It is extremely small - .21% is correct.

One of the problems is that "atheism" is difficult to define. It's sometimes hard to draw lines between atheism, agnosticism and "non-religious" beliefs. That .21% is "declared atheists."

(from Wiki) The most recent American Religion Identification Survey report, released March 9, 2009, found that in 2008, 34.2 million Americans (15.0%) claim no religion. Of which, 1.6% explicitly describe themselves as atheist or agnostic.