How to Calculate Your Financial Independence Number

FIRE · Retirement · 10 min read

Financial independence means your investment portfolio generates enough passive income to cover your living expenses indefinitely — without needing to work. The portfolio size required to achieve that is called your FI number (or FIRE number). Once you know it, every financial decision has a clear target: does this move me toward that number or away from it?

The 4% Rule: The Foundation

The 4% rule comes from the Trinity Study (1998), updated multiple times since. It states that if you withdraw 4% of your portfolio in year one of retirement and adjust for inflation each year, historically your portfolio will last 30+ years with a very high probability, even through recessions and market crashes.

This gives us the formula: FI Number = Annual Expenses × 25

The multiplier of 25 is simply the inverse of 4% (1 ÷ 0.04 = 25). The higher your withdrawal rate, the lower the multiplier — a 5% withdrawal rate uses a 20× multiplier, but with historically lower success rates.

Step 1: Calculate Your Real Annual Expenses

Be precise. Pull 12 months of actual spending from bank and credit card statements and categorize everything. Don't budget what you wish you spent — calculate what you actually spend. Common categories to include:

• Housing (mortgage, rent, property taxes, insurance, maintenance — budget 1–2% of home value annually for repairs)
• Food (groceries + dining)
• Transportation (car payment, insurance, gas, maintenance, or transit)
• Healthcare (premiums, out-of-pocket, dental, vision — especially important pre-Medicare)
• Travel and leisure
• Clothing and personal care
• Subscriptions and technology
• Gifts and charitable giving

Also project for lifestyle changes in retirement: more travel in early years, higher healthcare costs in later years, potentially lower transportation costs if you can drop a car.

Step 2: Adjust for What Changes in Retirement

What Typically Decreases

• Work commuting costs
• Professional clothing
• Childcare (if kids will be adults)
• Mortgage (if paid off before retirement)
• Payroll taxes (no earned income)

What Typically Increases

• Healthcare (before Medicare at 65)
• Travel and experiences (more free time)
• Home maintenance (more time at home)
• Hobbies and leisure

Step 3: Adjust for Other Income Sources

Your FI number only needs to cover the gap between your income sources and your expenses. If you'll receive Social Security, a pension, or rental income in retirement, subtract that from your annual expense number before calculating.

Step 4: Account for Early Retirement

The 4% rule was designed for 30-year retirements. If you're planning to retire at 45, you need a 40–50 year portfolio runway. Researchers suggest 3.3–3.5% as a safer withdrawal rate for 50-year retirements, which changes the multiplier to 28.5–30×.

Early retirees should also plan for a 2-phase withdrawal:

1. Pre-Social Security phase (age 45–67): Full portfolio withdrawal
2. Post-Social Security phase (67+): Partial withdrawal, SS income fills part of the gap

How Long to Reach Your FI Number

The variable that most dramatically affects your timeline isn't investment return — it's your savings rate. The higher the percentage of your income you invest, the faster you build the portfolio, and the lower your FI number becomes (because you're proving you can live on less).

Saving 50% of your income for 17 years produces financial independence regardless of your income level — because both the numerator (savings invested) and denominator (expenses needed from portfolio) move simultaneously.