Most retirement advice says you need "enough to live on." That's useless. The 4% rule gives you an actual number — a specific portfolio size at which, historically, you could stop working forever without ever running out of money.
It's the formula behind phrases like "I need $1.5 million to retire" or "I need $2 million." Here's exactly how that number is calculated, and what the research actually shows about whether it holds up.
What the 4% Rule Actually Says
The 4% rule states: if you withdraw 4% of your portfolio in year one of retirement, then adjust that amount for inflation each year, your portfolio has a very high probability of lasting 30+ years.
This comes from the Trinity Study (1998), which analyzed every 30-year retirement window in stock market history from 1926 to 1995. A portfolio of 50–75% stocks, 25–50% bonds survived nearly every historical period at a 4% withdrawal rate — including the Great Depression, multiple recessions, and the inflationary 1970s.
The Formula: Your Retirement Number
To find your retirement target, flip the 4% around:
- Decide what annual income you need in retirement
- Multiply that by 25
- That's your target portfolio size
Why 25? Because 1 ÷ 0.04 = 25. If you have 25× your annual expenses invested, you can withdraw 4% indefinitely.
| Annual Income Needed | Retirement Portfolio Target |
|---|---|
| $40,000/year | $1,000,000 |
| $60,000/year | $1,500,000 |
| $80,000/year | $2,000,000 |
| $100,000/year | $2,500,000 |
| $150,000/year | $3,750,000 |
Note that "annual income needed" refers to what you actually spend — not your current salary. Many people find they need 70–80% of their pre-retirement income once they're no longer commuting, saving for retirement, or paying for work-related expenses.
The 4% rule means your money works for you in retirement the same way compound interest worked for you during accumulation. A $1M portfolio earning 7–8% produces $70,000–$80,000 per year in returns. Withdrawing $40,000 (4%) leaves the rest reinvesting — which is how it sustains itself across 30+ years.
Where the 4% Rule Gets Complicated
The original Trinity Study assumed a 30-year retirement — roughly from age 65 to 95. If you retire earlier, the math changes:
- Retire at 65: 4% works well historically
- Retire at 55: 40-year horizon — some researchers suggest 3.5% to be safer
- Retire at 45 (FIRE): 50-year horizon — 3–3.5% is more conservative
The other complication is returns. The original research was based on U.S. equity returns of ~10% annually (historical average). If future returns are lower — say 6–7% — the 4% rule becomes less reliable, especially in the first decade of retirement.
Sequence-of-returns risk is the biggest practical danger: if markets fall sharply in your first 5 years of retirement while you're withdrawing, you deplete capital faster than it can recover. A 2008-style crash in year 2 of retirement is much more dangerous than the same crash in year 15.
Social Security Changes Everything
The 4% rule calculates how much you need from your investment portfolio. But if you have Social Security, a pension, or rental income, those count too — reducing how much your portfolio needs to cover.
Example: If you need $60,000/year and Social Security will provide $20,000/year, your portfolio only needs to cover $40,000/year. That means you need $1,000,000 (not $1,500,000) in invested assets.
How to Use This Number Practically
Think of your retirement target not as a finish line, but as a trajectory milestone. At $500,000 you're halfway to a $1M target. Each year, track how many "years of expenses" you have invested.
When that number hits 25×, you've hit your retirement number — regardless of your age. That's the core idea behind the FIRE (Financial Independence, Retire Early) movement: the 4% rule makes retirement a math problem, not an age problem.
Enter your current numbers and see exactly how many years until you hit your retirement target — and what changes get you there faster.
Calculate Your Retirement Number