We've mentioned several websites that help you crunch your "retirement number".   Tools that help you determine just how much you'll need to save for retirement are being developed by several mutual fund companies and financial providers right now.  

Here's another approach-pretty simple, the "rule of 25," I read about this in a Kiplingers magazine years ago.   Just estimate how much you'll need to withdraw from your savings during the first year of retirement and multiply by 25.  It's a basic ballpark estimate-but a  good starting point.

CNNMoney runs us through the retirement calculation.

The "$25 in assets per $1 of income" rule I mentioned earlier assumes you're going to spend 30 or more years in retirement. If you retire very late in life, you might be able to get by with a smaller nest egg. Conversely, if you retire early, at 50, say, you probably need a larger one.