William Bengen Built the Retirement Simulation in 1994. His 4% Answer Still Sets the Standard.
The median retirement savings for Americans between ages 55 and 64 is $185,000, according to the Federal Reserve's 2022 Survey of Consumer Finances. The most widely cited framework for deciding how much a retirement portfolio can safely provide each year says that $185,000 supports approximately $7,400 in annual withdrawals. Against an average annual cost of living in the United States above $60,000, those numbers do not appear to reconcile. Understanding why they can align anyway, and what the standard framework actually measures, is what retirement calculators are built to show.
William Bengen was a financial planner in El Cajon, California when he spent much of 1993 building spreadsheets from historical market data. His question was specific: what is the highest annual withdrawal rate a retiree could have used, in the worst possible historical starting year, and still not run out of money over 30 years? He published his answer in the Journal of Financial Planning in October 1994. The rate was 4.15 percent, rounded down to 4 percent in the publication.
Bengen's research covered every 30-year retirement period beginning from 1926. The worst starting years were the late 1960s and early 1970s, a period of high inflation and poor stock market returns. A retiree who withdrew 4 percent of their initial portfolio in year one, then adjusted that dollar amount upward for inflation each year, never depleted a 50/50 stock and bond portfolio within 30 years, even starting in those worst-case years. Four percent was not the average sustainable withdrawal rate across all historical scenarios. It was the floor: the rate that worked even in the worst case the historical data contained.
What the 4% Rule Actually Measures
The 4 percent rule is not a universal guarantee. It is derived from a specific asset allocation (50 percent U. S. large-cap stocks, 50 percent intermediate-term government bonds), a specific time horizon (30 years), and historical U. S. market data. Bengen himself later revised his estimate upward to 4.5 percent and then 4.7 percent after including small-cap stocks in the analysis and extending the dataset.
In 1998, researchers Philip Cooley, Carl Hubbard, and Daniel Walz at Trinity University published an independent analysis that became known as the Trinity Study. It examined portfolio survival rates for various withdrawal rates across the same historical data and produced probability-based conclusions rather than a single safe rate. For a 50/50 portfolio with a 4 percent withdrawal rate over 30 years, the Trinity Study found a 95 percent historical success rate. The 4 percent rate and the Trinity Study together established the framework that most retirement calculators now use.
The 25x rule follows directly from the 4 percent withdrawal rate as an inversion. If you can safely withdraw 4 percent per year, you need 25 times your annual expenses in total savings (because 1 divided by 0.04 equals 25). If you plan to spend $60,000 per year, the 25x rule says you need $1.5 million in savings, minus any Social Security or pension income that covers part of that spending.
How Retirement Calculators Use These Inputs
A retirement calculator's job is to model multiple inputs together and project outcomes over time. The inputs include current savings, expected monthly contributions, assumed investment return rate, planned retirement age, expected annual spending in retirement, and Social Security or other income that will offset portfolio withdrawals.
The calculation answers several related questions simultaneously. At a given withdrawal rate, how many years will this portfolio last? What annual savings rate is required to reach a target portfolio by a specific age? What is the breakeven portfolio size for a particular spending level? Each of these questions has a different answer, and the answers change significantly based on the assumptions used.
Investment return assumptions are particularly sensitive. A projected 7 percent annual return on a balanced portfolio reflects the long-run historical average for U. S. markets, but any specific 30-year period can deviate substantially. Sequence of returns risk, the specific pattern of good and bad years, matters more than the average return over the period. Two portfolios with identical 30-year average returns can produce very different outcomes if one experiences large losses in the early years of retirement when withdrawals are eroding the principal.
The Federal Reserve's data on median retirement savings ($185,000) appears inadequate until Social Security is included. The average Social Security retirement benefit in recent years has been approximately $1,900 per month, or roughly $23,000 per year. For a retiree spending $40,000 annually, Social Security covers more than half the budget, and the portfolio needs to supply only $17,000. At 4 percent, that requires $425,000, which is still more than the median but considerably different from the $1 million figure that appears in retirement planning discussions about a $40,000 spending need without Social Security.
The Compounding Gap Between Starting Ages
Retirement age is the input that most dramatically changes projected outcomes. Retiring at 62 versus 67 is not just a five-year difference in contributions. It adds five years to the withdrawal period, requires the portfolio to sustain 35 to 38 years of spending instead of 30, and in most cases means a lower Social Security benefit, since benefits increase by approximately 8 percent for each year of delay between 62 and 70.
The contribution period matters in the other direction through compounding. A common illustration: investing $5,000 per year from age 25 to 65 at a 7 percent return produces approximately $1.07 million. Starting at 35 instead of 25, with the same $5,000 per year and the same 7 percent return, produces approximately $510,000, despite contributing for 10 fewer years. Ten years of compounding time is worth more than 10 years of additional contributions in that scenario because the compounding effect is multiplicative. Each year of growth applies not just to the contributions but to all the growth that preceded them.
The quote often attributed to Albert Einstein that compound interest is the "eighth wonder of the world" cannot be verified in his writings and appears to have originated in print only in the 1980s. But the mathematics behind the sentiment is accurate regardless of who said it first: the exponential nature of compounding makes the starting date of an investment a more powerful variable than most people intuitively expect.
Social Security Timing and Its Effect on Calculations
Social Security benefits can be claimed from age 62, but claiming early permanently reduces the benefit. Claiming at 62 reduces the full retirement age benefit by approximately 30 percent. Claiming at 70, the maximum delay, increases the benefit by approximately 32 percent above the full retirement age amount. For a retiree with a full retirement age benefit of $2,000 per month, claiming at 62 produces roughly $1,400 and claiming at 70 produces roughly $2,640, a difference of $1,240 per month or nearly $15,000 per year for the rest of life.
The breakeven point between claiming at 62 versus 70 is typically around age 80. Retirees who expect to live past 80 generally benefit financially from delayed claiming. Those who have reason to expect shorter lifespans may benefit from early claiming. This calculation should be included in any serious retirement projection because the total lifetime Social Security income can vary by hundreds of thousands of dollars depending on claiming age.
Retirement calculators incorporate this variable by letting you specify both your Social Security benefit amount (typically provided in the annual SSA statement) and the age at which you plan to claim it. The difference in projected portfolio sustainability between claiming at 62 and claiming at 70 is large enough to shift the required portfolio balance by $100,000 to $200,000 for many people.
Conclusion
ToolHQ's retirement calculator lets you enter your current savings, monthly contributions, expected return, and target retirement age to see projected balances and sustainable withdrawal amounts. Adjust the retirement age by a few years to see how much the projections change. The relationship between the inputs is nonlinear, and the results rarely move in the increments that feel intuitive.
Frequently Asked Questions
What is the 4% rule in retirement planning?
The 4% rule says you can withdraw 4% of your portfolio in your first year of retirement, then adjust that amount for inflation each year. Based on historical data, this rate should sustain a 30-year retirement without depleting the portfolio.
How much do I need to retire comfortably?
A common benchmark is 25 times your annual expenses (the inverse of the 4% rule). If you plan to spend $60,000 per year, you need $1.5 million in savings, minus any Social Security or pension income.
At what age should I start saving for retirement?
The earlier the better due to compounding. Starting at 25 vs 35 can result in more than double the retirement balance by age 65, even with the same monthly contributions, due to 10 additional years of compound growth.
Is the 4% rule still valid?
Bengen updated his estimate to 4.5-4.7% (the SAFEMAX) after including small-cap stocks. Some researchers argue lower rates are safer given current valuations and lower expected returns, but 4% remains the most widely cited benchmark.