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.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why coast FIRE matters
FIRE stands for Financial Independence, Retire Early — reaching a portfolio large enough that investment income covers your living costs, so paid work becomes a choice rather than a necessity. It is not a specific account or product, just the point at which the figures below cross over.
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind coast FIRE is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
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
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
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.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
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.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.