How it works
FIRE stands for Financial Independence, Retire Early. Your FIRE number is the investment balance whose yearly withdrawals cover your spending indefinitely, or at least for a very long retirement.
FIRE number = yearly spending ÷ withdrawal rate
At a 4% withdrawal rate that's 25 times your yearly spending. The calculator then grows what you have invested, adding your yearly savings in monthly installments, until it reaches that number. It uses a return after inflation, so every figure stays in today's dollars.
Your Coast FIRE number is the amount that, left alone to grow, would reach your FIRE number by traditional retirement age with no more contributions. Pass it and you only need to earn enough to cover today's costs.
Worked example
You spend $40,000 a year, so your FIRE number at 4% is $1,000,000.
You're 30, with $50,000 invested, adding $20,000 a year and earning 5% after inflation. You reach your number in 23 years 0 months. Invest $30,000 a year instead and it takes 18 years 3 months.
Your Coast FIRE number is $181,290: that amount, untouched, grows to $1,000,000 by age 65.
About the 4% rule
The 4% rule comes from financial planner William Bengen's 1994 study of U.S. market history, later echoed by the 1998 Trinity study. Withdrawing 4% of a stock-and-bond portfolio in the first year, then the same amount plus inflation each year after, lasted at least 30 years in every historical period tested.
Early retirees may need their money to last 40 or 50 years, so many plan on 3.25% to 3.5% instead. The rule is a planning guide, not a guarantee. Fees, taxes and the order of market returns all matter.
Questions people ask
What are Lean FIRE and Fat FIRE?
Lean FIRE means retiring on a minimal budget, often well below the national average. Fat FIRE means retiring with a comfortable or generous budget. The maths is the same; only the spending figure changes.
What return should I assume?
Use a return after inflation. Many planners use 4% to 6% for a stock-heavy portfolio over the long run. Try a lower figure to see how sensitive your date is.
Does this include Social Security or pensions?
No. If you expect other income in retirement, subtract it from your yearly spending before you enter it.
Sources
- Bengen, W. P. (1994). Determining withdrawal rates using historical data. Journal of Financial Planning
- Investor.gov (U.S. SEC): free financial planning tools
Reviewed September 24, 2026 by The Dollars World team. How we build and check our calculators. This is general information, not financial advice.