How to Calculate Your FI Number
Financial Independence Writer
Your FI number is the total amount of invested wealth needed for your investment returns to cover your annual expenses indefinitely. It's the single most useful number in financial independence planning — everything else (savings rate, timeline, asset allocation) gets built around it.
The core idea: Once you know your number, "financial independence" stops being a vague goal and becomes a specific, trackable target.
The 25x Rule
The standard method multiplies your annual expenses by 25 — the inverse of the 4% withdrawal rate. This comes from historical research on how much a diversified portfolio can safely support in annual withdrawals without running out over a long retirement.
The formula: FI Number = Annual Expenses × 25
Quick example: Annual expenses of $50,000 → FI Number = $50,000 × 25 = $1,250,000.
Why Expenses, Not Income, Drive the Number
Two people earning the same salary can have very different FI numbers, because the calculation is based entirely on spending, not earning. Someone spending $30,000/year needs $750,000. Someone spending $80,000/year on the same income needs $2,000,000 — over 2.5x more, purely from lifestyle choices.
How Savings Rate Determines Your Timeline
Your savings rate is the percentage of take-home income you save and invest each year. It's the biggest lever on how long reaching your FI number actually takes — far more than most people expect.
| Savings Rate | Approx. Years to FI |
|---|---|
| 10% | ~51 years |
| 25% | ~32 years |
| 50% | ~17 years |
| 70% | ~9 years |
These are rough estimates assuming a typical long-term real (inflation-adjusted) investment return, and they ignore any starting portfolio balance — someone who already has savings invested will reach their number faster than these numbers suggest.
A Worked Example
Someone earning $80,000/year after tax, spending $50,000/year, is saving $30,000/year — a 37.5% savings rate. Their FI number is $1,250,000 (from $50,000 × 25). At that savings rate, they're on a path to financial independence in roughly 20-22 years, assuming consistent contributions and typical long-term market returns.
Adjusting the Number for Your Situation
The 25x rule is a starting point, not a fixed law. Someone planning a very long retirement (40+ years, common for early retirees) sometimes uses a more conservative multiple, like 28-30x, to add a safety margin. Someone with a paid-off house or a pension covering part of their expenses can often use a lower multiple, since not all expenses need to come from the portfolio.
Key Takeaway: Your FI number is annual expenses × 25 as a starting estimate, adjusted for your specific timeline and situation. Savings rate — not income — is what actually controls how fast you get there. Ready to map out your own plan? See our Financial Independence learning path.
Frequently Asked Questions
Is 25x always the right multiple to use?
It's a reasonable starting point based on historical research, but very early retirees planning a 40+ year retirement often use a higher multiple (28-30x) for extra safety margin.
Does my FI number include a paid-off mortgage?
It should be based on your actual expected expenses in retirement — if your mortgage will be paid off, your future annual expenses (and therefore your FI number) will be lower than your current spending.
What if my income and expenses change over time?
Recalculating periodically is normal — your FI number isn't fixed once and forgotten, it shifts as your actual spending and life circumstances change.