How long should the money last? The 20 to 40 year choice
Suppose you must pick one number from a list: 20, 25, 30, 35 or 40 years. The money has to last that long. Which would you choose?
Our money calculator offers those five, and a sixth choice: living on interest only, so the money is never spent. We used it for one example. The spending is $8,000 a month, or $96,000 a year, in today's dollars, before tax. Real yields on US inflation-linked Treasury bonds are from October 2, 2026.
Plan length is the first choice. The second is how the money is held. We tested each against every starting year from 1928 on.
Money needed by plan length and mix (today's dollars, no Social Security)
| Plan length | Ends at age, if you start at 65 | Bonds only | 40% stocks, 95 in 100 | 40% stocks, every past period | All stocks, 95 in 100 |
|---|---|---|---|---|---|
| 20 years | 85 | $1,481,000 | $1,626,000 | $1,646,000 | $2,099,000 |
| 25 years | 90 | $1,724,000 | $1,847,000 | $1,859,000 | $2,330,000 |
| 30 years | 95 | $1,925,000 | $1,991,000 | $2,026,000 | $2,433,000 |
| 35 years | 100 | $2,090,000 | $2,089,000 | $2,131,000 | $2,493,000 |
| 40 years | 105 | $2,221,000 | $2,185,000 | $2,216,000 | $2,618,000 |
Bonds are US inflation-linked Treasury bonds. The stock mix is rebalanced yearly. Results come from our calculator.
Compare the bonds-only column with the 40% stocks column. At 20 years, 40% stocks needed $145,000 more to reach 95 in 100. At 35 years the two were the same. At 40 years the stock mix needed $36,000 less.
All stocks needed more at every length: 42% more at 20 years and 18% more at 40. At these yields, bonds are hard to beat unless the plan runs long.
Why would a longer plan favor stocks? The longer the money is invested, the more years there are to recover from a bad start.
The history is short, though. From 1928 to 2025 there are 59 possible starting years for a 40-year plan, and they overlap. They behave like about two and a half separate periods. We would not rest much weight on the 40-year row.
Social Security changes every row. Take a payout of $2,900 a month in today's dollars. That is the middle of the range for a top 10% earner who starts at 67, two years after stopping work. We assumed it rises with prices and is not cut.
Bonds only, with and without Social Security (today's dollars)
| Plan length | Without | With $2,900 a month from year 3 | Less money needed | Share |
|---|---|---|---|---|
| 20 years | $1,481,000 | $1,013,000 | $468,000 | 32% |
| 25 years | $1,724,000 | $1,168,000 | $556,000 | 32% |
| 30 years | $1,925,000 | $1,296,000 | $629,000 | 33% |
| 35 years | $2,090,000 | $1,401,000 | $689,000 | 33% |
| 40 years | $2,221,000 | $1,484,000 | $737,000 | 33% |
The 30-year row matches the calculator's default with Social Security entered.
A payout for life shortens the gap between plan lengths. The step from 20 to 40 years costs $740,000 without Social Security and $472,000 with it.
Yields move the numbers. For the 30-year bonds-only plan, today's yields give about $1,924,000. With yields one point lower, it would be about $2,176,000. With yields one point higher, about $1,715,000.
The interest-only choice costs the most. At $96,000 a year and a real yield of 3.34% on 30-year bonds, it takes about $2,874,000, which is $653,000 more than the 40-year plan. In return, the principal stays.
The chance that the money is needed in the last years is small. For a couple who are both 65, at least one is alive at 95 about one time in five. At 100 it is about one in twenty-five. Another article in this series gives those chances.
Back to the list of five numbers. A longer number covers more of the years a person might live, and it costs more. The tables show how much more.
Which number would you pick, and what made you pick it? Tell us in the comments. Someone may be about to choose.
Sources we read
Our money calculator on this site, with US Treasury real yield curve rates for October 2, 2026. https://claude.ai/artifact/Hkp7mcbVE8k1ooB41X8tX6
US Treasury, daily real yield curve rates. https://home.treasury.gov/resource-center/data-chart-center/interest-rates/daily-treasury-rates.csv/2026/all?type=daily_treasury_real_yield_curve&field_tdr_date_value=2026&page&_format=csv
Aswath Damodaran, historical returns on stocks, bonds and bills, and US inflation, 1928 to 2025. https://pages.stern.nyu.edu/~adamodar/pc/datasets/histretSP.xls
Social Security benefit formula and 2026 figures (bend points and earnings cap), as summarized. https://wealthvieu.com/retirement/social-security/maximum-social-security-benefit/
Our life expectancy calculator on this site (chances that at least one partner is alive). https://claude.ai/artifact/UgScC4Cit4CGiZZMkP41Wi
Tell us in the comments