How to Calculate Your Retirement Number: The 4% Rule Explained

Most people think about retirement as "someday I'll have enough." But "enough" is a specific number, and most people have never calculated it. When they finally do, the number is almost always larger than they expected.

Your retirement number is the portfolio size that can sustain your desired lifestyle indefinitely — without running out of money before you run out of time. Getting to a specific number transforms vague anxiety into a concrete savings target.

The 4% Rule: The Foundation of Retirement Math

The 4% rule emerged from the "Trinity Study" (1998) by three professors at Trinity University. They analyzed historical stock and bond returns from 1926 to 1995 and found that a portfolio of 50–75% stocks could support annual withdrawals of 4% of the initial portfolio value, inflation-adjusted, for at least 30 years — with a success rate of 95%+.

The formula is disarmingly simple:

Examples:

• Spend $40,000/year → need $1,000,000
• Spend $60,000/year → need $1,500,000
• Spend $80,000/year → need $2,000,000
• Spend $100,000/year → need $2,500,000

What Annual Expenses Should You Use?

The most common mistake is using current pre-tax income instead of actual spending. Your retirement number should be based on what you actually plan to spend in retirement — which may be more or less than you spend today.

For most people, retirement spending is 80–100% of pre-retirement spending, not the often-cited 70%. Healthcare alone can add $10,000–$15,000/year per person before Medicare eligibility.

Adjusting for Inflation

The 4% rule already builds in inflation — the research assumed you'd increase withdrawals by inflation each year. But there's an important nuance: you should calculate your annual expenses in today's dollars, not future dollars.

If you're 35 and plan to retire at 65, your annual expenses of $60,000 today will feel like $145,000 in 30 years at 3% inflation. But your retirement number should still be calculated as $60,000 × 25 = $1,500,000 in today's dollars — because you'll also target a portfolio that has grown with inflation.

When the 4% Rule Needs Adjustment

The original study assumed a 30-year retirement. If you retire at 45 and live to 90, you need a 45-year runway. The 4% rule becomes less reliable over longer time horizons.

The Social Security Factor

Social Security reduces how much portfolio income you need. If you expect $24,000/year from Social Security and need $60,000/year total, your portfolio only needs to generate $36,000/year:

Portfolio needed: ($60,000 − $24,000) × 25 = $36,000 × 25 = $900,000 (instead of $1,500,000)

This is a significant difference. Include any pension income, rental income, or part-time work income in the same way to reduce your required portfolio size.

Calculating How Long to Reach Your Number

Once you have your target, you can work backwards to calculate how long it'll take given your current savings rate and expected returns. The rule of 72 is useful here: at 7% returns, money doubles every ~10 years.

$100,000 invested today at 7% average return:

• After 10 years: ~$197,000
• After 20 years: ~$387,000
• After 30 years: ~$761,000

If you also contribute $12,000/year to that portfolio, after 30 years you'd have approximately $1,340,000 — enough to support $53,600/year in retirement spending.

The Bottom Line

Most people will need between $1,000,000 and $2,500,000 to retire comfortably — a range that varies enormously based on actual spending habits. The single most useful thing you can do today is calculate your specific number and check whether you're on track to reach it.

If you're significantly short, you still have options: save more, spend less in retirement, retire later, or find part-time income in retirement. But you can only make those decisions with a real number in front of you.