The calculator assumes you never want to draw down your nest egg -- ie your savings will only grow larger. It also assumes you will spend the same amount in retirement that you spend now. Your rate of withdrawal is pretty much a wild guess as it depends on your rate of return, taxes in the future and inflation in the future. Historically 4% has been a good bet. See this article for a summary of this topic:

http://www.mrmoneymustache.com/2012/05/29/how-much-do-i-need...

and this article for more in depth writing:

http://financialmentor.com/free-articles/retirement-planning...