How is the FIRE number calculated?

Annual expenses × 25. The 25 comes from the 4% safe withdrawal rate in the Trinity Study — withdraw 4% of your portfolio per year and it has historically supported a 30-year retirement. Spend $50K/year, target $1.25M. The math doesn't care about income; it only cares about spending.

Why does Rachel hit FIRE before Marcus despite earning half as much?

Rachel saves $30K/year against a $1.26M target — 18 years to FIRE. Marcus saves $22K/year against a $1.8M target — 22 years. Higher income widened his target faster than it widened his savings. The variable that dominates FIRE timing isn't income; it's the gap between income and spending.

Does cutting $200/month really save 2 years?

It depends on where you are in the timeline. A $200/month cut at age 32 with $45K invested moves the FIRE date up by 1.5-2 years because it does two things at once: lowers the target by $60K AND adds $2.4K/year of investable surplus. Same cut at age 50 with $800K invested moves the date by ~6 months — the portfolio is doing most of the work by then, so reducing the target matters less than reducing required portfolio.

Rachel's FIRE Number Is $1.26M at 50 — Marcus Earns 2× and Hits FIRE 4 Years Later

Rachel’s spreadsheet moment

Rachel is 32. She earns $85,000 a year, saves $2,500 every month, and has $45,000 already invested. Her monthly expenses run to $4,200. One afternoon she opens a spreadsheet and asks the question that changes how she thinks about work: “When can I actually stop?”

The answer surprised her. The math surprised her more.

The FIRE number: 25× your annual expenses

The most widely cited FIRE rule is simple: multiply your annual expenses by 25. That gives you the portfolio size at which a 4% annual withdrawal — the “safe withdrawal rate” from historical research — covers your expenses indefinitely.

Rachel’s math:

Monthly expenses: $4,200
Annual expenses: $4,200 × 12 = $50,400
FIRE number: $50,400 × 25 = $1,260,000

That’s her target. Now the harder question: how long to get there?

Running the timeline

Starting with $45,000 invested, saving $2,500/month ($30,000/year), and assuming a 7% annual return on investments:

Using the future value formula with ongoing contributions, Rachel’s portfolio crosses $1,260,000 in approximately 18.3 years. She’d be 50.

That’s a real answer. Not a fantasy, not a vague “maybe someday” — a number with a year attached.

Two levers, one big difference

Here’s where it gets interesting. Rachel has two levers she can pull: spending and saving.

Lever 1: Cut monthly expenses from $4,200 to $3,500

New FIRE number: $3,500 × 12 × 25 = $1,050,000
Time to FIRE: 15.8 years (she’d be 48)

Dropping $700/month from her expenses shaves 2.5 years off her timeline and reduces the target by $210,000. That’s the power of the spending side.

Lever 2: Increase savings from $2,500 to $3,500/month

FIRE number stays at $1,260,000 (expenses unchanged).
Time to FIRE: 13.6 years (she’d be 46)

Saving an extra $1,000/month — roughly what a lot of people spend on dining out and subscriptions — shaves nearly 5 years off her timeline.

The insight here is worth repeating: savings rate matters far more than income level.

The high earner trap

Rachel’s colleague Marcus earns $150,000 — nearly double her salary. But he spends $120,000/year. His lifestyle has expanded to fill his income.

Marcus’s FIRE math:

Annual expenses: $120,000
FIRE number: $120,000 × 25 = $1,800,000
Annual savings: $150,000 − $120,000 (minus taxes, roughly) ≈ $22,000/year
Time to FIRE at 7%: approximately 22 years

Rachel, earning half as much, will hit FIRE first — by several years. The variable that dominates isn’t income; it’s the gap between income and spending.

What the 25× rule doesn’t tell you

A few things the simple formula glosses over:

Sequence of returns risk. If the market drops 40% in your first year of retirement, your portfolio may not recover the way historical averages suggest. The 4% rule is based on 30-year retirements; for someone retiring at 46, a 50-year horizon requires more caution — some researchers suggest 3.3–3.5% as a safer withdrawal rate, which means a FIRE number closer to 30× expenses.

Healthcare before 65. If you retire before Medicare eligibility, you’re buying private health insurance. In the US, that can run $500–$1,000/month for a single person.

Variable spending. Expenses often drop in early retirement (no commute, no work wardrobe) and rise again later (healthcare). A static number misses the curve.

None of this invalidates the exercise. It just means the FIRE number is a starting estimate, not a finish line etched in stone.

Where this scenario doesn’t apply

Three situations where Rachel’s clean math gets messy:

Variable income. Freelance, sales-commission, or equity-heavy comp doesn’t translate cleanly into “save $X/month.” Year-over-year savings can swing by 50%+, which means the timeline becomes a distribution rather than a single date. Worth running the calculator at both your good-year and bad-year savings rates to see the spread.
Single-income households with kids. Rachel’s $50K spend covers one person. A family of four hits $80-100K of expenses easily, which raises the FIRE number to $2-2.5M and pushes the timeline 8-12 years. Worth modelling separately rather than assuming a single number scales linearly.
Geographic moves planned for retirement. Anyone planning to leave a high-cost city at 50 has two FIRE numbers — the working-life one and the retirement one. The retirement number is what actually determines the target, but the working-life expenses determine how fast you get there.

Your turn

The most useful thing about the FIRE calculation isn’t the number itself — it’s what happens when you start pulling the levers. A $200/month reduction in expenses has a different impact at 32 than at 45. An extra $500/month in savings matters less when your portfolio is $20,000 than when it’s $500,000 (because the portfolio is doing more of the heavy lifting).

The FIRE Calculator lets you model all of this interactively — enter your current savings, monthly contribution, expected return, and target expenses, and see exactly how the timeline shifts as you move each variable. It’s the fastest way to answer “what if I saved just a little more?”

Open the FIRE Calculator → and run Rachel’s scenario, Marcus’s scenario, and yours. The lever that moves your timeline most isn’t always the obvious one.

The 4% withdrawal rule is derived from the Trinity Study (1998). Actual returns will vary. This calculator assumes a fixed annual return; real markets are volatile.