Those calculations seem like little more than wishful thinking when the experience of actual retirees is taken into account. A majority of retirees questioned in the same survey said their retirement income was at least 80% of what it had been while they were working.

The risks of not having enough

Once the cost of a retirement has been estimated, any number of analyses can be run to learn whether the prospective retiree has the resources. A common denominator seems to be an appreciation of worst-case scenarios and a recognition, as McNair says, that financial forecasting is "not an exact science." Still, you should consider several obvious stumbling blocks.

Can a bad economy destroy the value of a retirement nest egg? Gnuse cautions against being lulled into a false sense of security by projections of average returns. An average doesn't take into account the possibility that there will be several years of below-average returns that could force any retiree to dip into his investment principal. Returns can "revert to the mean later on," Gnuse says, "but by then there may not be enough assets to revert." The stock market's long stumble from its 2008 high vividly illustrates this problem.

Will inflation cut down your purchasing power? Watch out for the ravages of inflation. A portfolio can earn handsome returns, but if the cost of living increases at a faster clip, retirement can be jeopardized. So you will need assets that will grow over time and provide a hedge against inflation.

How reliable is your income? Weigh this carefully. The future viability of Social Security is an open question. And company pensions and retirement savings plans are not always the bedrock upon which a retirement can be built. So not only do you need assets that will grow, you also need assets that won't shrink -- bonds, bond funds and similar income-generating investments.

To capture "the vagaries of what the real world throws at us," the Schaumburg, Ill., firm of Balasa Dinverno & Foltz runs what Mark Balasa calls a sophisticated "Monte Carlo" analysis for clients.

Using a Monte Carlo simulation and other modeling tools, Balasa is able to tell prospective retirees the probability -- given their assets, savings rates and ability to generate income -- that they'll achieve the retirement they seek.

He says in cases where the probability is low -- 60%, for example -- "We say, gosh, either you need to work a few more years, go for a riskier portfolio with a higher rate of return, or you need to die sooner."

Balasa also recommends considering a number of other factors in any early retirement scenario:

  • The age for taking full Social Security benefits is increasing. Those born before 1938 reached full retirement age at 65; those born later have to wait longer.
  • Medicare benefits begin at age 65, so anyone retiring before that age may be on the hook for the cost of basic health insurance.
  • Avoiding a 10% penalty for early withdrawal of IRAs and pensions requires knowledgeable planning -- and the use of such things as the 72t election, which permits a fixed amount to be withdrawn at regular intervals before the age of 59 1/2.
  • Be aware of a portfolio's "tax efficiency." The lower an asset's cost basis, the worse the tax consequence when it is sold.

A final factor that must be taken into account is the possibility of failure. What if you retire and then, for whatever reason, can't make ends meet? How do you unretire?

One strategy is to maintain contact with the work world by lining up part-time employment during retirement. A majority of boomers -- nearly eight in 10, according to the AARP survey -- say they plan to do that, whether out of perceived economic necessity or simply for fun.

Click here to become a fan of MSN Money on Facebook

A variety of resources exist -- private and not-for-profit -- to help retirees find work. A good place to start is AARP's Work & Retirement homepage.

Even if all of this rumination and calculation yields an unfavorable result -- a finding that early retirement simply is out of reach -- it will not necessarily have been in vain.

A substantive benefit of such exercises, McNair says, "is finding out what you need to do to get in the ballpark."