How do I calculate my FIRE number?

Take your expected annual spending in retirement and multiply it by 25. That is the 4% rule expressed as a multiple. If you expect to spend $48,000 a year, your FIRE number is $1.2 million. The multiplier is the part worth thinking about: 25x assumes a 4% withdrawal rate, and a more cautious 3.33% rate means 30x.

Should I use 25x or 30x my expenses?

25x is the classic Trinity Study baseline and suits a roughly 30 year retirement with no dependents. 30x or 33x suits a retirement of 40 years or more, irregular expenses, a single income household, or anyone who would rather not go back to work at 60. The cost of caution is working longer; the cost of optimism is running out.

Does my FIRE number include my house?

Only if you intend to sell it or borrow against it. A home you plan to live in produces no income, so counting it inflates the number that is supposed to represent what you can spend. Most people track it in net worth but exclude it from the FIRE calculation, and keep the mortgage in their spending figure until it is paid off.

What if my spending changes after I retire?

It usually does, in both directions. Commuting, work clothes and lunches disappear; travel, hobbies and healthcare often rise. Estimate the spending you expect in retirement rather than the spending you have now, then recheck it every year as the picture gets clearer.

What Is My FIRE Number? How to Work Out the Amount You Actually Need

Your FIRE number is the amount of invested money that lets work become optional. The formula takes one line. The argument is entirely about one input, and most calculators pick it for you without telling you.

The one line

Your FIRE number is:

Annual spending in retirement, multiplied by a number.

If you expect to spend $48,000 a year and you use a multiplier of 25, your FIRE number is $1.2 million. That is the whole calculation.

Everything interesting is in the multiplier.

Where 25 comes from

The multiplier is the inverse of a withdrawal rate. A 4% withdrawal rate means you take 4% of your portfolio in the first year and adjust for inflation after that. One divided by 0.04 is 25.

The 4% figure comes from the Trinity Study, which tested historical US market returns against a 30 year retirement. It held up in the large majority of historical periods.

Two things about that are worth saying plainly:

It was tested against 30 years, not 50. Someone retiring at 40 is planning for a much longer stretch than the study covered.

It is a historical result, not a law. It describes what would have worked in the past, in one country, over one set of decades.

That does not make it useless. It makes it a starting point rather than an answer, which is covered in more depth in the 4% rule, honestly.

Choosing your multiplier

Multiplier	Withdrawal rate	Suits
25x	4.0%	A roughly 30 year retirement, no dependents, some flexibility to earn again
28x	3.5%	College costs, children, a longer safety margin
30x	3.33%	A 40 year retirement or more, irregular expenses, one income
33x	3.0%	Maximum buffer, uncertain healthcare, very long retirement

The gap between 25x and 30x on $48,000 of spending is $1.2 million against $1.44 million. At a $2,000 a month savings rate, that difference is roughly ten extra years of work.

So this is not a technical detail. It is the single biggest decision in the whole plan, and it deserves more thought than the default value in whatever calculator you opened first.

Four things most calculators get wrong

They use your current spending, not your retirement spending. These are different numbers. Commuting, work clothes and childcare fall away. Travel, hobbies and healthcare tend to rise. Use what you expect to spend, not what you spend now.

They include your house. A home you intend to live in produces no income. Counting it in a number that represents spendable capital makes the target look closer than it is. Track it in your net worth by all means, but leave it out of this calculation unless you genuinely plan to sell.

They ignore the mortgage. If you will still be paying one in retirement, it belongs in your annual spending figure. If it will be cleared, it does not. Getting this wrong moves the answer by hundreds of thousands.

They assume one fixed return forever. Markets do not deliver an average every year, and the order of returns matters enormously. A bad first five years hurts far more than a bad five years in the middle, which is a risk an average return quietly hides.

The number moves, and that is fine

Your FIRE number is not a fact about you. It is a function of what you expect to spend, and that expectation will change.

Which is why the useful habit is not calculating it once. It is watching two lines: the number you need, and the number you have. Recalculating annually, as your spending picture gets clearer, gives you a target that is actually about your life rather than about a default in a spreadsheet.

Working it out for yourself

You need three things: your expected annual spending, your chosen multiplier, and your current invested net worth. The first is the hardest and the most worth getting right.

Networo has a FIRE calculator that takes those inputs, shows the four common multipliers side by side with what each one assumes, and plots your current trajectory against the target. It is free to use, it does not require connecting a bank account, and the projection updates as your real numbers do.

The point is not to find a single perfect figure. It is to know roughly how far away you are, and whether the gap is closing.