What Is a FIRE Number and How Do You Calculate Yours?
Your FIRE number is the exact amount you need invested to retire early and live off returns forever. Here's the formula, real examples, and how to reach it faster.
The 4% Rule: What It Is, How It Works, and Its Real Limitations
The 4% rule is the foundation of FIRE retirement planning. Here's the research behind it, how to apply it correctly, when to use a more conservative rate, and what critics get wrong.
What is the 4% rule?
The 4% rule is the most widely used framework for determining how much you can safely withdraw from your investment portfolio in retirement without running out of money.
The rule states: withdraw 4% of your starting portfolio in year one of retirement, then adjust that amount annually for inflation. Do this, and historical data suggests your portfolio will survive for at least 30 years with a very high probability.
For FIRE planning, the 4% rule converts directly into the 25x formula: if you need $50,000/year, you need $1,250,000 invested ($50,000 ÷ 0.04 = $1,250,000). Multiply annual expenses by 25 to get your FIRE number.
Simple, elegant, and backed by serious academic research — but with important nuances that early retirees need to understand.
The Trinity Study: the research behind the rule
The 4% rule originates from the Trinity Study, published in 1998 by finance professors Philip Cooley, Carl Hubbard, and Daniel Walz at Trinity University.
Their methodology: analyze historical US market data from 1926 to 1995, test every 30-year rolling period, and calculate what percentage of portfolios at various withdrawal rates survived the full 30 years.
Results for a 50% stocks / 50% bonds portfolio:
| Withdrawal Rate | 30-Year Success Rate |
|---|---|
| 3% | 100% |
| 4% | 98% |
| 5% | 80% |
| 6% | 68% |
| 7% | 52% |
| 8% | 36% |
For a 75% stocks / 25% bonds portfolio, the 4% success rate over 30 years was also 98% — confirming the result across different allocations.
The study has since been updated multiple times (most recently by Cooley and Hubbard in 2011) with similar results. The 4% withdrawal rate has held up across the Great Depression, multiple recessions, the 1970s stagflation, Black Monday, the dot-com crash, and the 2008 financial crisis.
How to apply the 4% rule step by step
Applying the rule is straightforward:
Step 1: Determine your annual retirement expenses. Be thorough — include housing, food, transportation, healthcare, travel, insurance, and any debt payments.
Step 2: Divide annual expenses by 0.04 (or multiply by 25) to get your portfolio target. - $40,000/year → $1,000,000 - $60,000/year → $1,500,000 - $80,000/year → $2,000,000
Step 3: In your first year of retirement, withdraw 4% of your starting portfolio.
Step 4: Each subsequent year, increase your withdrawal by the inflation rate.
Example — applying the 4% rule over 5 years: - Starting portfolio: $1,250,000 - Year 1 withdrawal: $50,000 (4%) - Year 2 (3% inflation): $51,500 - Year 3: $53,045 - Year 4: $54,636 - Year 5: $56,275
Your withdrawals increase with inflation, maintaining your purchasing power. The portfolio continues to grow (in good market years) or draws down more slowly (in bad years).
Test your exact scenario with our 4% Rule Calculator →
The 4% rule for early retirement: why it doesn't fully apply
The Trinity Study was designed for 30-year retirements — the standard scenario where someone retires at 65 and the portfolio needs to last until 95.
If you retire at 35, 40, or 45, you need your portfolio to last 50-60 years. The 4% rule's historical success rate drops significantly over these longer horizons.
Estimated success rates for longer retirement periods:
| Retirement Duration | 4% Withdrawal Success Rate |
|---|---|
| 30 years | 95-98% |
| 35 years | 90-95% |
| 40 years | 85-92% |
| 50 years | 75-85% |
Estimates based on historical simulations; actual results vary by market conditions.
For a 40-50 year retirement, most FIRE practitioners use a 3-3.5% withdrawal rate instead: - 3.5% rate → multiply annual expenses by 28.6 - 3% rate → multiply annual expenses by 33.3
The cost: a larger required portfolio. But the benefit: dramatically improved long-term survival probability.
Withdrawal rate comparison: what each rate means in practice
| Withdrawal Rate | FIRE Multiplier | Portfolio for $50k/yr | Portfolio for $70k/yr |
|---|---|---|---|
| 5% | 20x | $1,000,000 | $1,400,000 |
| 4.5% | 22x | $1,111,000 | $1,556,000 |
| 4% | 25x | $1,250,000 | $1,750,000 |
| 3.5% | 28.6x | $1,429,000 | $2,000,000 |
| 3% | 33.3x | $1,667,000 | $2,333,000 |
| 2.5% | 40x | $2,000,000 | $2,800,000 |
The difference between 4% and 3% requires a portfolio that's 33% larger for the same spending level. This is a significant tradeoff — years of additional work vs. dramatically higher portfolio durability.
Criticisms of the 4% rule and what they miss
The 4% rule has attracted criticism, especially in low-yield market environments. Understanding these criticisms — and their limitations — helps you make better decisions.
Criticism 1: "Current valuations make future returns lower" This argument: because current stock valuations (price-to-earnings ratios) are elevated compared to historical averages, future returns will be lower, making the 4% rule too aggressive.
The response: This may be partially true. Researchers like Wade Pfau have suggested 3-3.5% is safer given current conditions. But market timing predictions have historically underperformed simply staying invested. A 3.5% withdrawal rate addresses this concern with a meaningful safety margin.
Criticism 2: "The rule ignores sequence of returns risk" This is valid. A major market decline in years 1-5 of retirement is far more damaging than the same decline later, because you're withdrawing at depressed prices.
The response: The Trinity Study actually accounts for this — their historical data includes starting periods that coincide with major crashes. The solution isn't abandoning the rule but maintaining a 1-2 year cash buffer and considering flexible spending strategies.
Criticism 3: "The rule uses US market data and ignores global portfolios" True — the US stock market has had exceptional returns historically. Other developed markets have had lower long-term returns.
The response: A globally diversified portfolio (US + international stocks) somewhat reduces this risk. Using a conservative withdrawal rate (3.5%) also provides a buffer for lower future returns.
Criticism 4: "Nobody actually spends a fixed inflation-adjusted amount" Also true — real retirees naturally spend less in bad market years and more in good ones.
The response: This actually makes the 4% rule more conservative than reality. Flexible spenders do better than the fixed withdrawal model suggests. If markets drop 30%, most retirees naturally reduce discretionary spending — travel, dining, entertainment — which reduces portfolio drawdown significantly.
Strategies to make the 4% rule more robust
The cash buffer strategy
Keep 1-2 years of expenses in cash or short-term bonds outside your investment portfolio. During market downturns, spend from the cash buffer rather than selling investments at depressed prices. Replenish the buffer when markets recover.
This effectively eliminates the sequence of returns risk for most market downturns.
The flexible spending strategy
Instead of fixed inflation-adjusted withdrawals, allow spending to fluctuate with portfolio performance. A simple rule: if your portfolio falls below your starting value, reduce spending by 10%. If it exceeds your starting value by 20%, allow spending to increase.
Research suggests this flexibility dramatically improves portfolio survival rates — allowing withdrawal rates closer to 4.5-5% with safety rates equivalent to 3.5% fixed.
The "one more year" buffer
Work one additional year beyond your calculated FIRE date. This accomplishes three things: adds more savings, reduces the retirement period by one year, and provides psychological confidence.
Supplemental income in early retirement
Maintaining even small supplemental income in early retirement dramatically reduces portfolio withdrawal pressure. $15,000/year from part-time consulting, rental income, or a passion project reduces a $60,000 spending need to $45,000 — lowering the FIRE number by $375,000 at a 4% withdrawal rate.
What rate should you actually use?
Based on current research and common FIRE planning practices:
Use 4% if: - You're retiring at 60+ with a 30-year horizon - You have flexibility to reduce spending in bad markets - You have supplemental income sources (Social Security, pension, part-time work) - You're comfortable with a small probability of portfolio depletion
Use 3.5% if: - You're retiring in your 40s-50s (30-40 year horizon) - You want a meaningful safety margin - Your spending is relatively fixed without flexibility
Use 3% if: - You're retiring in your 30s or younger (40-50+ year horizon) - Your spending has limited flexibility - You want near-certainty of portfolio survival
Use 4.5-5% if: - You have significant guaranteed income (Social Security, pension) - You'll inherit assets - You have flexible spending and would cut dramatically if needed - Your portfolio is primarily in annuities or other guaranteed instruments
The bottom line on the 4% rule
The 4% rule is not perfect — no planning tool is. But it is well-researched, historically validated, and provides a useful starting framework for retirement planning.
For early retirees, the key adjustment is the withdrawal rate: use 3-3.5% for 40-50 year horizons to account for longer time periods and reduced certainty about long-term returns.
Combined with a cash buffer, flexible spending, and any supplemental income, the 4% rule (or its more conservative variants) remains the most practical framework for determining retirement readiness.
Frequently asked questions
What happens if my portfolio runs out of money under the 4% rule? In the 2-5% of historical scenarios where a 4% withdrawal rate failed over 30 years, portfolios typically ran out in the final 5-10 years. This is manageable with supplemental income, reduced spending, or returning to part-time work. It's a risk, not a certainty.
Should I include Social Security in my 4% rule calculation? Yes — subtract expected SS income from annual expenses before calculating your FIRE number. If you spend $65,000/year and expect $18,000/year from Social Security at 67, calculate on $47,000/year: $47,000 × 25 = $1,175,000 FIRE number (vs. $1,625,000 without SS).
Does the 4% rule work for Roth IRA accounts? Yes — the 4% rule applies to total portfolio value regardless of account type. Roth accounts have the advantage of tax-free withdrawals, which effectively increases the real spending power of each dollar withdrawn.
What is the "4% rule" called in financial planning? It's formally called the "safe withdrawal rate" (SWR) or "sustainable withdrawal rate." The 4% figure is the most commonly cited safe withdrawal rate for 30-year retirement periods based on historical US market data.
Can I use the 4% rule if I have a variable income in retirement? Yes — this actually works in your favor. Variable income (freelancing, part-time work, rental income) reduces how much you need to withdraw from your portfolio, effectively making your actual withdrawal rate lower than 4% in years when outside income is available.