From Wheaton’s center for the study of american evangelicals:
Given the imprecision involved in defining exactly what—or who—an evangelical is, it is no surprise that it is extremely difficult to establish a precise estimate of their exact numbers in the United States. With so many different evangelical denominations, thousands upon thousands of independent evangelical churches, evangelical constituencies of varying sizes within historically evangelical “mainline” Protestant denominations–and even inside non-evangelical denominations–there is no single entity that can possibly serve as a representative gatekeeper (or census-taker) for the movement.
For this reason, the best approach to an evangelical headcount is a judicious triangulation of various polling and survey data. But, even this is fraught with problems. As the discussion about the intricacies of definition above would indicate, the framing of the definition or wording of survey questions are important variables that can produce varying results. Until a massive, definitive study is undertaken, estimates of the number of evangelicals in the United States, therefore, are just that: estimates.
One frequently relied-upon casual benchmark over the years has been an attempt to define evangelicals as those that label themselves as “born again.” Between 1976 and 2005 the Gallup organization asked roughly 1,000 adults some permutation of the question “Would you describe yourself as a ‘born-again’ or evangelical Christian?” In that first survey 34% of the people being surveyed responded “yes.” Over the years, the number fluctuated dramatically, reaching a low of 33% in 1987 and 1988 during the televangelist scandals, and a high of 48% in 2005. Overall, the numbers have averaged just under 39% of the population (See Gallup poll results >>).
Obviously, describing one’s self as “born again” as the definitive benchmark for identifying evangelical believers makes for a problematic statistical tool; the term “evangelical” is even less reliable (in one study, only 75% ofSouthern Baptists accepted either term!). For a variety of reasons, some groups and individuals which scholars would describe as “in the team picture” simply do not use those words to describe themselves. Unfortunately, attempts to use very tight definitional and behavioral criteria (church attendance, prayer, Bible-reading, evangelism, etc.) also prove frustrating because they ignore a very real slice of the American population which can only be described as “cultural evangelicals.” Similar in many ways to non-practicing Catholics, these lapsed-evangelicals do not show up as particularly pious or devout in studies that measure conventional religious behavior. But when these individuals do evidence any interest in church or seek spiritual change in their lives they almost inevitably gravitate toward points evangelical.
Another frustrating aspect of some studies is that they—for reasons having to do more with political demographics than religious characteristics—tend to separate out nearly all of the nation’s African American Protestant population (roughly 8-9% of the U. S. population) which is overwhelmingly evangelical in theology and orientation (for example, 61% of blacks—the highest of any racial group, by far—described themselves as “born-again” in the 2001 Gallup poll). In summary, when one lays a number of different studies side-by-side and considers the fact that many Americans could be described as “cultural evangelicals” (particularly within the African-American and Southern white populations), a general estimate of the nation’s evangelicals could safely be said to range somewhere between 30-35% of the population, or about 90-100 million Americans.