Both sites posted for calculators are good. However, just keep in mind that nothing is an exact science. In my experience, depending on the calculator, you can get very different answers.

For someone 40 years to retirement, maxing out a 401k ($10500/year, with a $0 initial balance) and an estimated 10% annual return...

I've seen figures from $1.5-6 million. That's a pretty wide variation.

There are lots of subtleties to projections, so I'm not knocking them. Just don't be upset if you end up at the bottom end of the range.

-H