How to Calculate Your FIRE Number (With Real Examples)
Your FIRE number is how much you need invested to make work optional. Here is the simple formula, why the 4% rule is debated, and worked examples in any currency.
Key takeaways
- Your FIRE number is roughly your annual spending times 25, the inverse of the 4% rule.
- The 4% rule came from Bengen (1994) and the Trinity Study (1998). It is a strong benchmark, not a guarantee.
- The safe withdrawal rate is genuinely debated today: a defensible range is about 3.7% to 4.7%.
- That range matters. A $40,000 lifestyle needs anywhere from roughly $851k to $1.14M depending on the rate you trust.
- Coast FIRE means you have invested enough that compounding alone reaches your number, so you can stop adding.
FIRE stands for Financial Independence, Retire Early. Your FIRE number is the single figure at the centre of it: the amount you need invested for work to become optional. The good news is that the core maths fits on a napkin. The interesting part is the debate over one percentage point.
What is a FIRE number?
Your FIRE number is the size of investment portfolio that can cover your annual spending more or less indefinitely, so you no longer depend on a salary. Reach it and work becomes a choice. It is defined by how much you spend each year, not how much you earn.
That last point trips people up. Two people on the same salary can have very different FIRE numbers, because the one who spends less needs a smaller portfolio to sustain their life. Your spending sets the target.
The formula: expenses × 25
The FIRE number is your expected annual expenses multiplied by 25. If you spend $40,000 a year, your FIRE number is about $1,000,000. This is the Rule of 25, and it is simply the inverse of the 4% rule: withdrawing 4% a year is the same as needing 25 times your expenses.
The 4% figure comes from two pieces of research. Financial adviser William Bengen showed in 1994 that a retiree could withdraw 4% in year one, adjust for inflation after that, and not run out over 30 years. The Trinity Study in 1998 confirmed the idea independently and gave it the “4% rule” name. It has been the planning benchmark ever since.
Is the 4% rule still safe?
It is genuinely debated. Morningstar’s recent forward-looking analysis points to a safer starting rate near 3.9%, while Bengen himself now argues that broader diversification supports around 4.7%. A sensible planning range today is roughly 3.7% to 4.7%, and which end you choose changes your number a lot.
This is the part most calculators hide. The withdrawal rate is an assumption, not a fact, and reasonable experts disagree. For a $40,000 lifestyle, the choice swings your target by nearly $300,000:
| Withdrawal rate | You need | For $40k / year |
|---|---|---|
| 4.7% (Bengen, updated) | about 21× | about $851,000 |
| 4.0% (classic Rule of 25) | 25× | $1,000,000 |
| 3.9% (Morningstar, recent) | about 26× | about $1,026,000 |
| 3.5% (cautious) | about 29× | about $1,143,000 |
There is no single correct answer. A common middle path is to plan around 4%, then keep some flexibility to spend a little less in bad market years.
Lean, Fat, Coast, and Barista FIRE
FIRE is not one target. Lean FIRE is early retirement on a frugal budget, Fat FIRE is a comfortable one, Coast FIRE means compounding will get you there without adding more, and Barista FIRE covers most costs from investments while part-time work fills the gap. The dollar thresholds are informal community conventions, not official rules.
- Lean FIRE. A deliberately frugal early retirement, often built on a smaller portfolio in the region of $500k to $1M.
- Fat FIRE. A comfortable or luxurious standard, usually needing $2.5M or more.
- Coast FIRE. You have invested enough that growth alone reaches your number by retirement age. You stop contributing and just cover today’s costs.
- Barista FIRE. Investments cover most expenses and part-time work covers the rest, often valued for benefits like health cover.
A worked example
Take someone who spends $40,000 a year. At the classic 4% rule their FIRE number is $1,000,000. If they are 30 and expect a 5% real return, they would only need about $181,000 invested today to Coast FIRE to that million by 65. A 45-year-old needs roughly double, because there is less time to compound.
The lesson is the power of time. Money invested in your twenties and thirties does most of the heavy lifting through compounding, which is why starting early beats saving hard later. If investing itself is new to you, the finance app’s guide to how to start investing is a good primer.
Run your own number
Estimate your annual spending, multiply by 25 for a baseline, then test a more cautious 28 to 29 times to see the range. A calculator makes the what-ifs quick, especially for Coast FIRE, where the return assumption matters so much.
Scroll: Personal Finance has a free FIRE number calculator that works in your currency and lets you flex the withdrawal rate, plus one-minute lessons on the investing and tax ideas behind it. Numbers like these are also easier to trust once you understand the biases that distort money decisions.
Sources
Frequently asked questions
How do I calculate my FIRE number?
Multiply your expected annual expenses in retirement by 25. If you expect to spend 40,000 a year, your FIRE number is about 1,000,000. This comes from the 4% rule: 25 times expenses is the same as withdrawing 4% a year.
Is the 4% rule still safe in 2026?
It is debated. Morningstar’s recent forward-looking work suggests a starting rate closer to 3.9% for a high chance of success, while William Bengen, who created the rule, now argues broader diversification supports around 4.7%. A defensible planning range is roughly 3.7% to 4.7%.
What is Coast FIRE?
Coast FIRE is when you already have enough invested that compound growth alone will reach your full FIRE number by traditional retirement age. You still cover your current expenses, but you no longer need to add to retirement savings.
How much do I need to retire early on 40,000 a year?
Using the 4% rule, about 1,000,000. Using a more cautious 3.5% rate, about 1.14M. Using Bengen’s updated 4.7%, about 851,000. The right figure depends on how conservative you want to be.
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