What “Coast FIRE” actually means
Most FIRE math asks how much you need to retire today. Coast FIRE flips the question: how much do you need invested right now so that, left untouched, it compounds into your retirement target by the time you actually stop working? Hit that number and you've reached Coast FIRE — you can stop saving for retirement entirely and simply earn enough to pay current bills while the portfolio does the rest.
The formula
Coast FIRE is just the present value of your future retirement target. If you expect to need a nest egg of N at retirement, your money has t years to grow, and you assume a real (after-inflation) return of r, then:
- Coast number = N ÷ (1 + r)t
- Example: target a $1,500,000 nest egg, 25 years out, at a 5% real return → you need about $443,000 invested today. After that, you never have to contribute another dollar to retirement accounts.
The two assumptions that move this number the most are the return rate and the time horizon. A single percentage point of return, compounded over decades, swings the answer dramatically — which is exactly why a year-by-year model beats a one-line formula.
Why model it instead of using one formula
A closed-form Coast FIRE number assumes a single fixed return and a single retirement target. Real plans aren't that tidy: returns vary, expenses change, you may keep contributing for a few more years, and taxes eat into withdrawals later. Modeling the full path — contributions, growth, inflation, and eventual drawdown — shows you not just the Coast number but what happens on either side of it.
Model your Coast FIRE number
FIRE Planner runs the whole projection in your browser. Set your current balances, expected return, and the year you'd like to stop contributing, and watch whether the portfolio coasts to your target — or where it falls short. It's free, private, and requires no signup; your numbers never leave your device.
Related reading: the 4% rule and safe withdrawal rates covers how much that nest egg can actually pay out once you're there.