FIRE stands for Financial Independence, Retire Early. The core idea is simple: accumulate a portfolio large enough that investment returns can cover your living expenses indefinitely, without ever touching a job again. Your FIRE number is the specific portfolio size that makes that possible.
The 4% Rule and Where It Comes From
The 4% rule originated from research by William Bengen in 1994, later reinforced by the Trinity Study in 1998. Both studies found that a retiree who withdraws 4% of their portfolio in year one, then adjusts for inflation each year, had a very high probability of not running out of money over a 30-year retirement period, even across historical market downturns.
The 4% figure is often called the Safe Withdrawal Rate (SWR). Some planners use 3.5% for longer retirements (40+ years) or for people retiring in a low-return environment.
The FIRE Number Formula
The formula is straightforward:
FIRE Number = Annual Expenses / Safe Withdrawal Rate
At 4%:
$50,000 annual expenses / 0.04 = $1,250,000
$40,000 annual expenses / 0.04 = $1,000,000
$80,000 annual expenses / 0.04 = $2,000,000
At a more conservative 3.5% SWR:
$50,000 annual expenses / 0.035 = $1,428,571
The single most powerful lever is your annual expenses. Cutting $10,000 from your spending does not just save $10,000 per year. It also reduces your FIRE number by $250,000 at a 4% SWR.
Lean FIRE vs. Fat FIRE
People in the FIRE community generally cluster into two camps. Lean FIRE targets a minimalist lifestyle, often below $40,000 per year, which results in a smaller target number but less spending flexibility. Fat FIRE targets $80,000 to $100,000 or more per year, providing a more comfortable retirement with less reliance on frugality.
Neither is objectively better. The right target is the one that matches the life you actually want to live.
How Monthly Contributions and Returns Affect Time to FIRE
Once you have your FIRE number, the next question is how long it takes to get there. This depends on your current portfolio balance, monthly contributions, and expected investment return.
A rough example: starting with $50,000, contributing $2,000 per month, targeting $1,000,000, at a 7% annual return:
Time to FIRE: approximately 17 years
Increasing contributions to $3,000 per month brings that closer to 14 years. The relationship is not linear because contributions and returns both compound together.
The "One More Year" Problem
One well-documented trap for people approaching FIRE is the tendency to keep working for "just one more year" to build a larger cushion. This often happens because the fear of running out of money feels more painful than the cost of additional years of work.
The math usually supports stopping. A $1.25 million portfolio at 4% is well-supported by historical data. A $1.5 million portfolio at 3.33% is roughly equivalent in terms of sustainable withdrawals but requires substantially more time to accumulate.
The decision to retire is ultimately personal, but understanding the numbers makes it easier to identify when you have actually reached the finish line.
Calculate Your FIRE Number
To find your exact target and time-to-FIRE based on your current savings, contributions, and return assumptions, use the FIRE Number Calculator.