How It Works
- Your retirement number is 25× your annual expenses (the 4% rule)
- Your portfolio is simulated month by month with your current contributions
- Each month a growth-only look-ahead checks: would today’s balances, with no further contributions, compound past the retirement number by your retirement age?
- The first month that check passes is your Coast FIRE date
- After that date the chart shows the portfolio coasting on growth alone
Key Assumptions
- 1Expected returns remain consistent until retirement
- 2Monthly contributions continue at current levels until the coast date
- 3The 4% withdrawal rule (25× annual expenses) defines the retirement number
- 4Your retirement expenses stay flat in nominal terms, unless the optional inflation simulation is enabled — then the FIRE number reflects what they will cost at retirement age
- 5Returns compound monthly
Mathematical Approach
Retirement (FIRE) number: FIRE = Monthly Expenses × 12 × 25 Each simulated month t, the coast check tests: Σ Valueᵢ × (1 + rᵢ/12)^(T − t) ≥ FIRE Where: Valueᵢ = simulated value of asset i at month t rᵢ = expected annual return of asset i T = months from now until your retirement age Until the check passes, each asset grows monthly: New Value = Value × (1 + rᵢ/12) + Monthly Contribution
Important Limitations
- Market volatility and sequence-of-returns risk not modelled
- The 4% rule is a heuristic, not a guarantee
- Inflation only raises your FIRE number when its simulation is enabled — asset returns are always nominal
- Taxes on withdrawals are only considered when the tax simulation is enabled
- Assumes you truly stop contributing at the coast date
Important: Coast FIRE is a planning milestone, not a promise. Reaching it under these assumptions does not guarantee the portfolio will actually cover retirement — revisit the numbers as markets and your expenses change.
Frequently Asked Questions
What is Coast FIRE?
Coast FIRE is the point where your portfolio is large enough that compounding alone — with no further contributions — will carry it to your retirement number (25× annual expenses) by your target retirement age. After reaching it, you only need to earn enough to cover living costs.
How is my Coast FIRE number calculated?
Your FIRE number is monthly expenses × 12 × 25 (the 4% rule). Your Coast FIRE number today is the portfolio value that, compounded monthly at your expected return until retirement age, reaches that FIRE number.
Does Coast FIRE account for inflation?
Optionally. With the inflation simulation on, the FIRE number is grown to reflect what your expenses will actually cost at retirement age, so the target is in future money rather than today’s.
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The information and calculations provided are for informational and educational purposes only and should not be considered financial, investment, or professional advice. Projections are hypothetical, based on assumptions that may not reflect actual market conditions. Past performance does not guarantee future results, and all investments carry risk of loss. Consult a qualified financial professional before making financial decisions.