People on the verge of retirement underestimate the length of their retirement years considerably–along with how long their savings will need to last, according to a new study.

Pre-retirees believe they will need to make their retirement savings last until an average of age 83, according to a report on the study, conducted by Fidelity Investment Research, part of Fidelity Investments, Boston.

The problem is that a healthy 65 year-old man has a 24% chance of living to at least 90 and a healthy woman a 35% chance of reaching that same age, according to estimates cited in the study.

The report, “Structuring Income for Retirement,” found that 53% of preretirees are concerned about outliving their savings, yet 61% admit that they have not made a formal calculation of how much they can afford to spend each month in retirement to prevent that from happening.

For those who have no idea how much they can afford to spend, the most popular reported income planning strategy is simply to “live as they did before retirement and make adjustments later if necessary,” the report states.

To help aging boomers get their retirement planning in order, the report suggests a number of approaches to embrace in their financial planning, including lifetime income annuities with fixed or variable payments, variable annuities with guaranteed living income benefits for life and a traditional systematic withdrawal plan with investments in stocks, bonds and cash.

Each of the three investment options plays a different role within a portfolio designed to provide structured income in retirement, the report maintains.