Coast Retirement · Research

How Coast FIRE is calculated

Coast FIRE is calculated by estimating your future retirement number, then discounting it back to today using an expected real return over the years until retirement.

What this means

The math answers: “How much do I need right now so I never have to save another dollar for retirement?”

How it works

Estimate annual spending in retirement (today's dollars).
Choose a safe withdrawal rate (e.g. 4%).
FIRE number = spending ÷ SWR.
Years until retirement = retirement age − current age.
Coast number = FIRE number ÷ (1 + real return) ^ years.
Compare Coast number to current portfolio.

FIRE Number = Annual retirement spending ÷ Safe withdrawal rate
Coast FIRE Number = FIRE Number ÷ (1 + real return) ^ years until retirement

Example

$90k/yr spending, 4% SWR → $2.25M FIRE number. Age 40 to 60 (20 years), 7% real → Coast ≈ $581k.

Assumptions

Single real return (no distribution of outcomes on quick calc)
No contributions after today
No taxes on growth in simplified view

Limitations

Real life has variable spending, tax brackets, and account types. Use the full retirement planner for NPV of expenses and drawdown paths.

Common mistakes

Using nominal returns without adjusting inflation
Forgetting employer match already invested
Mixing Coast number with FIRE number when comparing balances

Related tools

FAQ

What real return should I use?

Many FIRE planners use 5–7% real long-term for diversified stock-heavy portfolios; conservative users go lower.

Why divide by SWR?

That converts annual spending into a portfolio target using the 4% rule / 25× heuristic.

Can I use a different withdrawal rate?

Yes — lower SWR raises both FIRE and Coast numbers.

Does inflation appear twice?

Using real returns and real spending avoids double-counting if you're consistent.

Where is this in Coast Retirement?

Homepage quick calculator and Models tab in the full planner.

Run the Coast FIRE calculator →

CoastRetirement.com provides educational calculators and planning models. It does not provide individualized financial, investment, tax, or legal advice. Calculator outputs depend on user-provided assumptions and are not guarantees of future results. Consult a qualified professional before making financial or tax decisions.