Most people don't know their financial independence number. They have a vague sense that they need "a lot" saved to retire, or they've heard the generic advice to save 15% of their income. Neither of those tells you anything useful.
Your FI number is a specific dollar amount. Once you hit it, your investment portfolio generates enough passive income to cover your living expenses indefinitely. That's it. That's the finish line.
Here's how to calculate it.
The Foundation: The 4% Rule
The FI number comes from research on sustainable withdrawal rates from investment portfolios. The key study is the Trinity Study from 1998, which looked at 30-year periods in US market history and asked: what withdrawal rate from a diversified stock-and-bond portfolio survived every 30-year period?
The answer was 4%. A portfolio withdrawing 4% of its initial value annually, adjusted for inflation, survived virtually every historical 30-year window including the Great Depression, the 1970s stagflation, and the dot-com crash.
Flip that: if you need your portfolio to last indefinitely, you want to withdraw no more than 4% per year. Which means you need a portfolio equal to 25 times your annual expenses. That's the Rule of 25.
FI number = annual expenses x 25
This is the foundation. For a deeper look at where this formula comes from and how it changes for early retirees, see The Rule of 25: Plan Your Financial Exit. Everything else is calculating how long it takes to get there.
Calculating Your FI Number
Start with your actual monthly expenses. Not your ideal budget. What you actually spend.
If you're not sure, add up your last three months of spending across all categories: rent/mortgage, food, transportation, insurance, subscriptions, entertainment, clothing, everything. Divide by three. That's your real monthly baseline.
Here are three common spending levels and what they mean for FI numbers:
$2,500/month: Annual expenses = $30,000. FI number = $750,000.
$4,000/month: Annual expenses = $48,000. FI number = $1,200,000.
$6,000/month: Annual expenses = $72,000. FI number = $1,800,000.
Notice that the FI number scales directly with spending. This is one of the most important levers in financial independence: every $100/month you permanently reduce your expenses cuts $30,000 from your FI number. That's a meaningful impact.
How Long Does It Take to Get There?
Now you need to know the timeline. This depends on three variables: how much you have saved now, how much you add each month, and what return you assume.
For the return assumption, the US stock market has returned about 10% annually on average over long periods. After inflation (which averages about 3%), the real return is roughly 7%. Use 7% real return for this calculation. It's the most commonly used and most defensible long-term assumption.
Here's the timeline starting from $0 in savings at different monthly savings rates:
Saving $500/month at 7% real return: Time to $750K = 44 years. Time to $1.2M = 51 years.
Saving $2,000/month at 7%: Time to $750K = 24 years. Time to $1.2M = 29 years.
Saving $4,000/month at 7%: Time to $750K = 18 years. Time to $1.2M = 22 years.
Saving $7,000/month at 7%: Time to $750K = 12 years. Time to $1.2M = 16 years.
The numbers jump significantly between $500/month and $2,000/month because compound growth needs a meaningful base to work with. Run your exact timeline with your current savings balance at Stack's free calculator → Getting from $500 to $2,000/month in monthly savings. Whether by increasing income, reducing expenses, or both, is the most impactful thing most people can do for their FI timeline.
The Double Compounding Effect of Reducing Expenses
This is the insight most people miss.
When you permanently reduce your monthly expenses, two things happen simultaneously:
- Your FI number drops (lower expenses mean you need a smaller portfolio)
- Your monthly savings go up (more money available to invest)
Let's quantify this. Say you're currently spending $4,500/month and saving $1,500/month, with a FI number of $1,350,000. You make one change: you reduce a recurring expense by $400/month (downsize your apartment, cut a subscription bundle, eliminate a car payment).
Now you're spending $4,100/month and saving $1,900/month. Your FI number dropped to $1,230,000.
With $1,900/month savings at 7%, time to $1,230,000 from $0: about 26.5 years. With $1,500/month savings at 7%, time to $1,350,000 from $0: about 30 years.
That $400/month reduction saved you 3.5 years. The double compounding effect is real.
Inflation Adjustment
One caveat worth understanding: the 4% rule and the Rule of 25 assume your withdrawals increase with inflation over time. When you hit your FI number and start drawing down, you're not withdrawing exactly 4% every year. You're withdrawing 4% of the initial balance and adjusting that amount for inflation annually.
This is why the 7% real return (nominal return minus inflation) matters. If your portfolio grows at 7% real, it's outpacing inflation by 7% annually, which is more than enough margin to support 4% real withdrawals indefinitely.
If you want to be conservative, use 3.5% as your withdrawal rate and 28 as your multiplier. This gives you more margin, especially if you retire early and need the portfolio to last 40-50 years instead of 30.
Starting Points and Actual Numbers
Here's a practical calculation for three different starting situations:
Starting at 23 with $5,000 saved, spending $2,800/month, saving $800/month: FI number = $840,000. Timeline at 7% real: 35 years. Freedom year: 2061. If savings rate increases to $1,500/month at 27: freedom year moves to 2050.
Starting at 28 with $40,000 saved, spending $4,200/month, saving $2,000/month: FI number = $1,260,000. Timeline at 7% real: 26 years. Freedom year: 2054. Adding $1,200/month side hustle: freedom year moves to 2047.
Starting at 32 with $90,000 saved, spending $5,500/month, saving $3,500/month: FI number = $1,650,000. Timeline at 7% real: 22 years. Freedom year: 2048. Reducing expenses by $600/month: freedom year moves to 2044.
The pattern: for most people in their 20s and early 30s, a combination of modest expense reduction and one meaningful income boost (raise, side hustle, better-paying job) can move the freedom year by 5-10 years.
What to Do With This Number
Calculate your FI number today. Seriously, right now: multiply your monthly expenses by 12, then by 25. Write it down.
Then calculate your timeline with your current savings balance and monthly surplus. Most people find the number is either closer or further than they expected, both are useful information.
If it's closer than you thought: you might be nearly there, or you might realize that increasing your savings rate by 15% gets you there significantly faster than you expected. Often a small improvement has an outsized impact because you're closer to the finish line than the rough estimate suggested.
If it's further than you thought: the calculation tells you exactly what to change. For the full plan on retiring before 40, see FIRE at 35: The Exact Financial Plan. Reduce expenses to lower the FI number and raise the monthly surplus simultaneously. Build a second income stream. Both together.
The FI number isn't a magic number that makes your life perfect. It's a finish line you can actually measure your progress against. That's more useful than a vague goal to "save more."
FAQ
What is a good FI number for someone in their 20s? That depends entirely on your spending. The formula is simple: multiply your annual expenses by 25. If you spend $3,000/month ($36,000/year), your FI number is $900,000. If you spend $4,500/month, it's $1,350,000. There's no universal "good" number, just your number.
What is the Rule of 25 for financial independence? The Rule of 25 says you need 25 times your annual expenses saved in investments to be financially independent. It's the inverse of the 4% safe withdrawal rate from the Trinity Study. At 25x your annual spending, a 4% annual withdrawal covers your expenses indefinitely based on historical market returns.
What is the 4% rule? The 4% rule comes from the Trinity Study, which analyzed historical portfolio survival rates across 30-year periods. It found that withdrawing 4% of your initial portfolio per year, adjusted for inflation, survived nearly every historical 30-year window including major crashes. It's the mathematical basis for the FI number calculation.
How long does it take to reach financial independence? It depends on your savings rate. At 10% savings rate, roughly 40+ years. At 20%, around 30 years. At 30%, about 22 years. At 50%, as few as 15-17 years. The most powerful lever is getting your savings rate up fast, especially early in your career when compound growth has the longest runway.
Can I retire early with $1 million? At a $1 million portfolio and 4% withdrawal rate, you can withdraw $40,000/year indefinitely. Whether that's enough depends on your expenses. If you spend $3,200/month or less ($38,400/year), $1 million technically covers you. Most people retiring early use a more conservative 3.5% rate for a longer time horizon, which means $35,000/year from $1M.
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