The FIRE number formula
FIRE number = (Annual spending − Other income) ÷ Withdrawal rate
At a 4% withdrawal rate this is the famous "25× rule": multiply your annual spending by 25. At 3.5% it's about 28.6×, and at 3% it's 33.3×.
Lean FIRE, FIRE and Fat FIRE
The calculator also shows three lifestyle versions of your number, because most people aren't sure exactly what they'll spend:
- Lean FIRE (75% of your spending): a frugal retirement with little room for surprises.
- Your FIRE number: your stated spending.
- Fat FIRE (150% of your spending): extra room for travel, gifts and big-ticket items.
How to estimate retirement spending
- Start with the last 12 months of actual spending from your bank and card statements.
- Remove costs that will end, such as a mortgage you'll pay off, commuting and retirement contributions.
- Add costs that will start, especially health insurance before Medicare at 65, which can be $6,000 to $15,000+ a year per person on the ACA marketplace.
- Add income taxes on withdrawals and a buffer for irregular costs (cars, roofs, family help).
Frequently asked questions
Is 25 times expenses enough to retire?
Historically, 25× annual spending (the 4% rule) has lasted at least 30 years in almost all US market periods. For retirements of 40+ years, many people target 28× to 33×.
Do I count my 401(k) and IRA?
Yes, all invested retirement and brokerage accounts count. Remember that traditional accounts will be taxed when you withdraw.
How does other income change my FIRE number?
Every dollar of reliable annual income reduces what your portfolio must cover. At 4%, $10,000 a year of pension income lowers your FIRE number by $250,000.
Last reviewed: 2026-10-09