FIRE calculator

What is a FIRE calculator?

A FIRE calculator is a free online tool that estimates how much money you need to become financially independent and how many years it will take you to get there. FIRE stands for Financial Independence, Retire Early — a movement built around saving and investing aggressively so that the returns from your portfolio can eventually cover your living expenses, freeing you from the need to work for a paycheck.

The calculator answers two questions at once: your FIRE number (the size of the portfolio you are aiming for) and the years to financial independence (how long your current savings and contributions need to grow before they reach that target).

How does the FIRE calculator work?

The calculator takes five inputs:

• Annual expenses in retirement — how much you expect to spend each year once you stop working.
• Safe withdrawal rate — the percentage of your portfolio you plan to withdraw each year (commonly 4%).
• Current savings and investments — the value of the portfolio you already have.
• Annual contribution — how much you add to your investments every year.
• Expected annual return — the average yearly growth rate you assume for your portfolio.

From the annual expenses and the safe withdrawal rate it derives your FIRE number, then projects your portfolio forward year by year — adding contributions and compounding returns — until the balance reaches that number.

The 4% rule and the safe withdrawal rate

The FIRE number is based on the 4% rule, a guideline drawn from historical market research suggesting that a retiree can withdraw about 4% of their portfolio in the first year, adjust for inflation thereafter, and have a high chance of the money lasting for decades. A 4% withdrawal rate is equivalent to needing 25 times your annual expenses invested.

Choosing a lower safe withdrawal rate (for example 3%) makes your plan more conservative and raises your FIRE number, while a higher rate lowers the target but carries more risk of running out of money.

Formula

The FIRE number is the portfolio size that supports your spending at the chosen withdrawal rate:

$FN = \frac{E}{w}$

Where:

• $FN$ is your FIRE number (target portfolio value).
• $E$ is your annual expenses.
• $w$ is the safe withdrawal rate expressed as a decimal (4% = 0.04).

Assuming a portfolio that starts at $S$, receives a fixed contribution $C$ each year and grows at annual return $r$ (as a decimal), the number of years $t$ to reach the FIRE number is:

$t = \frac{\ln\!\left(\dfrac{FN \cdot r + C}{S \cdot r + C}\right)}{\ln(1 + r)}$

When the return is zero, this simplifies to a straight line, $t = \dfrac{FN - S}{C}$.

Examples of use

1. A saver spends 40,000 per year and plans to use the 4% rule. They currently have 100,000 invested, add 30,000 each year, and expect a 7% annual return.

   • FIRE number $FN$ = 40,000 / 0.04 = 1,000,000
   • Current savings $S$ = 100,000
   • Contribution $C$ = 30,000
   • Return $r$ = 0.07

   Calculation: $t = \frac{\ln\!\left(\dfrac{1{,}000{,}000 \cdot 0.07 + 30{,}000}{100{,}000 \cdot 0.07 + 30{,}000}\right)}{\ln(1.07)} \approx 14.70 \text{ years}$

2. The same saver assumes no growth (a 0% return) but raises contributions to 90,000 per year:

   • FIRE number $FN$ = 1,000,000
   • Current savings $S$ = 100,000
   • Contribution $C$ = 90,000
   • Return $r$ = 0

   Calculation: $t = \frac{1{,}000{,}000 - 100{,}000}{90{,}000} = 10 \text{ years}$

Notes

The result is an estimate: real returns vary from year to year, inflation erodes purchasing power, and taxes and fees reduce net growth. Treat the years-to-FI figure as a planning benchmark rather than a promise, and revisit it whenever your income, spending, or return assumptions change. A slightly higher savings rate or a lower withdrawal rate can shorten the timeline dramatically thanks to compounding.

FAQs

What is a FIRE number?

Your FIRE number is the total amount you need invested so that a safe withdrawal from it covers your annual expenses indefinitely. At a 4% withdrawal rate it equals 25 times your yearly spending.

Why is the 4% rule so common?

The 4% rule comes from studies of long historical periods showing that withdrawing 4% of a diversified portfolio in the first year, then adjusting for inflation, rarely depleted the savings over a 30-year retirement. It is a rule of thumb, not a guarantee.

How can I reach financial independence faster?

The two most powerful levers are increasing your annual contribution and reducing your annual expenses — lower expenses shrink your FIRE number while freeing up more money to invest. A higher expected return helps too, but it comes with more risk and is less within your control.