I'm 67 years old, retired and have $600,000 in investments. Will this be enough if I live to age 95? -- Judy

It's impossible to give a yes or no answer to your question because the length of time your nest egg will last depends on several factors, including how much you withdraw each year, how you invest and how sure you are that you can count on those withdrawals in the future.

A few online tools can give you a sense of the trade-offs involved. You can then come up with a plan for getting a reasonable amount of income from your investments without too high a risk of running through your money too soon.

One tool is Vanguard's Retirement Nest Egg calculator. Even though this tool uses a sophisticated forecasting technique know as Monte Carlo simulation, it's simple to use and interpret.

Begin by dragging the first of the tool's three sliders to specify how long you want your portfolio to last. Given today's long lifespans, you want to be careful not to enter too short a period. Your assumption of age 95, or 28 more years, seems sound.

Next, move the second slider to indicate the current size of your retirement portfolio -- $600,000 in your case. Then drag the third slider to show how much inflation-adjusted annual income you'd like. You didn't mention a figure, but let's assume $30,000 a year, adjusted for inflation.

Once you've done that, go to the portfolio pie chart at the top of the page and enter how you want to divvy up your retirement investments. I'd start with a moderate mix of 50% stocks-50% bonds. Click "Run Simulation," and voila!

How I'm easing into retirement

You'll get an estimate of the probability your nest egg supporting you for 28 years given the withdrawals and investment strategy you've chosen, or a 77% chance in this case.

You'll also see a graph showing a range of possible portfolio balances year by year over that 28-year span, forecasting what your portfolio's value might be in good and bad markets. Click on that graph, and it shows how rapidly the odds of your nest egg lasting tail off in the later years.

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