The FIRE number is the portfolio size at which you can stop working and live off your investments indefinitely. For salaried employees with stable income, calculating it is straightforward. For freelancers, business owners, and anyone with variable income, the math needs more care. Here's the complete guide.
What Is a FIRE Number?
FIRE stands for Financial Independence, Retire Early. Your FIRE number is the amount of money you need invested to cover your living expenses forever — without depleting your portfolio.
The concept rests on the 4% Rule, derived from the Trinity Study (1998): historical data shows that a portfolio invested in a mix of stocks and bonds can sustain annual withdrawals of 4% of the initial portfolio value for at least 30 years, even through major market downturns.
The basic formula:
FIRE Number = Annual Expenses ÷ 0.04
Or equivalently:
FIRE Number = Annual Expenses × 25
Example: If your annual expenses are $60,000, your FIRE number is $60,000 × 25 = $1,500,000.
Calculating FIRE Number with Variable Income
Variable income changes the equation in a specific way: your expenses are what matters, not your income. Whether you earned $80,000 or $120,000 last year is less relevant than what you spent.
Step 1: Calculate your true annual expenses
Don't use a single month × 12. Use 12–24 months of actual spending data.
Variable income often produces variable spending — a good income year might mean more restaurant meals, upgrades, or discretionary purchases. Use your average annual spending, not a best-case estimate.
Include:
- Fixed costs (rent/mortgage, insurance, subscriptions, loan payments)
- Variable necessities (groceries, utilities, transportation)
- Irregular but predictable costs (annual insurance premiums, car maintenance, dental)
- Discretionary spending (travel, entertainment, dining out)
Common expense categories to review:
- Housing (rent or mortgage, property taxes, insurance, maintenance)
- Food and groceries
- Transportation (car payment, insurance, fuel, public transit)
- Healthcare and insurance premiums
- Utilities and internet
- Entertainment and hobbies
- Travel
- Personal care
- Gifts and miscellaneous
Step 2: Adjust for expected changes in retirement
Your expenses in early retirement may differ from today:
- Lower: No commuting costs, no work clothing, possibly lower housing if you move
- Higher: More travel and leisure, potentially higher healthcare costs if retiring before 65 (especially in the US without employer-sponsored health insurance)
Step 3: Apply the formula
FIRE Number = Adjusted Annual Expenses × 25
Use DevZone's Retirement/FIRE Calculator to model different scenarios — vary your annual expenses, expected return rate, and withdrawal rate to see how your FIRE number changes.
The 4% Rule: Assumptions and Limitations
The 4% rule is a starting point, not a guarantee. Understanding its assumptions helps you apply it intelligently.
It assumes:
- A 30-year retirement (not suitable for retiring at 35 and living to 90)
- A 50/50 to 75/25 stock/bond mix
- Historical US market returns (which may not repeat)
- No other income sources in retirement
Adjustments to consider:
For longer retirements (30+ years), many FIRE practitioners use 3–3.5% instead of 4%. This increases the FIRE number:
$60,000 ÷ 0.035 = $1,714,286 (at 3.5% rule)
$60,000 ÷ 0.03 = $2,000,000 (at 3% rule)
For multiple income sources in retirement (Social Security, part-time work, rental income), reduce your portfolio withdrawal requirement accordingly:
Annual Expenses: $60,000
Expected Social Security: $15,000
Portfolio Withdrawal Needed: $45,000
FIRE Number: $45,000 × 25 = $1,125,000
Variable Income and Saving Rate
With variable income, your saving rate fluctuates — which makes projecting time to FIRE harder. Two approaches:
Base expenses target: Set your FIRE number based on your average annual expenses. In high-income years, invest the surplus. In low-income years, draw on cash reserves rather than your investment portfolio. Maintain a 6–12 month emergency fund to bridge lean periods.
Conservative income baseline: Project your FIRE timeline using your lower-range income (say, the 25th percentile of your last 5 years' earnings). This gives a conservative but more reliable target.
How Long Will It Take?
Saving rate is the biggest lever. The higher the percentage of your income you invest, the faster you reach FIRE.
| Saving Rate | Years to FIRE (approximate) |
|---|---|
| 10% | 40+ years |
| 25% | 32 years |
| 50% | 17 years |
| 65% | 11 years |
| 75% | 7 years |
These numbers assume 7% average annual investment returns and starting from zero.
FAQ
What if my expenses change dramatically after I retire?
Build in a buffer. Many FIRE practitioners target a number that would cover 110–120% of their expected expenses, giving room for lifestyle drift or unexpected costs.
Should I include my home equity in my FIRE number?
Only if you plan to access it — by downsizing, renting, or taking a reverse mortgage. A paid-off home reduces your housing expenses in retirement, which indirectly improves your FIRE number by lowering annual expenses.
Does the 4% rule work outside the US?
The Trinity Study used US market returns. International evidence is mixed — some countries have seen periods of sustained underperformance relative to US historical averages. International diversification in your portfolio (global index funds) helps mitigate this.
What about taxes in retirement?
Withdrawals from traditional 401(k) or IRA accounts are taxed as ordinary income. Withdrawals from Roth accounts are tax-free. The FIRE number should account for taxes on withdrawals — your gross withdrawal target should cover both your expenses and your tax liability.