The standard FIRE number is presented as a finish line.
Annual spending times 25. Hit the number. Retire.
This is simple, memorable, and wrong in a way that keeps people working longer than necessary.
Your FIRE number is not fixed. It should shrink every birthday.
The reason is brutal and clarifying: each year you remain alive is one fewer year your portfolio has to fund. The denominator changed. The plan should change with it.
The static-number error
Suppose you spend $50,000 a year. The default FIRE calculation says you need $1.25 million, using the 4% rule.
That number might make rough sense for a 35-year-old trying to fund 50 or 60 years. It makes less sense for a 52-year-old. It makes very little sense for a 61-year-old with Social Security seven years away and Medicare four years away.
But most people keep carrying the same number forward.
They calculate it once in a moment of anxiety, pin it somewhere in their mind, and spend the next decade treating it as a law. Meanwhile, the actual obligation is shrinking. The bridge to Social Security shortens. The years before Medicare shorten. The probability-weighted number of healthy years changes. The retirement horizon changes. The portfolio requirement changes.
The number should move.
Retirement is a liability
Think like an actuary for one minute.
Retirement is not a mood. It is a liability: a stream of future expenses that must be funded. A 40-year-old with $50,000 of annual spending and no pension has a large liability. A 63-year-old with the same spending and Social Security arriving soon has a smaller liability. Same lifestyle. Different math.
This is obvious in every domain except personal finance, where people prefer round numbers because round numbers feel moral.
The 25x rule is not moral. It is a rough shortcut for a specific horizon. Change the horizon and the shortcut breaks.
The birthday recalculation
Once a year, on your birthday, recalculate three numbers:
- Years until Medicare.
- Years until Social Security at 62, 67, and 70.
- Honest expected years of portfolio-funded spending.
Then divide your spending into phases.
Phase one: now until Social Security or other income begins.
Phase two: Social Security begins, portfolio covers the gap.
Phase three: lower or different spending later in life, if that is your realistic plan.
This is not more complicated than pretending you need one giant number forever. It is just more honest.
The bridge is often the plan
For many readers, the real plan is not “fund 40 years from a portfolio.”
The real plan is “fund the bridge.”
Bridge from 50 to 62. Bridge from 57 to Medicare. Bridge from burnout to part-time work. Bridge from the high-cost city to a lower-cost place. Bridge from full employment to a life where some income may still appear, but the old job no longer owns the week.
The bridge number is smaller than the forever number. Sometimes much smaller.
A person spending $50,000 a year who needs a 12-year bridge does not necessarily need $1.25 million. At zero real return, the bridge costs $600,000. With reasonable returns, flexible spending, and Social Security later, the required number can be lower still. The exact answer depends on the details, but the direction is not subtle.
The bridge changes everything.
Why people resist the shrinking number
People resist this because the shrinking number feels like cheating.
It is not cheating. It is time passing.
The culture has trained you to treat every birthday after 35 as bad news. For a FIRE planner, a birthday is mixed news. You lost a year. You also reduced the financial obligation attached to the remaining years. Mortality is doing arithmetic in the background whether you acknowledge it or not.
The refusal to update your number is not prudence. It is a superstition with a spreadsheet.
The cost of not updating
Assume your real required number has fallen from $1.25 million to $900,000, but you still believe the old number. You currently have $920,000.
The plan is done.
But because you are using stale math, you keep working. One more year for safety. Then another because markets look expensive. Then another because the bonus is good. Then another because you are close to a rounder number.
Four years pass.
The portfolio is larger. The remaining healthy window is smaller. You bought safety with the only currency safety cannot refund.
The rule
Recalculate annually.
Use conservative assumptions, but current ones. Include Social Security, even if haircut. Include Medicare. Include lower work-related spending. Include the fact that you are older. Include the fact that older is not only a tragedy; it is also a shorter bill.
Your FIRE number should shrink every birthday unless your spending grows, your obligations expand, or your risk tolerance collapses.
If the number is not moving, you are probably not planning. You are worshipping an old estimate.
The goal is not to hit the number you invented five years ago. The goal is to stop selling years once the remaining years have already been paid for.
Leave a Reply