The "FIRE Number" is perhaps the most significant figure in any early retirement plan. It represents the total amount of invested assets you need to have accumulated before you can consider yourself financially independent. The standard method for calculating this target is known as the Rule of 25. This rule provides a quick, reliable estimate of the "nest egg" required to support your lifestyle without ever needing another paycheck.

Calculating Your Target: The Step-by-Step Formula

The Rule of 25 is elegantly simple: you need to save 25 times your annual expenses to retire . To arrive at this number, you must follow a specific sequence of calculations to ensure your estimate is grounded in reality.

Determine Monthly Expenses: Track every dollar you spend in a typical month. This includes essentials (housing, food, utilities, insurance) and discretionary spending (entertainment, travel, hobbies).
Calculate Annual Expenses: Multiply your monthly total by 12.
Apply the Multiplier: Multiply that annual figure by 25.

Example: The $6,000 Monthly Lifestyle

If your monthly expenses are $6,000, your annual expenses would be $72,000 ($6,000 x 12). Applying the Rule of 25, you multiply $72,000 by 25 to get a FIRE number of $1.8 million .

Why 25x? The Inverse Relationship

The Rule of 25 is the mathematical inverse of the 4% Rule (which we will cover in the next section). If you divide 1 by 0.04 (4%), you get 25. This means that if you have 25 times your annual expenses saved, withdrawing 4% of that total in your first year of retirement will exactly equal your annual spending needs. This symmetry is what makes the Rule of 25 such a powerful tool for beginners; it allows you to work backward from your desired lifestyle to a concrete savings goal.

Variables That Influence Your FIRE Number

While the Rule of 25 is a great starting point, it is a "generalization" and not a "one size fits all" solution . Several factors can shift your target higher or lower:

1. The Lifestyle Choice (Lean vs. Fat)

Your FIRE number is entirely dependent on your spending. A "Lean FIRE" adherent who lives on $30,000 a year only needs $750,000 ($30,000 x 25). Conversely, a "Fat FIRE" enthusiast who wants to spend $200,000 annually will need a staggering $5 million ($200,000 x 25) .

2. Inflation and Cost of Living

The Rule of 25 assumes your expenses remain relatively stable in "today's dollars." However, over a 30- or 40-year retirement, inflation will erode the purchasing power of your money. Most calculators use a default assumption of a 3% average annual inflation rate . If you live in a high-cost-of-living area or expect your expenses to rise (e.g., due to healthcare), you might choose a more conservative multiplier, such as 28x or 30x.

3. Other Retirement Income

If you expect to receive other forms of income in retirement—such as Social Security, a pension, or rental income—you can subtract that amount from your annual expenses before multiplying by 25 . This effectively lowers your FIRE number because your portfolio doesn't have to do all the heavy lifting.

FIRE Number Comparison Table

Monthly Expenses	Annual Expenses	FIRE Number (25x)	Lifestyle Category
$2,000	$24,000	$600,000	Ultra-Lean FIRE
$4,000	$48,000	$1,200,000	Lean/Standard FIRE
$6,000	$72,000	$1,800,000	Standard FIRE
$8,000	$96,000	$2,400,000	Comfortable FIRE
$12,000	$144,000	$3,600,000	Fat FIRE
$20,000	$240,000	$6,000,000	Ultra-Fat FIRE

Frequently Asked Questions: The FIRE Number

Q: Does my FIRE number include my home equity?
A: Generally, no. Your FIRE number should consist of "investable assets"—money in brokerage accounts, IRAs, and 401(k)s that can produce income or be sold to pay for expenses. Unless you plan to sell your home and move to a cheaper one (downsizing), your primary residence doesn't help pay for your groceries .

Q: What if my expenses change after I retire?
A: This is a common concern. Many people find that their expenses drop in retirement because they no longer have commuting costs or work-related wardrobes. Experts often suggest budgeting for 70% of your pre-retirement income as a baseline, assuming some expenses will disappear . However, FIRE followers often prefer to use their actual current expenses to ensure a safety margin.

Q: Is $1.8 million really enough?
A: It depends on your "Post-retirement rate of return." If your investments earn 5% during retirement and inflation is 3%, you are only gaining 2% in real value . The 25x rule is designed to balance these factors, but if you are worried, you can aim for a higher multiplier.

The Importance of Tracking

To accurately calculate your FIRE number, you must be an expert on your own spending. This requires "intense budgeting" . You cannot guess your annual expenses; you must know them. This includes the "hidden" costs like annual car registrations, home maintenance, and the occasional emergency. By mastering your budget now, you ensure that your FIRE number is a solid foundation rather than a shaky estimate.