What Is the FIRE Number and Why Does It Matter?

FIRE stands for Financial Independence, Retire Early. Your FIRE number is the total amount of invested capital you need so that a 4% annual withdrawal covers your living expenses indefinitely. This rule, derived from the Trinity Study at Trinity University, is based on historical stock market data showing that a diversified portfolio can sustain a 4% withdrawal rate for at least 30 years in virtually all historical scenarios.

The formula is simple: FIRE Number = Annual Expenses divided by 0.04, or equivalently Annual Expenses multiplied by 25. If you spend $40,000 per year, your FIRE number is $1,000,000. Once you reach that number with your invested assets, you can theoretically live off the portfolio returns without ever depleting the principal. This figure is based purely on your expenses — if Social Security, a pension, or rental income is expected during retirement, the more accurate move is to subtract that expected monthly income from the monthly expenses entered above, rather than treating the FIRE number as a complete picture on its own.

Lean FIRE, Regular FIRE, and Fat FIRE

Lean FIRE means retiring on a reduced budget — typically 70% of current expenses — accepting a more frugal lifestyle in exchange for reaching financial independence much earlier. Regular FIRE targets your current spending level. Fat FIRE aims for 150% or more of current expenses, providing a generous buffer for travel, healthcare, and lifestyle upgrades in retirement.

Whichever target applies, the savings rate is the single lever that moves the timeline more than any other input. A savings rate of 50% or more can reach financial independence in roughly 15-17 years from any starting point, and even a jump from 10% to 20% can shave a decade off the date — increasing income and trimming fixed expenses both push on the same lever.

What Is Coast FIRE?

Coast FIRE is the point at which your current savings, left untouched, will grow to your full FIRE number by your target retirement age through compound interest alone. Once you reach Coast FIRE, you no longer need to save aggressively — you just need to cover your current living expenses with your income, and time does the rest of the work.

The 4% Rule — Is It Still Valid?

The 4% safe withdrawal rate comes from the 1994 Trinity Study and has been widely discussed and debated. In historical back-tests, a portfolio of 50-75% stocks never ran out of money over a 30-year retirement period when withdrawing 4% annually. Some researchers suggest a 3-3.5% rate for longer retirements (40+ years), while others argue 4-4.5% remains reasonable with a flexible spending approach.

The Expected Return and Inflation fields in Advanced settings work together to produce a real return — nominal growth minus inflation, which represents actual purchasing power rather than a raw percentage. A portfolio growing 8% against 3% inflation has a real return of roughly 5%, and this calculator runs entirely on that real figure so every number on screen is expressed in today’s purchasing power rather than inflated future dollars. As a starting point for that input, US stocks have historically returned around 10% nominally, or about 7% after inflation, over long periods; a diversified global portfolio is often modeled more conservatively at 6-8% nominal or 3-5% real.