The formula
How to calculate coast FIRE
Coast FIRE is the point where you can stop contributing entirely and still retire on time, because compounding alone closes the remaining distance. You still need to cover current living costs, but nothing more has to go into the portfolio.
It is the FIRE number discounted back to today. Reaching it at 32 with a target of 60 requires roughly a quarter of the eventual figure, because 28 years at 5% real multiplies what you hold by almost four.
Here is what each field means:
- Your age now (years)
- Target retirement age (years)
- Annual spending in retirement
- Real return (%)
- Invested today
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
Worked example
A concrete run-through, using the values already in the fields:
- Your age now: 32 years
- Target retirement age: 60 years
- Annual spending in retirement: 40,000
- Real return: 5 %
- Invested today: 180,000
That gives:
- Coast FIRE number: 255,093.64
- Surplus or shortfall today: -75,093.64
- Your portfolio at retirement if you stop saving now: 705,623.24
Reading the result
A surplus means you are past the coasting threshold: any further saving buys an earlier retirement rather than a possible one. That is the moment a lower-paid job, a four-day week or a career change stops being a financial risk.
Where this goes wrong. Coast FIRE is exquisitely sensitive to the return assumption because it compounds over decades. At 4% real rather than 5%, the coast number at 28 years out is roughly 30% higher. Leave a margin.
Coast FIRE means the portfolio no longer needs contributions but you still work to cover living costs. Barista FIRE means the portfolio already covers part of your spending, so part-time income can fund the rest.
That is the point of the milestone — it becomes optional. Continuing brings the retirement date forward or raises the eventual income; stopping frees up cash flow now. Both are defensible.
The headline figure is coast FIRE number. With 32 years your age now, 60 years target retirement age and 40,000 annual spending in retirement, that comes to 255,093.64. Change any field and the figure moves with it.