The formula
How to calculate monthly expenses
Annual budgets and monthly cash flow are different mental models, and most people are fluent in only one. Converting between them — and into the capital they imply — makes the same spending easier to reason about.
A month is not four weeks. Dividing by twelve and dividing by four give figures roughly 8% apart, which is the source of a surprising number of budgeting errors.
What to enter:
- Annual spending
- Withdrawal rate (%)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
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 monthly expenses 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 expenses 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.
This kind of calculation rarely stands entirely alone. A monthly expenses figure usually feeds into a wider decision — how it compares with a competing offer, whether it fits inside a monthly budget, what it does to a longer-term plan — and the value of having it as an exact number rather than a rough guess is that those follow-on comparisons stop being guesswork too. Once one figure in a decision is precise, it is worth making the effort to get the others precise as well, rather than mixing an exact calculation with several estimates and treating the result as equally reliable.
In practice, most people arrive at a page like this one having already tried a version of the calculation by hand or in a spreadsheet, and use the calculator here to confirm it rather than replace it. That is a reasonable way to use it — the two should agree to the last decimal place if the same inputs and the same formula are used, and if they do not, the formula shown above is the one to check your own working against first.
Worked example
Take the figures the calculator starts with:
- Annual spending: 38,000
- Withdrawal rate: 4 %
That gives:
- Per month: 3,166.67
- Per week: 730.77
- Portfolio required: 950,000
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
The capital line is the useful one for planning. At a 4% withdrawal rate, every £1 of annual spending needs £25 of portfolio, so an extra £50 a month costs £15,000 of capital.
Where this goes wrong. Assuming an even monthly spread. Real spending is lumpy — December, the summer holiday and the insurance renewal month can each run 50% above average, and a budget built on the average will feel wrong in most months.
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.
Because a year has 52.18 weeks, not 48. Using four weeks per month overstates the weekly figure by about 8%, which compounds into a meaningful error over a year.
About £900,000 at a 4% withdrawal rate, before tax and before any pension income. Guaranteed income reduces the portfolio requirement pound for pound at 25 times the annual amount.
The headline figure is per month. With 38,000 annual spending and 4 % withdrawal rate, that comes to 3,166.67. 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.