FIRE Calculator
FIRE — Financial Independence, Retire Early — asks one question: when can your investments cover your spending forever? Enter your numbers to find your FI number and your timeline.
Your inputs
Net worth trajectory to FI
What moves the needle most?
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What is FIRE?
FIRE stands for Financial Independence, Retire Early. The core idea is simple: once your invested savings are large enough that a sustainable withdrawal covers your annual spending, work becomes optional. You don't have to retire — you just can. Many people in the movement keep working but switch to part-time, passion projects, or lower-stress jobs.
The math: your FI number
This calculator uses the classic rule of thumb: FI number = 25 × annual spending. That's the inverse of the 4% rule — if you can withdraw 4% of your portfolio per year, you need 25 times your spending saved (because 1 ÷ 0.04 = 25). Spend $60,000/year and your FI number is $1.5 million.
balancet+1 = balancet × (1 + r/12) + monthly savings
until balance ≥ 25 × annual spending.
Worked example
Start with $50,000, save $2,000/month, spend $60,000/year, earn 7%. Your FI number is $1,500,000. Month-by-month compounding gets you there in about 22 years — having contributed roughly $582,000 out of pocket. Notice the gap: over $900,000 of the total comes from growth, which is why starting early and saving aggressively both matter enormously.
Caveats worth knowing
- 25× is a guideline, not a guarantee. It comes from research on 30-year retirements (see our 4% rule calculator). Retiring at 40 means planning for 50+ years, which is stricter.
- Spending is the biggest lever. Cutting spending helps twice: it lowers your FI number and frees more to save. A 10% spending cut can shave years off the timeline.
- Returns are assumed constant. Real markets crash and surge. A bad decade early in your journey (sequence-of-returns risk) hurts more than the average suggests.
- Life happens. Healthcare, kids, housing changes — budgets shift. Revisit the math yearly rather than treating the output as a fixed destiny.
Background reading: U.S. SEC investor.gov compound interest tools; the 25×/4% framework originates from financial planner William Bengen's 1994 research on sustainable withdrawal rates.