How to Calculate Coast FIRE
What is Coast FIRE?
Coast FIRE is a financial independence strategy where you save aggressively early, then let compound interest grow your portfolio to your FIRE number by retirement — without contributing another dollar.
Formula
FV = PV × (1 + r)^n; years_to_FIRE = log(target_amount / current_savings) / log(1 + annual_return)
- PV
- Current savings ($) — Amount already saved
- r
- Annual return (%) — Investment return (e.g., 7% stock market)
- n
- Years (years) — Time until retirement
- FV
- Future value ($) — Amount at retirement (compound growth)
Step-by-Step Guide
- 1FIRE Number = Annual Expenses × 25 (4% withdrawal rule)
- 2Coast Number = FIRE Number ÷ (1 + real return)^years to retirement
- 3Typical real return assumption: 7% nominal (5% real)
- 4Once you hit the Coast Number, you only need to cover living expenses
Worked Examples
Input
$40K expenses, age 30, retire 65
Result
FIRE number $1M. Coast number ~$95K today at 7% growth
Input
$60K expenses, age 35, retire 60
Result
FIRE number $1.5M. Coast number ~$430K
Frequently Asked Questions
What is Coast FIRE?
Stop contributing but let invested savings grow to retirement target. Works if you have enough time and can live on earned income without saving.
What return rate should I assume?
Historical stock market: ~7–10% annual. Conservative: 5%. Bonds: 3–5%. Inflation typically 2–3%. Subtract inflation for real returns.
Is Coast FIRE realistic?
Requires discipline to stop saving and spend all earned income. Best for people early in career with decades until retirement.
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