If you’ve spent any time in the FIRE community, you’ve heard the term “FIRE number.” But what does it actually mean — and more importantly, how do you calculate your FIRE number with confidence?
The 4% Rule (and Its Limits)
The classic FIRE number comes from the 4% rule: take your annual expenses, multiply by 25, and that’s the portfolio size at which you can theoretically retire. It’s a useful shortcut, but it’s built on historical averages — and your life is not average.
Your sequence-of-returns risk, tax situation, healthcare costs, and withdrawal strategy all change the picture significantly.
Coast FIRE vs. Lean FIRE vs. Fat FIRE
Not all FIRE is created equal:
- Lean FIRE means retiring with a minimal lifestyle — usually under $40k/year in spending.
- Fat FIRE means retiring with a more comfortable budget — $100k+ per year.
- Coast FIRE is the underrated middle ground: accumulate enough that your portfolio grows to your full FIRE number on its own, even if you never contribute another dollar.
Coast FIRE is particularly powerful because it gives you optionality much earlier than traditional FIRE — you can switch to a lower-stress job, go part-time, or pursue passion projects without worrying about contributions.
What Most Tools Get Wrong
Spreadsheets and simple calculators assume static income and static expenses. Real life doesn’t work that way. Your income changes. You have a kid. You pay off your mortgage. You inherit money or face a medical event.
Real financial planning needs to model events over time — not just your current snapshot.
That’s exactly what PlanyFI is built for: a multi-year timeline model that adapts as your life changes, so you can always see exactly where you stand and what it takes to get free.