Under the Hood: The Math Behind FIRE
The Financial Independence, Retire Early (FIRE) movement is built on a very simple mathematical concept: if you can save a large percentage of your income and invest it, your investments will eventually generate enough passive income to cover your living expenses indefinitely.
The 4% Rule
The cornerstone of FIRE is the Safe Withdrawal Rate (SWR), most commonly set at 4%. Based on the Trinity Study, a portfolio of stocks and bonds can historically sustain a 4% inflation-adjusted withdrawal rate for at least 30 years without running out of money.
To find your "FIRE Number" (the total portfolio size needed to retire), you simply divide your annual expenses by 0.04. Equivalently, you can multiply your annual expenses by 25. If you spend $40,000 a year, your FIRE number is $1,000,000.
Savings Rate is Everything
The most critical factor in determining your time to retirement isn't your absolute income—it's your savings rate (the percentage of your income you save).
If you save 10% of your income, you have to work 9 years to pay for 1 year of retirement. If you save 50% of your income, you only have to work 1 year to pay for 1 year of retirement. Increasing your savings rate exponentially decreases your time to financial independence.
Disclaimer
This calculator is provided for informational and entertainment purposes only. Every individual's financial situation, lifestyle, and local market conditions are unique, and there are many variables that a purely mathematical tool cannot account for. The results produced here are simulations based on your inputs and our assumptions—not professional financial advice. Always apply your own critical thinking and consult with a qualified advisor before making major life or financial decisions.