If you spend $45,000 a year, the classic 4% rule says you reach financial independence when your investments hit $1,125,000. That’s your FIRE number: annual spending ÷ safe withdrawal rate = $45,000 ÷ 0.04. At that point, in theory, you can withdraw 4% of your portfolio every year and it should keep funding your life.
This article walks through where that number comes from, how long it takes you to reach it, and — just as important — where the rule gets fragile.
What is the 4% rule?
The 4% rule comes from research by financial planner William Bengen in 1994, later expanded by the Trinity Study (1998). Looking at historical U.S. stock and bond returns, they asked: what’s the highest withdrawal rate that would have let a portfolio last at least 30 years in every historical period they tested? The answer, historically, was about 4%.
That one number became the shorthand for the FIRE (Financial Independence, Retire Early) movement. Withdrawing 4% each year is roughly the same as saying your portfolio needs to be 25 times your annual spending — because 1 ÷ 0.04 = 25.
How to calculate your FIRE number
The formula is deliberately simple:
FIRE number = annual spending in retirement ÷ safe withdrawal rate
| Annual spending | At 4% (25×) | At 3% (33×) |
|---|---|---|
| $30,000 | $750,000 | $1,000,000 |
| $45,000 | $1,125,000 | $1,500,000 |
| $60,000 | $1,500,000 | $2,000,000 |
| $80,000 | $2,000,000 | $2,667,000 |
A lower withdrawal rate (3%) is more conservative — it means a bigger target but a higher probability your money lasts. There’s no single “right” rate; it’s a risk dial you choose.
The second question: how long until you get there?
Your FIRE number is only half the picture. The other half is how many years of investing it takes to reach it. This depends on:
- how much you’ve already invested,
- how much you save each year,
- and the return your portfolio earns (compounding).
This part needs a simulation, not a single formula, because your balance compounds and grows from contributions at the same time. As an example, with $50,000 already invested, saving $35,000 a year, a 7% expected return, and a $45,000 spending level (FIRE number $1,125,000), you’d reach financial independence in roughly 16 years — at age 46 if you start at 30.
Our FIRE Calculator runs exactly this month-by-month projection for your own numbers: it shows your FIRE number, the years to financial independence, your projected FI age, and the monthly spending your portfolio could support.
Where the 4% rule gets fragile
The 4% rule is a planning tool, not a guarantee. A few things can break it:
- Sequence-of-returns risk. If the market drops right after you retire, withdrawing 4% from a shrunken portfolio early is the scenario that historically hurt most. This is why many FIRE planners use 3–3.5%.
- A 30-year assumption. Bengen’s research looked at 30-year periods. Retire at 45 and your money may need to last 50+ years — that calls for a more conservative rate.
- Taxes and fees. The 4% is typically the gross withdrawal; taxes and investment fees come out of it. If you pay 0.5% in fees, your effective withdrawal cost is higher. (See how much investment fees cost.)
- Inflation. A fixed $45,000 buys less in 20 years. Most plans assume withdrawals rise with inflation, which makes the math harder — our inflation calculator shows what that does to purchasing power.
The takeaway
- Your FIRE number = annual spending ÷ withdrawal rate. At 4%, that’s 25× your spending.
- The timeline depends on your savings rate and compounding. That’s where the FIRE Calculator does the heavy lifting.
- Treat 4% as a starting point, not a promise. The lower your withdrawal rate and the more flexible your spending, the safer your plan.
- This is educational math, not financial advice. A withdrawal rate that works in history may not fit your personal situation, health costs, or market future.
Run your own numbers on the FIRE Calculator — current portfolio, annual savings, spending level, and it returns your FIRE number and years to financial independence. For the retirement math more generally, also see the Retirement Calculator.