For years, I saved and invested without knowing whether I actually needed $1 million, $2 million, or $5 million to retire. Every month, I'd contribute to my 401(k), pick up a few stocks, and trust that things would work out. No target. No finish line. Just a vague assumption that someday the balance would be enough.
Then in 2018, I discovered a formula backed by decades of research with a 95% historical success rate. The 4% rule gave me the mathematical clarity I desperately needed as an engineer, a single calculation that told me exactly when I could stop working. That's the difference between investing toward a number and just investing, which I'd been doing. Let's break down calculating your personal FIRE number, and why flexibility matters more than rigid adherence.
What Is the 4% Rule and Why It Works
The 4% rule gives you the one thing most retirement advice never does: the math for how big a portfolio you need before you can stop working. The rule itself is one line: withdraw 4% of your portfolio in your first year of retirement, adjust that dollar amount for inflation each year, and it should last at least 30 years.
Flip it around, and you get your target. Multiply your annual expenses by 25 to get your FIRE number, the portfolio size required for retirement. If you spend $100,000 annually, you need $2.5 million ($100,000 × 25). Withdraw 4% of that $2.5 million, and you get your $100,000 to live on each year!
One critical detail that trips people up: the 4% applies to your initial portfolio value, not to whatever the portfolio is worth each year. So if you retire with $2.5 million and withdraw $100,000 in year one, year two you'd withdraw $100,000 plus inflation, say $103,000 at 3%, regardless of whether your portfolio grew to $2.8 million or dropped to $2.2 million.
This isn't some made-up principle. Financial planner William Bengen published his research in 1994 analyzing historical market data from 1926 to 1992. He discovered that retirees could safely withdraw 4% of their initial portfolio value, adjusted annually for inflation, and never run out of money over a 30-year retirement, even during periods like the Great Depression and the 1970s stagflation. The Trinity Study later validated these findings, showing a 95% success rate with a 50/50 stock-to-bond mix.
Expenses times 25 equals freedom. My engineering brain grabbed onto that instantly, the same way it had with compound interest growth. Finally, I had a concrete target instead of vague advice like “save for retirement and figure it out later.”
Calculating Your Personal FIRE Number
Let's get practical. Every year without fail, I sit down with my wife and review our family expenses. We project forward, making adjustments for inflation. If we need $100,000 today but inflation is running at 3%, next year's target is closer to $103,000, which means our FIRE number shifts from $2.5M to $2.575M.
It cuts the other way too: trim $4,000 of spending in that review and the target drops from $2.5M to $2.4M, which is $100,000 we no longer need to save ($4,000 × 25). This annual ritual keeps us honest about our spending and ensures our target aligns with reality. I use Monarch to categorize expenses, see exactly where our money goes, and monitor our overall net worth across investment accounts.
Once you know that number, the same 25x math drives the other milestones, from Coast FIRE to your time to FIRE.
Free tools like cFIREsim and FI Calc let you model various withdrawal rates against every historical market period since 1871. I ran dozens of simulations when first calculating my number. Plug in your portfolio size, asset allocation, and withdrawal rate, and these tools show you how many historical periods your plan would have survived.
Adjusting the 4% Rule for Your Situation
Here's where it gets interesting. Bengen's math assumes your portfolio covers every dollar you spend, but what if you have rental income or a pension? Your situation dramatically affects the calculation.
I don't expect my entire living expenses to be covered by my stock portfolio, as I also have rental properties generating income. If my annual expenses are $100,000 and I receive $10,000 cash flow from rental properties, my portfolio only needs to cover $90,000. That means my target is $2.25 million ($90,000 × 25) instead of $2.5 million, a difference of $250,000!
The same goes for any income that arrives from outside the portfolio: a pension, Social Security, a side business, part-time work.
Every account counts toward the number: 401(k), IRA, brokerage, and the one most people forget, the HSA. I treat mine as part of my retirement portfolio, since HSA withdrawals in retirement work like a traditional IRA once you turn 65. And I count every liquid asset regardless of type, stocks and crypto investments alike; the rentals sit outside the portfolio and show up as income instead.
Which account you draw from first, and how to tap a 401(k) or IRA before 59½ without penalties, matters in early retirement and regular retirement alike, so settle your withdrawal strategy before you need it.
Here's something most 4% rule explanations skip: taxes. If most of your portfolio is in a traditional 401(k) or IRA, withdrawals are taxed as ordinary income, so treat that tax as one more line in your annual expenses before you multiply by 25; how big a line depends on your tax bracket. The good news: without a paycheck, most people land in a lower bracket than they worked in, and with the right mix of accounts, withdrawals can add up to a zero-tax retirement. And if you're retiring before 65, don't forget healthcare costs: without employer coverage or Medicare, health insurance premiums can easily add $15,000–$25,000 per year for a family.
What about retiring decades before a traditional retirement age, as I plan to? Bengen's original research covered 30-year retirements, but I plan for 40-50 years, and I still plan on 4%. His 2025 research backs that up: with a diversified portfolio, the worst-case rate is 4.7% for 30 years and 4.2% even for 50. If a longer retirement worries you, don't shrink the rate. Get to your number sooner by saving and investing more, and keep some income flowing in the early years of retirement, from real estate investments or part-time work, so a bad first decade never forces you to sell at the bottom.
Your asset allocation matters enormously. I don't own bonds at all. In the portfolio the 4% rule is applied to, low-cost index funds make up 90-95%, with the remaining 5-10% in crypto; the rental properties sit alongside it as an income stream, not a slice of the pie. In retirement, I'll keep a year or two of expenses in cash instead; that's my buffer against sequence-of-returns risk, a bad market in the first few years, which does more damage than a bad market later. Different allocations yield varying success rates with the 4% rule. Understanding your risk tolerance and asset allocation helps you build the mix that lets 4% hold; the thing to adjust is your portfolio, not the rate.
When and How to Be Flexible with the 4% Rule
The 4% rule is guidance, not a rigid mandate! Bengen himself emphasized that retirees should adjust their withdrawals based on market conditions.
I've had moments of serious doubt, especially during market downturns. When the stock market crashed in 2020 or inflation spiked in 2022, I was still working and still buying, and wondered whether the whole plan was a mistake. What settled me was the flexibility principle: when I do start drawing on the portfolio, a bad year won't mean rigidly withdrawing 4%. It will mean cutting back on discretionary spending and using strategies to handle market volatility.
If we hit a year where the market drops 25%, I won't withdraw my full 4% and deplete my portfolio faster. We'll cut back on dining out, skip the big vacation, eliminate subscription services, and reduce other non-essential spending. That might mean living on $75,000 to $80,000 instead of $100,000 that year. This flexibility dramatically improves your long-term success rate by avoiding the sale of assets at depressed prices, crucial for navigating different stages of financial independence.
In a normal or good year, you spend the full plan. The vacation and the dinners out are part of the $100,000, not a reward for a bull market; they only get trimmed in a bad year, and they come back as soon as the market does. The growth in good years builds the cushion that makes those cuts rare. The 4% plan is the ceiling: you flex below it when you have to, back up to it when you don't, and never past it.
Common Misconceptions and Current Debates
Despite this built-in flexibility, you'll hear “the 4% rule is outdated!” everywhere, mainly from financial advisors. Critics argue that with today's lower expected returns and higher valuations, we should use 3% or 3.5% instead. I don't buy it. The rule already survived the Great Depression and 1970s stagflation, and the newest research says 4% is too cautious, not too bold.
Bengen himself has moved in the opposite direction: his 2025 update, based on a more diversified portfolio, puts the safe rate at 4.7%. So even the man who wrote the rule thinks 4% is conservative! Dismissing the entire concept because researchers are still arguing about the decimal is like throwing out your GPS because the traffic patterns changed.
The argument for lower expected returns deserves a look. If returns average 7–8% instead of the historical ~10% nominal average since 1928, does that break the 4% rule? No. The rule wasn't built on average years; it's the rate that survived the worst starting years on record, when returns ran far below 7–8% for a decade or more. Lower returns mean it takes longer to reach your number, not that you can withdraw less once you're there.
Through conversations with people in the FIRE community, I've seen how differently everyone applies the 4% rule. Some opt for aggressive 95% stock allocations because they're comfortable with volatility and plan to work part-time in retirement. Others obsess over every market fluctuation until they realize the 4% rule gives you a research-backed starting point, not a crystal ball. Their anxiety disappears once they accept uncertainty and build flexibility into their plan.
Your Personal 4% Rule Strategy
The 4% rule turned my unfocused saving into a plan with clear milestones. Start today by tracking your expenses and calculating your personal FIRE number. The gap between where you are and that number is your roadmap.
The whole experiment in one line: plan on 4%, save and invest more to get there sooner, and be ready to spend less in a bad year. Then make it yours: subtract any rental, pension, or Social Security income before you multiply, check whether Bengen's 50/50 allocation matches your risk tolerance, and know which of the various FIRE strategies you're aiming at.
The 4% rule isn't magic, but it has held through every crash on record. It provides an evidence-based framework backed by three decades of academic research. Apply it intelligently, remain flexible during market turbulence, diversify your income sources by building passive income, and you'll have a solid foundation for financial independence. Calculate your FIRE number today, then compare it with your net worth to see how far you are from it.
What You Need to Remember
- Multiply your annual expenses by 25 to calculate your FIRE number.
- Plan on 4% for any retirement length; Bengen's 2025 update puts the safe rate above it, so 4% is conservative, not bold.
- Subtract rental income, pension, Social Security, or side-hustle earnings from your expenses before you multiply by 25.
- During market downturns, cut spending instead of rigidly withdrawing the full 4%, so you're not selling assets at depressed prices.
- Recalculate your FIRE number annually as expenses and life circumstances change.
Questions I Always Get
Does the 4% rule still work if I'm retiring before 40? Yes. Bengen's 2025 research shows it holds even for a 50-year retirement with a diversified portfolio. Save more to get there sooner, be ready to spend less in a bad year, and keep part-time work options open during your first decade, since the early years are when a bad market does the most damage.
How do I handle the 4% rule if I have a pension or Social Security? Subtract guaranteed income from your annual expenses before you multiply by 25. If you need $100,000 annually and expect $25,000 from Social Security, your portfolio only needs to cover $75,000, meaning you need $1.875 million instead of $2.5 million. If that income starts years after you retire, your portfolio covers the full $100,000 until it does, so the smaller target applies only from that point. Guaranteed income dramatically reduces the required portfolio size and can move your FIRE date up by years.
Do I subtract dividends from my expenses like rental income? No. Dividends come from the portfolio itself. When you withdraw $100,000 a year from your $2.5 million, some of it comes from dividends and the rest from selling shares; the dividends are included in the $100,000, not on top of it, so subtracting them would count the same money twice. Rent, a pension, and Social Security are different: they're paid by something other than your portfolio, which is why they reduce your number. I don't even think about dividends: I hold total-market and growth index funds with dividends reinvested, and the 4% covers everything.
Does the 4% rule work with an aggressive all-stock portfolio? Bengen's original research used a 50/50 stock-bond allocation. A 100% stock portfolio increases both potential returns and volatility, and it has historically supported higher withdrawal rates in strong markets. The tradeoff is larger drawdowns during crashes, which can be devastating if they hit in your first few years of retirement. It’s close to the no-bonds mix I hold today (index funds plus a small crypto slice), and I'll add a cash buffer for the early years once I stop working. Understand your risk tolerance before you follow, because the first bad decade tests your nerve, not the math.
Does the 4% rule account for taxes on retirement withdrawals? Not on its own; taxes are an expense you add. Withdrawals from traditional 401(k)s and IRAs are taxed as ordinary income, so if you need $100,000 to live on and expect $20,000 in tax on those withdrawals, your annual expenses are $120,000, and your number is $3 million, not $2.5 million. Having a mix of pre-tax, Roth, and taxable accounts gives you flexibility to manage your tax burden in retirement and keep more of each withdrawal.