How to Calculate Your FIRE Number (And What It Actually Means)

$40,000 a year. That’s what a $1 million portfolio can safely produce, if you believe the research, trust the assumptions, and don’t retire into the worst sequence of returns in history.

Most people asking “how much do I need to retire?” are really asking: what’s my number? The answer is a formula. Whether the formula holds for your specific situation is a different question.

The 4% rule

In 1994, financial planner William Bengen analyzed 75 years of U.S. market history and found that a retiree who withdrew 4% of their portfolio in year one, then adjusted that amount for inflation each year, never ran out of money in any 30-year period he tested. Four years later, researchers at Trinity University ran a broader study and confirmed the finding.

The 4% rule became the standard.

It’s not a law. It’s a finding from historical data: hold a diversified portfolio of stocks and bonds, withdraw 4% per year adjusted for inflation, and you’ve historically had a very high probability of not outliving your money over 30 years.

The 25x multiplier

Flip the 4% rule and you get the FIRE number formula:

The multiplier comes directly from the math: 1 ÷ 0.04 = 25. The variable you actually control isn’t the withdrawal rate. It’s the expenses.

Is $1 million enough?

Depends entirely on what you spend.

$1M × 4% = $40,000/year. If that covers your life (housing, food, healthcare, travel, everything) then yes. If you need $80,000 a year to live the way you want to live, $1M gets you halfway there.

“Is $1 million enough?” is the wrong question. “What do I actually spend, and what will I spend in retirement?” is the right one.

The limitations the rule doesn’t advertise

The Trinity Study modeled 30-year retirement periods. If you retire at 40, you might need 50 years of portfolio survival. That changes the math.

At longer horizons, the failure rate at 4% creeps up. Many in the FIRE community use 3.5% (a 28.6× multiplier) or 3% (a 33.3× multiplier) to build in extra margin. Conservative but rational, especially if you’re retiring decades before traditional retirement age.

Sequence of returns risk is the other issue that doesn’t get enough attention. The order market returns arrive in matters enormously. A portfolio with a 7% average return that crashes 30% in years one and two is a very different beast than one that gains steadily for a decade before a dip. You sell shares at the bottom to fund your life. The portfolio never fully recovers.

Bengen’s data includes bad sequences: the Great Depression, 1970s stagflation. The 4% rule survived those historically. But “survived historical worst cases” is different from “guaranteed to work.”

Expenses are the lever most people underestimate

There’s no faster way to change your FIRE number than to change what you spend. Cutting $500/month does two things simultaneously: it lowers the amount you need to generate from your portfolio, and it frees up $500/month to invest.

That’s not a rounding error. Shaving $500/month off your life costs you $150,000 less to retire, and your savings rate goes up at the same time.

This is why expenses are the most powerful lever in FIRE math. Income matters, but spending has a multiplied effect in both directions.

The fastest way to reduce expenses is eliminating debt payments. If you’re carrying high-interest debt, choosing between the avalanche and snowball methods is the first decision.

What the number tells you, and what it doesn’t

Knowing your FIRE number tells you the portfolio size you’re aiming for. It doesn’t tell you how long it will take to get there, whether healthcare costs will run higher than planned, how your expenses will shift as you age, or what happens if inflation runs hotter than history suggests.

Use the 4% rule and the 25× multiplier as a starting point. Build in margin. A small amount of income in early retirement (even a few thousand dollars a year) dramatically reduces the stress on your portfolio.