The formula
How to calculate fIRE age
This puts a date on financial independence. Given what you have invested, what you add each year and what you expect to spend, it solves for the point where the portfolio reaches 25 times your annual costs.
Everything runs in real terms — the return is after inflation, so the answer is in today's money and the spending figure does not need to be inflated. That avoids the most common source of confusion in retirement projections.
Fill in the following:
- Your age now (years)
- Invested today
- Saved per year
- Annual spending
- Real return (%) — after inflation, so the answer stays in today's money
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind fIRE age works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why fIRE age 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.
The formula behind fIRE age is standard and has not changed in decades; what changes is the situation it gets applied to. Two households can run the identical calculation and land on very different conclusions once their own numbers — income, rate, term, balance — are dropped in, which is why a generic textbook example is less useful than a calculator you can adjust to match your own circumstances.
This tends to come up when comparing two concrete alternatives — two lenders, two savings products, two ways of structuring the same decision — rather than in the abstract. Run both scenarios through the same calculator with the same assumptions and the comparison becomes fair, because the only thing changing between the two results is the number you are actually trying to test.
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 fIRE age 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
Here is the calculation with the starting values:
- Your age now: 34 years
- Invested today: 120,000
- Saved per year: 24,000
- Annual spending: 40,000
- Real return: 5 %
That gives:
- Age at financial independence: 52.5 years
- Years from now: 18.5 years
- FIRE number to reach: 1,000,000
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
Test the sensitivity before trusting the date. Drop the return from 5% to 4% and the answer typically moves a year or two; cut annual spending by 10% and it moves considerably more, because that change lowers the target and raises the saving at the same time.
Where this goes wrong. Using a nominal return with today's spending. Mixing a 7% nominal return with an un-inflated expense figure understates the target badly and can pull the date forward by five years or more.
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.
Long-run global equities have delivered roughly 5% after inflation. Portfolios holding bonds should assume less. Since the whole answer hinges on it, run the calculation at 4% as well and treat the range as the honest result.
No. State and workplace pensions arriving later reduce what the portfolio has to cover from that point, which usually pulls the FIRE age forward. Bridging the years before they start is the harder part of the problem.
The answer it gives you is age at financial independence. With 34 years your age now, 120,000 invested today and 24,000 saved per year, that comes to 52.5 years. 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.