Immediately after 9/11, many journalists in the United States and Europe came out with several glib generalisations that I found, and still find, irritating, and furthermore, I am very skeptical of the claims contained in them. We were told that either 40,000  or even 60,000 people had converted to Islam in the wake of the attacks on the Twin Towers in Manhattan, and that Islam was the fastest growing religion in the world. The figures 40,000 and 60,000 come up time after time. A quick Google search will give you many journalists evidently copying each other without any verification or independent evidence. For example, author Ed Rogers, writing in 2009, tells us that “…60,000 Americans who were raised in a “Christian” home are converting to Islam every year” .
But this turns out to have been copied without acknowledgement from a book written in 2004 by Joel Richardson, Will Islam Be Our Future? A Study of Biblical and Islamic Eschatology, Chapter 1: “But here’s the other sad aspect of these figures: Over 80% of these American converts to Islam were raised in a Christian Church. If the higher figures of conversion are accurate, that would mean that as many as 60,000 Americans, who were raised in Christian homes, are converting to Islam annually.”  So now the figure 60,000 is claimed for annual rates of conversion.
Another report, published in 2011 by the Religion News Service, based in Boston, tells us, “The majority of post-9/11 converts are women, according to experts, Hispanics and African-Americans, who were already converting well before 9/11, are the most common ethnic groups to convert. Though exact numbers are difficult to tally, observers estimate that as many as 20,000 Americans convert to Islam annually”. Notice, first, the new figure for the number of annual converts: 20,000, and, second, those ubiquitous “experts”, but we are not told which “experts” or how they have made their calculations. They are now joined by “observers”. 
Another quote that comes up over and over again is from The Times of London, January 7, 2002: “There is compelling anecdotal evidence of a surge in conversions to Islam since September 11, not just in Britain, but across Europe and America. One Dutch Islamic centre claims a tenfold increase, while the New Muslims Project, based in Leicester, [England] and run by a former Irish Roman Catholic housewife, reports a “steady stream” of new converts.”
“Anecdotal evidence” in the form of personal testimonies can be powerful, moving, and very useful, though one has to treat such evidence with caution when trying to calculate national number of converts. As for the number of Muslims in the United States, there have often been unscientific guesses, with many Muslim organizations inflating their figures for political reasons. As Dr Tom W. Smith of the University of Chicago wrote in October 2001, “None of the 20 specific estimates during the last five years is based on a scientifically-sound or explicit methodology. All can probably be characterized as guesses or assertions. Nine came from Muslim organizations such as the Islamic Society of North America, the Muslim Student Association, the Council on American-Islamic Relations, the American Muslim Council, and the Harvard Islamic Society, or unspecified ‘Muslim sources.’ None of these sources gave any basis for their figures.” 
Certainly the most sober assessment of numbers and conversions seems to come from The Economist. In an article published in 2013, The Economist cites the research of Kevin Brice, of the University of Wales, who calculated that 5200 Britons convert to Islam every year, and that the total number of converts is about 100,000; presumably over a period of twenty years, though this is not made clear. For the United States, The Economist turns to the Pew Research Center, and says: “In 2007 the Pew Research Centre (sic) estimated that there were around 2.4 million American Muslims…Pew reckons that just under a quarter are converts”. Actually, the Pew Research Center did no such thing. First, The Pew Research Center only gives percentage estimates, and second, they are for adults only, children are not included.
According to The Pew Research Center, basing themselves on the Religious Landscape Study [RLS], Muslims made up 0.4 % of the adult population in 2007.  How did The Economist arrive at the figure of 2.4 million Muslim Americans? According to the Pew Research Center, in 2007, there were 227 million adult Americans , in which case, 0.4% of them were Muslim Americans, that is, 908,000. And the adult population of the USA in 2014 was approximately 245 million, of which 0.9% were Muslims, that is, 2,205,000.