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, the 4% rule provides a historical planning framework for withdrawals; it does not guarantee that the portfolio will fund every future retirement path.
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 historical withdrawal research by William Bengen and later work commonly associated with the Trinity Study. The research tested inflation-adjusted withdrawals across historical U.S. market periods; 4% became a widely used starting point for a roughly 30-year horizon, not a promise for future markets. Source: Stanford-hosted review of Bengen / Trinity research.
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 scenario such as 3% requires a larger target and can be useful for stress-testing a longer retirement. It is not guaranteed to be sustainable, and there is no single official “right” rate.
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 modeled 7% annual 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 a lower withdrawal-rate scenario such as 3%–3.5% is worth stress-testing for a longer horizon.
- 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.
Sources & assumptions
- 4% rule: historical withdrawal-rate research associated with Bengen and the Trinity Study; historical outcomes do not guarantee future results. Research review.
- Return assumptions: examples such as 7% are model inputs, not forecasts. The SEC's Investor.gov compound-interest calculator likewise treats the interest rate as an estimated input and allows users to test a variance range. Source: Investor.gov.
- Inflation: use BLS CPI data for historical consumer-price changes rather than assuming any single future inflation rate. Source: BLS.
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.