Coast FIRE Number vs FIRE Number: Understanding the Math
Reference FIRE Number
$1.5M
Target Age
65
Monthly Needed
$600
The FIRE number is the total portfolio you need at retirement to sustain your lifestyle indefinitely — typically 25× annual spending (the 4% rule). For $5,000/month ($60,000/year) in spending, the FIRE number is $1,500,000. The Coast FIRE number is the present value of that $1,500,000 target — the amount you need invested today so that compounding carries it to $1,500,000 by your target retirement date with no additional contributions.
The calculation: Coast Number = FIRE Number ÷ (1 + r)^n, where r = expected real return and n = years until retirement. At 35 targeting retirement at 65 (30 years), with 7% real returns: $1,500,000 ÷ (1.07)^30 = $1,500,000 ÷ 7.61 = $197,000. So $197,000 invested at 35 grows to $1,500,000 by 65 at 7% returns — no additional contributions needed. $197K is the coast number; $1.5M is the FIRE number.
The gap between the coast number and the FIRE number shrinks as you approach retirement. At 35 (30 years to go), the coast number ($197K) is only 13% of the FIRE number ($1.5M). At 50 (15 years to go), the coast number ($543K) is 36% of the FIRE number. At 60 (5 years to go), the coast number ($1,069K) is 71% of the FIRE number. The earlier you can reach the coast threshold, the more compounding power works in your favor.
The return assumption is critical. At 5% real returns instead of 7%, the coast number at 35 for a $1.5M goal is $1,500,000 ÷ (1.05)^30 = $347,000 — nearly double the 7% calculation. The difference between optimistic and conservative return assumptions on the coast number is dramatic. Most financial planners recommend using 5–6% real returns for conservative planning and 7% as a best-case scenario, then targeting somewhere between the two calculations.