same math instantly with your own numbers.
Your coast fire number isn't a finish line for saving — it's a threshold. Below it, you still need to keep contributing for your retirement plan to stay on track; above it, continued growth alone is enough, and any further contributions just add a margin of safety or pull your retirement date earlier. Crossing it doesn't mean you're done working or done saving in general — it means your retirement savings specifically no longer require new money to hit their target. Most people who reach this point either keep contributing anyway (to retire sooner or with more cushion) or redirect that money toward other goals, like paying down a mortgage faster, funding a career change, or simply spending more today.
Two inputs move your coast fire number more than any other: your expected annual return and your time horizon. A higher assumed return lowers the number sharply, since more of the work happens through compounding rather than contributions — but it also means the year-to-year path is less certain, so many people intentionally use a slightly conservative return estimate rather than a long-run market average to stay realistic. Time horizon matters just as much: someone 30 years from retirement needs far less invested today than someone 10 years out, since compounding has decades rather than years to close the gap. Your safe withdrawal rate and desired retirement spending also matter, but indirectly — they set your retirement number first, before the discounting step even happens. Because all of these inputs shift over time, it's worth recalculating your coast fire number at least once a year, or any time your savings, income, or retirement timeline changes meaningfully.
Your coast fire number is the amount you'd need invested today, with zero further contributions, to grow through compounding alone into your full retirement number by your target retirement age. It's smaller than your retirement number because it accounts for years of growth still ahead of you.
Divide your desired annual retirement spending by your safe withdrawal rate to get your retirement number. Then divide that by (1 + expected annual return)^(years until retirement). Example: a $60,000/year spend at a 4% withdrawal rate gives a $1,500,000 retirement number. Discounted 30 years at 7% annual return: $1,500,000 / 1.07^30 ≈ $197,000 — that's the coast fire number.
Yes. A higher assumed annual return means your money compounds faster, so you need less invested today to reach the same future target — which lowers your coast fire number. It also makes the projection riskier, since real returns vary year to year.
Your retirement number is the total nest egg you need at retirement to sustain your desired spending. Your coast fire number is what you need today — a smaller figure — for that retirement number to be reachable through growth alone, without any further contributions.
Reaching it means your current invested savings, left alone, are on track to hit your full retirement number by your target age. From there it's a choice: keep contributing to retire earlier or build in more cushion, or redirect that money elsewhere — paying down debt, a career change, or spending more now. Many people choose a middle path and keep contributing at a lower rate rather than stopping entirely.
At least once a year, or any time something meaningful changes — a raise, a market swing that moves your savings balance significantly, a change in when you want to retire, or a change in your expected return assumption. The number isn't fixed; it shifts as your inputs do.