Financial Independence Number
The financial independence number is the total amount of invested assets a person needs to accumulate in order to live off portfolio income and growth indefinitely without requiring earned income, typically calculated as annual expenses divided by a safe withdrawal rate.
The financial independence number — sometimes called the FI number or FIRE number — is the central target in personal wealth-building strategy. The most widely referenced framework uses the 4% rule, derived from the Trinity Study, which found that a diversified portfolio of stocks and bonds could sustain annual withdrawals of 4% of the initial balance, inflation-adjusted, over a 30-year retirement period without being depleted. Dividing annual expenses by 0.04 gives the target portfolio size: $50,000 in annual spending requires $1.25 million; $100,000 requires $2.5 million.
The 4% rule has been both widely adopted and widely debated. Critics point out that it was calibrated for a 30-year retirement and may be too aggressive for someone retiring at 40 who needs the portfolio to last 50 years. Lower expected returns from current equity valuations and bond yields have led some planners to advocate for a 3% or 3.5% withdrawal rate, which correspondingly raises the target number by 14% to 33%.
Personal circumstances make each individual's FI number unique. A person who owns their home outright, has paid-off vehicles, lives in a low-cost area, and maintains flexibility to earn occasional income can operate on a lower number than someone with high fixed expenses. Healthcare costs are a major variable for those who achieve financial independence before Medicare eligibility at age 65.
The compounding power of saving and investing early makes the FI number achievable at a range of timelines depending on the savings rate. A person saving 50% of their income in a diversified portfolio can typically reach financial independence in 15 to 17 years regardless of absolute income level, because the same ratio of savings to expenses that produces rapid wealth accumulation also implies a lower absolute dollar target.
Tracking progress toward the FI number using the concept of a financial independence ratio — current portfolio value divided by the target — provides a measurable metric that focuses behavior on the controllable variables: savings rate, investment costs, and portfolio allocation.