How to Calculate Your FIRE Number in 5 Minutes
The simple formula, real examples, and why you should stress-test the result.
At the core of every FIRE plan sits one deceptively simple question: how much money do I actually need to never work again? That target is your FIRE number, and calculating it is far easier than most people assume.
What Is a FIRE Number?
Your FIRE number is the total amount of invested assets you need to sustain your desired lifestyle indefinitely, without employment income. Once your portfolio reaches that number, the returns it generates cover your annual spending. Think of it as the price tag on your freedom.
The concept rests on the Trinity Study (1998), which found that a diversified portfolio historically survived a 4% annual withdrawal rate over 30-year periods with a very high success rate.
The 25x Rule
If you can safely withdraw 4% of your portfolio each year, you need a portfolio equal to 25 times your annual expenses:
FIRE Number = Annual Expenses × 25
It's about spending, not income
Two people earning the same salary can have wildly different FIRE numbers if one spends significantly less. This is why the FIRE community emphasizes tracking expenses and optimizing your savings rate.
Example Calculations
$750K
$30K/yr spending
$1.25M
$50K/yr spending
$2M
$80K/yr spending
$3M
$120K/yr spending
Notice how a $20,000 reduction in annual spending drops your target by $500,000. Small lifestyle changes compound into enormous differences in how quickly you reach financial independence.
Adjusting for Pensions & Social Security
Most people will eventually receive guaranteed income — state pension, Social Security, or rental income. These reduce the amount your portfolio needs to provide:
Adjusted FIRE Number = (Expenses − Guaranteed Income) × 25
| Scenario | Annual Expenses | Pension Income | FIRE Number |
|---|---|---|---|
| No pension | $50,000 | $0 | $1,250,000 |
| With Social Security | $50,000 | $18,000 | $800,000 |
| With SS + private pension | $50,000 | $28,000 | $550,000 |
Mind the gap
Pensions typically don't start until a specific age. You still need your full FIRE number to bridge the gap between early retirement and when payments begin. Also factor in inflation erosion for pensions that aren't inflation-indexed, and policy risk — benefits could change.
Why Stress-Test with Monte Carlo?
The 25x rule is a great starting point, but real markets don't deliver smooth returns. They crash, surge, and stagnate in unpredictable sequences. The order of returns — especially in the first years of retirement — can make or break your plan. This is sequence-of-returns risk.
A Monte Carlo simulation runs your retirement plan through 1,000 randomized market scenarios. Instead of a single number, you get a probability distribution — for example, "your plan succeeds in 87% of simulated scenarios."
Questions Monte Carlo answers
What happens if a major downturn hits in your first two years?
How much does dropping from 4% to 3.5% withdrawal improve your odds?
What if inflation runs hotter than average for a decade?
How does part-time income in early retirement change the picture?
Key Takeaways
- Your FIRE number is 25× annual expenses, based on a 4% safe withdrawal rate.
- Every dollar cut from spending lowers your target by $25.
- Pensions and Social Security can significantly reduce the portfolio you need, but timing and inflation matter.
- Monte Carlo simulation stress-tests your plan against real-world volatility, giving you a success probability rather than a single-point estimate.
Plug in your real numbers and get a probability-based FIRE plan in minutes.
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