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.
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
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.
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.