While there is no one right answer, to ensure funds last for decades, it is usually recommended that no more than 4% of the balance be withdrawn each year, an answer only 10% of the respondents gave. Thus, if you need to generate $50,000 of annual retirement income in addition to Social Security and other pension benefits, you will need to save $1,250,000 by retirement age. That is a conservative number meant to ensure you never outlive your retirement funds.

That assurance, however, comes at a very steep price — many people will have difficulty saving 25 times the amount they need to withdraw annually. Thus, it is probably best to go through a detailed analysis of how much you can withdraw. This amount can be calculated based on your life expectancy, expected long-term rate of return, expected inflation rate, and how much principal you want remaining at the end of your life. Guess wrong on any of those variables and you risk depleting your assets too quickly. Yet, your life expectancy, rate of return, and inflation are difficult to predict over such a long time. Keep these points in mind when making your calculations:

• Your life expectancy: While it is easy to find out your actuarial life expectancy, life expectancies are only averages. Approximately half of the population will live longer than that. You can gauge your life expectancy by how long close relatives lived and how healthy you are. Just to be safe, you might want to add five or 10 years to that age. After all, you don’t want to run out of money at age 75 or 80, when you might not be able to return to work.

• Rate of return: Expected rates of return are often derived from historical rates of return and your current investment allocation. Historical rates of return are averages of returns over a period of time. Actual returns may be better than that in some years and less than that in other years. You might want to be more conservative than that, assuming a rate of return that is lower than long-term averages. Even if you get the average return right, the pattern of those returns can significantly affect your portfolio’s balance. For instance, if you experience high returns in the early years of retirement when your portfolio balance is lower and then lower returns in the later years when your portfolio balance is higher, you will have a lower ending value than if the opposite occurred.

• Expected inflation: While inflation has been relatively tame recently, that has not always been the case. Inflation can have a dramatic impact on your money’s purchasing power. For instance, at 2.5% inflation, $1 is worth 78¢ after 10 years, 61¢ after 20 years, and 48¢ after 30 years. Since your retirement is likely to last decades, use an inflation estimate encompassing a long time period.

Use conservative estimates when making your withdrawal calculations. That will result in a lower withdrawal amount, but it will also help ensure that your funds don’t run out. You should review your calculations every couple of years in retirement, especially during the early years. If you find you are depleting your assets too rapidly, you may be able to go back to work on at least a part-time basis.