Every guide on FIRE eventually shows you the same equation. Take your annual spending. Multiply by twenty-five. That’s your number. Save it, retire, the math works out.
The equation is right, in a very narrow sense. It is also misleading in a way that has cost a lot of people a decade of their lives.
Here is what the equation actually says, and what it is hiding.
Where the number comes from
The 25 is the inverse of 4%. Multiply by 25 and you’ve calculated 4% of your portfolio — the amount you can “safely” withdraw each year. Safely, here, means: based on William Bengen’s 1994 study, this withdrawal rate would not have exhausted any historical American retiree’s portfolio over a 30-year retirement, across every starting year on record.
Three details inside that sentence matter.
First, thirty years. Bengen wasn’t writing for FIRE. He was writing for people retiring at 65 who might live to 95. The 30-year window is a function of conventional retirement age and conventional mortality. The FIRE community took the 4% rule and extrapolated it to 40, 50, even 60-year horizons. The math doesn’t break exactly, but the margin of safety thins fast.
Second, not exhausted. The 4% rule does not aim at zero. It aims at survival. In a large fraction of the historical scenarios Bengen studied, retirees following 4% died with more money than they started with. Sometimes much more. The 4% rule is designed to leave a legacy.
Third, the worst case. The 4% number is the floor of historical outcomes. The median outcome was closer to 7%. Half of all historical retirees could have withdrawn nearly twice as much and still finished with money on the table.
So when someone hands you “$40k spending × 25 = $1M,” they are giving you a number that:
- Was designed for someone twenty years older than you
- Aims to leave money for heirs you may not have or care about
- Solves the worst historical case rather than the typical one
Whether this is your number depends on which of those three you accept.
What changes with an honest horizon
The single biggest lever on the FIRE number is how long you intend to live off it.
For most people, the honest answer is not “forty years.” The honest answer is “I want to live well for as long as I’m able to live well, and the math of the back end can take care of itself once I’m old enough to qualify for the safety net.”
If you accept a finite, chosen horizon — say, fifteen good years before you re-engage with the world at a different pace — the math changes dramatically.
Karsten Jeske, who runs the most rigorous public FIRE simulations available, has shown that for a 15-year retirement using historical data, the worst-case safe withdrawal rate sits around 6.5%. For 20 years, around 5.2%. For 25 years, around 4.5%. For 30 years (Bengen’s territory), the familiar 4%.
The numbers, then:
| Annual spending | 30-yr (4%) | 25-yr (4.5%) | 20-yr (5.2%) | 15-yr (6.5%) |
|---|---|---|---|---|
| $30,000 | $750,000 | $667,000 | $577,000 | $462,000 |
| $40,000 | $1,000,000 | $889,000 | $769,000 | $615,000 |
| $50,000 | $1,250,000 | $1,111,000 | $962,000 | $769,000 |
| $80,000 | $2,000,000 | $1,778,000 | $1,538,000 | $1,231,000 |
For someone spending $50k a year, the difference between planning for thirty years versus fifteen is $481,000. That’s roughly a decade of working at $50k a year saved. It is also, for most people, the difference between I can leave now and I’m stuck.
The honest method
Here is how you actually arrive at your number.
Step 1. Calculate your real spending. Not aspirational spending. Not “what I think I should spend.” Pull your last twelve months of bank and credit card statements and add them up. That number is your starting point. Most people are surprised — usually downward.
Step 2. Decide your active horizon, honestly. Look at your parents and grandparents. Look at your body’s current state. Look at what you actually want to be doing at 65, 70, 75. Most readers, sitting with this question, land somewhere between fifteen and twenty-five active years. Pick yours.
Step 3. Pick the withdrawal rate that matches. Use the table above. If you’re 45 and your honest active horizon is fifteen years, you’re a 6.5% planner. If you’re 35 with twenty-five years ahead, 4.5%. If you’re 50 and want to plan for twenty, 5.2%.
Step 4. Divide. Annual spending ÷ withdrawal rate = your real FIRE number.
That’s it. That is the math the FIRE forums won’t quite let themselves do, because it requires admitting that the safety net at the back end exists, that mortality is real, and that the optimum isn’t infinity.
A worked example
Take Lena. She is 42, a software engineer in Seattle, single, no children. Her actual annual spending, audited from her statements, is $46,000 — lower than she expected, because she had stopped looking at it for years.
She has been told her FIRE number is $46,000 × 25 = $1.15M.
Lena does the honest calculation.
She’s 42. She has seen her father deteriorate after 70 in ways that scare her. Her grandmother is in a nursing home with no idea where she is. Lena’s honest active horizon is about seventeen years — through age 59 — during which she wants real freedom and energy.
For seventeen years, the safe withdrawal rate is roughly 5.7%. Her real FIRE number is:
$46,000 ÷ 0.057 = $807,000
That is $343,000 less than the FIRE forum number. At her current savings rate, that’s about seven years of work eliminated from her timeline. Seven Augusts she gets back.
After 59, she has Social Security (she’ll get something), Medicare at 65, and the option to work part-time at a fraction of her current intensity. The plan doesn’t have to terminate at 59. It has to reach 59.
The number you’ll actually arrive at
The number people land on, doing this honestly, falls into a narrower range than the FIRE forums suggest. Most readers I’ve talked to, when they do the audit, arrive at a real FIRE number somewhere between 16× and 20× their annual spending, depending on age and horizon.
For most of you reading this, that means your real number is somewhere between 35% and 60% smaller than the one you have been chasing.
That gap is not abstract. It is years. It is, very literally, life.
What changes once you have it
Once you have your real number, three things happen.
First, you re-anchor. The FIRE number you have been chasing — the one that probably felt impossible, or close to it — drops to something visible. Visible is a different psychological state than impossible. You can plan against visible numbers.
Second, you start asking different questions. Not how do I save more? but how do I spend less without diminishing what matters? Not when can I retire? but what would I actually do with the years I free up? The latter questions are the ones that matter; the FIRE forums skip them because they’re harder than spreadsheets.
Third, you start to feel the weight of the trade. Every year you continue working past your real FIRE number is a year you have explicitly chosen to spend earning money you have no plan to use. You may decide that’s worth it — for the security, the identity, the relationships at work. But you should be choosing it, not drifting into it.
That last sentence is the entire point of this essay.
The math is small. The math is straightforward. The math has been hiding in plain sight inside a paper from 1994 that almost everyone has misread.
The math is yours. Now you have it.
Next in this series: “The 4% Rule Is Lying to You — Here’s What to Use Instead” — a deeper dive into the assumptions behind Bengen’s number and why the FIRE community’s version of it has drifted from the original.
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