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 comes from two steps. First, find your retirement number — desired annual spending divided by your safe withdrawal rate. Second, 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 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.