FIRE Number Calculator (4% / 3.5% safe withdrawal)

Understanding the FIRE number calculator — concept and principle

Your FIRE number is the portfolio size at which investment withdrawals can cover your living costs, so work becomes optional. People use it to set a savings goal, to compare the 4% rule with more cautious rates, and to see how far they still are from financial independence.

How it is calculated

FIRE number = annual spending ÷ safe withdrawal rate. The calculator also shows the target at three common rates: 4% (25× spending), 3.5% (about 28.6×) and 3% (about 33.3×).

• Gap today = FIRE number − current savings (never below zero).
• Years to FIRE: each year, balance = balance × (1 + real return) + yearly saving, until the balance first reaches the FIRE number.

All amounts are in today's money because the return you enter is a real (after-inflation) return.

The principle

William Bengen (1994) tested withdrawals against US market history and found that starting at about 4% of the portfolio, then adjusting for inflation, lasted through every 30-year period he examined. The Trinity study (1998) reached similar conclusions using success rates. Turning the rule around, a sum equal to 25 times your spending is the pot that historically supported that spending for 30 years. A longer early retirement needs a lower rate and therefore a larger multiple.

Worked example

1. Inputs: spending 40,000, savings 150,000, saving 30,000 a year, real return 5%, rate 4%.
2. FIRE number = 40,000 ÷ 0.04 = $1,000,000.
3. Alternatives: 40,000 ÷ 0.035 = $1,142,857 and 40,000 ÷ 0.03 = $1,333,333.
4. Gap today = 1,000,000 − 150,000 = $850,000.
5. Growing 150,000 by 5% and adding 30,000 a year gives about 959,196 after 15 years and 1,037,156 after 16, so the number is reachable in about 16 years.

How to read the result

The multiple tells you how cautious your plan is: 25× is the classic benchmark, 28–33× suits retirements of 40 years or more. The gap and the years show the effort still needed. Small spending cuts matter twice: they shrink the target and free up more money to save.

Common mistakes

• Using a nominal return with today's spending, which overstates progress.
• Leaving out taxes on withdrawals, insurance and replacement costs such as cars.
• Counting your home's value in savings even though it does not produce cash to spend.

Frequently asked questions

What is the FIRE number for 40,000 a year?

1,000,000 at 4%, about 1,142,857 at 3.5% and about 1,333,333 at 3%.

Is the 4% rule still safe?

It held for 30-year periods in US history, but lower expected returns and longer horizons lead many planners to prefer 3–3.5%.

Should I include part-time income in retirement?

You can subtract reliable side income from annual spending; the lower spending figure gives a smaller FIRE number.

Why does the calculator count whole years?

It adds savings once a year, so the answer is the first full year in which the balance reaches the target.

Sources: Bengen, W. P. (1994), "Determining Withdrawal Rates Using Historical Data", Journal of Financial Planning. · Cooley, Hubbard & Walz (1998), "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable", AAII Journal.