The formula
How to calculate monthly savings for FIRE
Set the date first and the monthly saving falls out of it. Given your target spending and what is already invested, this is the contribution that gets the portfolio to 25 times expenses by your chosen year.
Existing investments do part of the work on their own. The calculation compounds the current balance forward to the target date first, then sizes the payment to cover only the remaining gap.
Fill in the following:
- Annual spending in retirement
- Invested today
- Years until independence (years)
- Real return (%)
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 monthly savings for FIRE 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 monthly savings for 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.
The formula behind monthly savings for FIRE 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.
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.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable monthly savings for FIRE result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
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
Take the figures the calculator starts with:
- Annual spending in retirement: 40,000
- Invested today: 150,000
- Years until independence: 15 years
- Real return: 5 %
That gives:
- Save each month: 2,555.08
- FIRE number to reach: 1,000,000
- Provided by growth on what you already hold: 167,055.59
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
Compare the required saving against your income. If it exceeds what is plausible, the honest levers are a later date or lower retirement spending — and the second one helps twice, because it lowers the target as well as freeing up cash.
Where this goes wrong. A negative result is not an error. It means what you already hold grows past the target on its own within the timeframe: you have reached Coast FIRE.
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.
The target is not a number you pick — it is derived from your spending at 25 times, so changing your retirement budget changes the goalposts as well as the contributions.
Real, matched with spending in today's money. That keeps the entire calculation inflation-neutral and means the answer is a monthly figure you should increase with inflation each year.
The answer it gives you is save each month. With 40,000 annual spending in retirement, 150,000 invested today and 15 years years until independence, that comes to 2,555.08. 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.