How to Calculate Your FIRE Number: The Exact Math

Q: What is the formula for the FIRE number?

FIRE Number = Annual Spending divided by 0.04. The 0.04 is the 4% Safe Withdrawal Rate (SWR). For a retirement horizon longer than 30 years, use 0.035 instead: FIRE Number = Annual Spending divided by 0.035, roughly 28.6 times annual expenses.

Your FIRE Number Is Your Spending Divided by 4%

Your FIRE number is your annual spending divided by 0.04. That is the whole calculation. Spend $50,000 a year and your FIRE number is $1,250,000. Spend $40,000 and it is $1,000,000. The 0.04 is the Safe Withdrawal Rate (SWR), from the Trinity Study (Cooley, Hubbard & Walz, 1998): a portfolio withdrawing 4% a year survived every 30-year market period they tested. No retirement calculator on the internet does anything more than this formula with extra variables attached.

The number in your head — the round million, the figure from a headline — was never calculated from the only input that matters: what you spend.

The Number People Pick Instead

Ask someone for their retirement number and they do not hand you math. They hand you a round figure: $1 million, $2 million, "as much as possible." These numbers come from headlines, dinner-table talk, and a vague sense that a million is a lot. None of them survive contact with the formula above.

Here is what happens instead. Someone plans around $2 million, spends $85,000 a year, and retires fine — the math says $2,125,000, and they are close enough. Someone else plans around $1 million, spends $55,000 a year, and retires at a 5.5% withdrawal rate. They are the ones who get surprised when the portfolio starts eating itself in a bad decade.

The mistake is not the target. The mistake is building the target without the spending input. A retirement number you cannot derive from your own expenses is not a plan. It is a guess with a dollar sign.

Your Spending Is the Engine

Think of your portfolio as a car and your annual spending as its fuel consumption. The 4% rule says: your fuel tank must hold 25 times what you burn in a year. A $40,000-a-year lifestyle needs a $1,000,000 tank. A $75,000-a-year lifestyle needs a $1,875,000 tank. You can argue about the size of the tank all day. The variable that actually moves the number is the fuel consumption — and it is the only one you control directly.

Most people have never measured their fuel consumption. They know their salary to the dollar and their rent to the dollar, and could not tell you within $500 a month what they spend overall. That is the gap this whole calculation lives in. You cannot divide by 0.04 what you cannot count.

This is why I call the FIRE number a Spending-Driven Number. It is not a wealth target. It is a spending number wearing a math costume.

The Math, Run Three Times

The formula, once, in plain English:

FIRE Number = Annual Spending ÷ 0.04 (the 4% SWR)

Or the equivalent version you will see everywhere: annual spending × 25. Same number, same rule, two notations.

Run it for three realistic households:

Household A: single, $40,000/year. FIRE Number = $40,000 ÷ 0.04 = $1,000,000.

Household B: couple, $60,000/year. FIRE Number = $60,000 ÷ 0.04 = $1,500,000.

Household C: couple with a paid-off house, $30,000/year. FIRE Number = $30,000 ÷ 0.04 = $750,000.

Now the full table, so you can find your row:

Annual Spending	FIRE Number (25x)
$30,000	$750,000
$40,000	$1,000,000
$50,000	$1,250,000
$60,000	$1,500,000
$75,000	$1,875,000
$90,000	$2,250,000
$100,000	$2,500,000

Assumptions: 7% real return before withdrawal, 4% SWR, 30-year retirement horizon. Every calculator on this site runs these same defaults.

The table is the whole argument. Cut $10,000 a year of spending and your FIRE number drops $250,000. That is why frugality beats salary — a higher savings rate retires you faster than a higher income, because it works both ends of the equation at once.

Your actual number depends on your real spending, your age, and your target retirement date. Run the math with your own inputs in the FIRE Calculator at CoastFIRE Hub — it applies the 4% rule, handles inflation, and shows which FIRE tier your plan lands in.

One Refinement: The Longer the Retirement, the Lower the Rate

The 4% rule was designed for a 30-year retirement. Retire at 65 and it works as stated. Retire at 50 and you are asking the money to last 40+ years, and the honest adjustment is a 3.5% withdrawal rate — about 28.6x expenses instead of 25x.

Annual Spending	FIRE at 4% (retire at 65)	FIRE at 3.5% (retire at 50)
$40,000	$1,000,000	$1,143,000
$50,000	$1,250,000	$1,429,000
$60,000	$1,500,000	$1,714,000

The gap is real but not brutal. An extra 14% of portfolio for a decade of extra freedom. Most people overpay more than that in investment fees over their career without noticing.

How to Find Your Real Spending in an Afternoon

You cannot calculate the number until you can count the spending. The good news: this takes one afternoon, not an app subscription.

Go through the last 12 months of bank and credit card statements and sort everything into four buckets: fixed housing and utilities, food, transportation, everything else. Sum the totals. Divide by 12. That monthly number, times 12, is the annual spending you divide by 0.04. Do not estimate. Estimates are how people discover a $400-a-month gap between their budget and their bank statement.

Two honest notes on the result. First, include healthcare and taxes if you will still owe them in retirement — the classic error is calculating on your working-life expenses and discovering Medicare does not cover everything. Second, if your number embarrasses you, that is the point of the exercise. It is the one number you can move by changing your life, which is exactly why the FIRE community targets spending by age instead of income by age.

Your FIRE Number Is a Range, Not a Verdict

The math above produces one clean number. Your real number is a range, because your real spending will not stay frozen for 40 years. A paid-off mortgage drops it. Kids, travel, and medical surprises raise it. Recalculate every year or two; the formula takes 30 seconds.

One more thing. The FIRE number is the finish line. Most people do not need to run the whole race — once your investments pass the point where they will grow to the FIRE number on their own, you are Coast FIRE, and the job changes from saving to waiting. What is Coast FIRE explains that threshold, and the Coast FIRE Calculator shows how much closer it is than the full number.

Bottom Line

Your FIRE number is not a mystery and not a headline. It is your annual spending divided by 0.04, and the spending side is the part you actually control. Measure it this afternoon, divide it tonight, and you will know the exact figure your retirement plan is built on. Everyone else is still guessing.

Frequently Asked Questions

How do I calculate my FIRE number?

Take your annual spending and divide it by 0.04, or multiply it by 25. Both give the same number. If you spend $50,000 a year, your FIRE number is $1,250,000. The formula comes from the 4% safe withdrawal rate, which the Trinity Study showed survived every 30-year market period tested.

What is the formula for the FIRE number?

FIRE Number = Annual Spending ÷ 0.04. The 0.04 is the 4% Safe Withdrawal Rate (SWR). For a retirement horizon longer than 30 years, use 0.035 instead: FIRE Number = Annual Spending ÷ 0.035, roughly 28.6 times annual expenses.

Is my FIRE number based on income or spending?

Spending, never income. Two people earning $150,000 can have FIRE numbers that differ by hundreds of thousands of dollars, because one spends $50,000 a year and the other spends $90,000. The number follows the money that leaves your account, not the money that enters it.

What is the 25x rule?

The 25x rule is the FIRE number expressed as a multiple: your annual spending times 25. It is the same math as dividing by 0.04, written differently. Spend $60,000 a year and 25x is $1,500,000. The rule assumes a 30-year retirement at a 4% withdrawal rate.

Does my FIRE number include taxes?

Only the taxes you will still pay in retirement. If you draw from a traditional 401(k), withdrawals are taxable income, so your spending number should include the tax on those withdrawals. If you plan on Roth accounts and taxable brokerage only, tax can be minimal — but the calculation is the same either way: total withdrawals needed, divided by 0.04.

What if my spending changes in retirement?

Recalculate. The FIRE number is a snapshot of one spending level, and the honest plan treats it as a range: lower with a paid-off house, higher with travel or medical costs. Re-run the division once a year or after any large life change. It takes 30 seconds and it keeps the plan honest.