Coast FIRE number: formula and examples
How to calculate a Coast FI number, with a clear discount formula and numeric examples you can re-run in the calculator.
Your Coast FIRE number is the portfolio you need today so that, with no further contributions, compound growth may reach full FIRE by a traditional retirement age under an assumed real return.
The discount formula
If full FIRE is T, real return is r, and years until traditional retirement is n: coast number ≈ T ÷ (1+r)^n. You are discounting the future target back to today.
Example A — 20-year runway
Full FIRE $1,500,000, r = 5%, n = 20. Coast number ≈ $1,500,000 ÷ (1.05)^20 ≈ $565,000. With roughly that amount invested today (under those assumptions), the model says growth alone could finish the job by traditional retirement age.
Example B — shorter runway
Same $1,500,000 target and 5% real return, but only 10 years left: coast ≈ $1,500,000 ÷ (1.05)^10 ≈ $921,000. Less time means less help from compounding, so you need more capital today.
Stress-test the return
At 4% real over 20 years, coast ≈ $1,500,000 ÷ (1.04)^20 ≈ $684,000. Lower assumed growth raises the coast number. When in doubt, use a more conservative r rather than a heroic one.
What Coast FIRE is not
- Not early retirement by itself
- Not a substitute for an emergency fund or healthcare plan
- Not a guarantee of constant real returns
Run your own numbers in the Coast FIRE calculator, compare with full FIRE and Barista FIRE, and read the Methodology page for model details.